TRANSCRIPT Eat This, Not That: The Surprising Science of Personalized Nutrition

This is a transcript of the Gastropod episode, Eat This, Not That: The Surprising Science of Personalized Nutrition, first released on June 10, 2019. It is provided as a courtesy and may contain errors.

RESEARCHER: Five, four, three, two, one. Enjoy!

CYNTHIA GRABER: Wow, “enjoy” was a little bit of an exaggeration. LAUGHTER

NICOLA TWILLEY: They’re even bluer inside! Look at that color. That’s not found in nature, I’m gonna say.

GRABER: Ok, it’s oily and sweet, is my response.

TWILLEY: It’s left a little blue oil slick on the plate, which is charming.

GRABER: Yes, we ate muffins the color of a Smurf, just for you, dear listeners. And for science.

TWILLEY: It wasn’t a muffin, it wasn’t a meal, it was a metabolic challenge. And our goal was to find out how our bodies—mine, and yours, Cynthia—responded differently to that challenge. How we handled those bright blue muffins.

GRABER: We of course are Gastropod, the podcast that looks at food through the lens of science and history, I’m Cynthia Graber—

TWILLEY: And I’m Nicola Twilley. And this episode, we are doing an experiment. Alongside several hundred other people.

TIM SPECTOR: it’s the world’s largest nutrition study of its of its kind about personalized nutrition.

GRABER: Tim Spector is the scientist running this study, and of course you’re going to hear a lot more from him this episode. He wants to know—and we want to know—do we each respond to different foods differently? You’ve heard of personalized medicine, will there be such a thing as personalized diets?

TWILLEY: And should there be? Will figuring out the nuances of how my individual body processes different foods make me healthier? And if we all do it, can it cut down on diabetes and other obesity-related diseases? It’s an exciting idea, but what’s the science?

GRABER: This episode, we answer these big questions, and also eat a lot of muffins, weigh our meals, learn to take our own blood samples, and so much more. So much fun.

TWILLEY: This episode was made possible thanks to generous support from the Alfred P. Sloan Foundation for the Public Understanding of Science, Technology, and Economics, as well as the Burroughs Wellcome Fund for our coverage of biomedical research.


GRABER: Tim Spector is a genetic epidemiologist at Kings College in the UK. We met him at the hospital where he works, it’s called St Thomas’s.

TWILLEY: Which explains why you’ll hear ambulances in the backgrounds once or twice while he’s talking.

GRABER: Tim’s specialty is twins.

SPECTOR: Yeah. Twins UK is a cohort of 14,000 twins that I started myself in 1993. I only have had one good idea in science and that was it. But it’s lasted me a career. And we follow up these twins every three or four years, bring them into the hospital, do a whole range of clinical tests on them, give them questionnaires, bleed them, prod them, and we’ve basically looked at all kinds of things about what’s nature and what’s nurture.

TWILLEY: Tim has asked lots of questions of his multiplicity of twins. Because identical twins have the same DNA, any differences between them can’t be totally genetic. And then fraternal twins share an early childhood environment. So put a bunch of twins together and you can start to tease out which has more influence—genes or the environment.

SPECTOR: And we’ve looked at things like the genetics of sense of humor and political views. And even in belief in God. So virtually everything you look at has some genetic basis. So the twin model is a really nice way to balance out—it’s like doing a randomized controlled trial for life.

GRABER: When it comes to their weight, the identical twins, who again, have exactly the same genetics, they don’t necessarily follow the same patterns in life. One might be 30 to 40 pounds heavier than the other.

TWILLEY: That heavier twin—turns out they would often have a very different gut microbiome to their lighter but genetically identical twin. Over the past five years, Tim has used his twins to isolate a handful of particular microbes that seem to have an effect on weight—he tested them by putting the gut microbes from the skinny twin in mice, and watching the mice get thinner.

GRABER: Obviously there is a genetic component to whether or not you’ll gain weight easily, some people do, and so do their parents, and some people just don’t. I do want to take a moment to point out, as we always do when we’re discussing issues of nutrition and weight—we know that people can be healthy at a variety of weights. But as we’ve also pointed out in past episodes, there is a rise in diet- and weight-related diabetes, and there are other diseases linked to, again, diet and weight, like heart disease.

TWILLEY: Tim’s research on the twins shows that about seventy percent of their chance of gaining weight more easily or instead more easily keeping it off is linked to their genes. Other researchers, looking at other populations, have pegged that number at anywhere from 35 to 80 percent—it’s hard to pin down to a precise number, but the point is, it’s not entirely down to your genes.

SPECTOR: The rest is due to things that you can modify, or environment, or random.

TWILLEY: Tim wanted to figure out how different each of our particular responses to food really is—how we metabolize it. And he wanted to know why, and he wanted to know how those differences are connected to weight and other health outcomes. So he launched a huge study—the study we took part in. It’s called PREDICT, which sounds like a “Black Mirror” episode, and it is kind of surveillance-y and futuristic, to be honest.

SPECTOR: PREDICT study is really a series of studies trying to inform people what type of food is best for them. So it’s personalizing nutrition in a way that’s never been done before. And what we’ve decided to do is to individually test lots of people with standardized food challenges, to see how the body responds to those, by measuring particularly blood markers, and see how the levels go up and down over time.

GRABER: Tim started off with 700 of his twins, and when he analyzed the results, he found some surprising differences.

SPECTOR: So one will respond differently to a banana. The other to a cookie. And this is not something you’d ever predict. We always believed that identical twins who are clones should behave in the same way.

TWILLEY: Next up, Tim decided to do what’s called a validation study, testing the algorithm on a different population—and on people who didn’t share all their DNA. And he decided to do that on one hundred lucky people in America. At the Massachusetts General Hospital.

GRABER: And we were two of those lucky ones!

TWILLEY: Obviously, when we heard there was an experiment that was studying the effect of our gut microbes…

GRABER: Yes, drink—

TWILLEY: …and other factors, on how our bodies respond to food, we volunteered ourselves as guinea pigs. Our commitment to microbes knows no bounds.

GRABER: We had a couple of long phone calls with some folks at Mass Gen, and they asked us repeatedly whether we were really up for the challenge of participating. They asked us this question in like ten different ways, but Nicky and I were determined.

TWILLEY: And we got in! And then they kindly sent us a welcome gift.

SPECTOR: What individuals have to do is they get a pack sent to them at home with a whole list of instructions that looks a bit daunting. Various pots to pee in and poo in.

TWILLEY: I had actual anxiety dreams about my ability to produce an adequate sample for the feco-tainer. Which is what it sounds like.

GRABER: It was not a fun part of this study to fulfill, let me tell you.

TWILLEY: And then came the paperwork.

GRABER: Basically, they wanted to know everything we’ve been eating. For the past year.


GRABER: Sour cream one tablespoon. How many times average, per month per week per day, have I had a tablespoon of sour cream? I like sour cream, I use it in things. Okay. This is gonna be fun.

TWILLEY: Question: Like one tablespoon. So if you make a thing that uses five tablespoons, but it’s only on one day of the month.

GRABER: Well I think you can average it.


TWILLEY: Yes. These are the kind of scintillating conversations with Cynthia that I live for. We had to figure out the same thing for broccoli—how many cups per week, and bread—how many slices, and ice cream—how many scoops.

GRABER: Just think for a minute about how you would answer these questions. It’s not easy.

TWILLEY: We really struggled. But these kind of food frequency questionnaires are the bedrock of nutrition research. Tim’s been using them for 20 years.

SPECTOR: They were invented in the U.S. There are a lot of critics of them. But to my point of view they serve a number of good purposes. So on the whole they’ve been positive. They do give you patterns that we think are real.

GRABER: They can’t be super exact, of course. We all have a hard time remembering what we’ve eaten—a recent study even showed that when people tried to remember what they ate over the past 24 hours, they misjudged the sheer amount of food they consumed by hundreds of calories.

SPECTOR: And we know that people systematically do lie about them, and It’s caused a lot of spurious data.

TWILLEY: For PREDICT, Tim has developed a new way of tracking food intake: Basically tracking our food in real time, as we eat it. But to get a baseline sense of our diets—what we normally eat—he says the food frequency questionnaire is the only tool he’s got.

GRABER: And then after we got done with all of that, the next morning, we headed off to the hospital.

SPECTOR: Then the fun starts.

TWILLEY: For me, the fun actually started with my blood draw. Turns out I have no blood in my veins.

GRABER: Or at least it’s apparently really hard to find.

SHEILA: So I’m going to put a little tourniquet on here… and I’m just going to look right now. And this is gonna get a little tight. And I’m going to give you a little squeezy ball. It is a tiger. LAUGHS Okay, go ahead and squeeze.

TWILLEY: This is my lovely nurse. She was very patient. She tried my left arm. And then my right arm. And then my left arm again.

SHEILA: I might go back over to the other side actually. Yeah you can squeeze more. Yeah. SQUEEZING SOUNDS

GRABER: I was getting a little nervous at this point, Nicky’s nurse had even called my nurse over to help her out.

TWILLEY: Finally, they got some blood out of me. And they took our blood pressure too.


TWILLEY: They did a full body fat composition scan. And they managed to get a glucose monitor in, although again, mine was much more traumatic than yours, Cynthia.

GRABER: My process was pretty easy, my nurse popped two of these quarter-sized glucose monitors onto me


GRABER: A tiny needle slid into my skin under each one. And then she put a big piece of tape over them both so they’d stay stuck in place for two weeks.

TWILLEY: Blood glucose levels—they’re also called blood sugar levels—this is one of the big markers that Tim wanted to study in PREDICT. How they go up and down over time is called your glycemic response.

JENNIE BRAND MILLER: It’s how your blood glucose changes when you start to eat a food.

GRABER: Jennie Brand Miller is professor of human nutrition at the University of Sydney, and she is one of the world experts when it comes to blood sugar and insulin levels in our bodies. She’s been studying this for three decades.

TWILLEY: Anytime you eat something that contains any carbohydrate at all—and most things do—your blood sugar will go up a little and then back down again. That’s normal. It’s all well and good.

GRABER: What’s not normal, or at least what’s bad, is if your blood sugar spikes at a particularly high level, and then also if it doesn’t come down quickly.

BRAND MILLER: And the harm it does is simply because glucose leads to these inflammatory responses that are harmful in terms of the fact that they might cause over the longer term blindness or kidney failure or nerve damage.

TWILLEY: If you have a single blood sugar spike after eating something, your body is perfectly capable of bouncing back from that.

BRAND MILLER: It has to be a kind of constant background insult going on day after day, week after week, month after month. It will do harm.

SPECTOR: if one person has a high glucose response, another one has a low glucose response, we now know that the person with the high response, if that occurs repetitively, is more likely to get fatter and more likely to get diabetes. And we think this is because it’s causing more stress on the system that the insulin is also under stress, it’s overproduced and so eventually will start to fail.

TWILLEY: Insulin is a hormone that your body produces to manage your blood glucose levels. So there are two problems: One, the repeated spikes in blood sugar cause inflammatory responses. Two, the repeated spikes puts so much pressure on your insulin production system that it pegs out.

GRABER: This is why our nurses put those glucose monitors on us, and this is the main thing that Tim’s PREDICT study is testing over the full two weeks—he and his team are looking at whether our blood sugar levels spike particularly high after a given food or meal. And whether I spike higher or lower in response to different foods than you do, Nicky.

TWILLEY: Once we had our all-important glucose monitors in, we were outfitted with a stylish activity monitor, which tracked our exercise as well as how long we slept.

GRABER: And then we were finally able to break our more than 12 hour fast with those glorious blue muffins and a delicious cup of chocolate milk.

DAVID DREW: They are blue because we are trying to understand your GI transit time. They are very blue, so prepare yourselves. LAUGHTER

GRABER: The color of the muffins, Nic!

TWILLEY: Love it.

GRABER: It’s like this blue-green algae color.

TWILLEY: The muffins were so blue for a reason. As our doctor, David Drew said, the point of the color was to measure transit time.

GRABER: So—transit time. What we were supposed to do after eating the blue muffin is check in the toilet every day until we had the unique pleasure of staring at blue poop. It’s one factor they’re looking at to try to understand our gut health.

SPECTOR: And people’s recall of how long it takes and how often they go is not very reliable. And so having a test where you see this blue marker up in the toilet pan is actually really useful for analyzing the results and adjusting for that.

TWILLEY: Yum. We gazed at our blue muffins until we reached the scheduled time for eating them.

GRABER: And then we ate them in the ten minutes we were given to finish it all. It was kind of hard to get them down even as hungry as we were. The muffins were sweet and dense and super oily.

TWILLEY: And then the poor nurses had to draw more blood.

DREW: Yeah. So the first muffin’s a metabolic challenge muffin. So it’s high fat, high sugar. We really want to measure responses in your body to really shock it first thing in the morning with that. So I’m sure it was nice right after fasting. But you guys finished it, so good job on that. LAUGHTER

GRABER: With difficulty!

TWILLEY: Our post-muffin blood should have had a peak in insulin, in response to all the sugar, but also a spike in something called triglycerides, which is what any fat you eat that you don’t need right away becomes in your blood.

GRABER: They took our blood every hour, that was a lot of fun.

TWILLEY: Especially for me and my nurse because it took almost an hour to get a vial’s worth of blood out of me every time!

GRABER: Then in the afternoon—this was a really long day—

TWILLEY: Really long.

GRABER: We had a little break and we learned how to use the PREDICT app to log our food.

TWILLEY: The app was designed by a company called Zoe. Zoe was founded by Tim, along with a couple of internet entrepreneurs.

SPECTOR: And unlike any other food logging device or questionnaire I’ve done, I found this an amazing advance over all the other methods I’ve done. And the good news is about 95 percent of all the participants lasted at least 13 days of food logging, which is unheard of in the nutritional field. Most people say about 50 percent drop out after two days.

GRABER: We wanted to be one of those 95%. So we spent more than an hour learning how to use the app from a nutritionist named Shannon.

TWILLEY: We learned how to look up ingredients on the app, how to put in all the ingredients for a given dish, how to weigh and log it all separately on the scale before cooking, and then weigh and log our personal portion after cooking.

GRABER: How to take a photo of our dish with the little card next to it to give a sense of perspective and size, how to put in the time of when we started eating, how to copy and paste if we ate leftovers, and then go back in and edit the actual amount of food, EVERYTHING.

TWILLEY: You have to log some ingredients with their raw weight and some with their cooked, you have literally weigh every drop of oil or shake of salt, forget taking an extra spoonful of something or tasting while you’re cooking—you’d have to stop, weigh, photograph, and enter it all in the app. Even with Shannon’s help, it took us 20 minutes to enter a practice meal—an imaginary ham and cheese sandwich.

GRABER: Finally, after our fun-filled day at the hospital, they sent us home with the app, and with a goody bag filled with frozen muffins and weird drinks and needles and another feco-tainer.

TWILLEY: And to be honest, that’s when things really got hard.


GRABER: We should have taped our first attempt to make dinner that same evening back at my place with my partner Tim. But frankly, we were hungry and exhausted, and we were totally caught up in trying to figure out how to work the app!

GRABER: So we made it through our first dinner and we weighed and measured absolutely everything. And it was really hard.

TWILLEY: It was a nightmare. The food was cold by the time we ate it because I would say it probably took half an hour to weigh—not just weight, but weigh and take a picture with the stupid picture card of every single separate thing.

TWILLEY: And then, after all that, Shannon emailed to say we’d done it wrong.

GRABER: It was a learning experience. We fell into bed when dinner was over, then we woke up in the morning, and we ate muffins.

TWILLEY: Well, but first we had to give blood samples. The at home blood sampling was more like what a diabetic person has to do with finger pricks—and now I have a whole new level of sympathy for them. We had to jab our fingertips and then squeeze our fingers like milking a cow to get four big blots of blood on a special card.

GRABER: It stung when I jabbed, but even worse was how sore our fingers were for days afterwards. We had to do it three times a day for the first four days. And we moved from finger to finger, so a couple of days in, it got really hard to type.

TWILLEY: So the routine was: Jab, squeeze, blood, app, eat the breakfast muffin. And then do it all over again, three hours later: Jab, squeeze, blood, lunch muffin.


TWILLEY: They’re so dense, they’re so rubbery.

GRABER: Oh my God. I can barely get down the first two this morning in 10 minutes. I can’t believe I have to eat all three of these in 10 minutes now.

TWILLEY: It’s like a dead lump of mattress stuffing in my mouth.

GRABER: After eating five or six muffins a day for the first couple of days—after that we only had to have the muffins at breakfast—well, first of all, I already never wanted to see another muffin ever. But more importantly, why muffins?

SPECTOR: This came from the Nutrition Department here, and they found it the best way to very accurately deliver precise amounts of a certain fat and a certain carbohydrate and sugar and balance. And do it in a very standardized way that was palatable, transportable. You could freeze it.

TWILLEY: This was a fascinating aspect of our daily lives in the experiment—the muffins were obviously different each meal. Some had risen, some were flat like hockey pucks, some were sticky, some were dry. But that was intentional.

SPECTOR: There were basically, as far as I remember, the three different ones. So there was a high fat, low fat and an average American British meal ratio of carbs and fat.

TWILLEY: The muffins, as delicious as they were, they were not the worst part. The worst part was the non-muffin meals. For the first couple of days of the experiment, we were traveling—we had to go to DC for a conference—but we couldn’t eat any food that we couldn’t weigh the raw ingredients of and photograph.


GRABER: Here we are at a reception at a party. We can’t eat anything here.

TWILLEY: There are passed hors d’oeuvres. There’s fried artichoke with aioli.

GRABER: There was a really beautiful like fried cigar that had gorgonzola and some other lovely things inside.

TWILLEY: Mashed potato.

GRABER: Oh, mashed potato.

TWILLEY: And then deep fried.

GRABER: Oh. I really want it! I’m so hungry.

GRABER: Instead, we had to cobble together our dinner from ingredients that we could weigh and log in the app.

TWILLEY: We set up a little station at a cocktail table and got out our scales and our phones, and prepared dinner.

GRABER: And we have cheese, we have almonds, we have crackers. And we have chocolate. Oh we have carrot—no—where the carrots???


GRABER: We didn’t bring the carrots!

TWILLEY: I think you had to be there, not drinking the complimentary cocktails and not eating the delicious hors d’oeuvres to realize how sad forgetting the carrots really was.

GRABER: My parents live in DC and I visited them a few days into the experiment. I of course had to be in charge of preparing any meal we ate. They were, well, perplexed.

CYNTHIA’S DAD: I want to know why anybody would do this.

GRABER: LAUGHS It’s a really good question. And Mom, what do you think about the study?

CYNTHIA’S MOM: I think you’re both nuts. I think everybody’s nuts. Why would you do—I don’t see the benefit of it.

TWILLEY: As always, Cynthia, your mom is right. This was one of the unintended side effects of the study: Social isolation. I basically didn’t see any friends for two weeks, because who wants to hang out with someone who can’t even have a drink without weighing it. There was a real low when I made chana masala from scratch and weighed and photographed everything and then when I finally sat down to eat it, the chickpeas were undercooked because I hadn’t been able to taste it as I went along! I nearly cried. I dumped it out and had toast. Without butter because I couldn’t handle weighing and entering anything else.

GRABER: One time I had almost no time for lunch and I forgot to log the salsa I was putting on a dish so I basically gave up and ate chocolate. And then one night at dinner, my partner Tim was not thrilled when we were cooking a meal we make usually a couple times a month. It’s our easy go-to.


GRABER: OK. So it’s time to make our usual stir fried rice. What are some of the challenges you’re running into in making this dinner together tonight.

TIM: So how do I measure like the sauces that I’m going to squirt in? Like I don’t know how much I’m going to squirt in, am I going to squirt in a tablespoon? No. Am I going to squirt in three cups? No. So tell me what you want me to weigh. I don’t know what to weigh!!

GRABER: Everything! That was the answer. Tim, weigh every last drop of soy sauce and sriracha.

TWILLEY: And of course photograph it.

GRABER: There were some fun surprises thrown in. For one morning, we had to eat two muffins PLUS these two like snack bars that were extra loaded with a type of fiber called inulin. One bar tasted like a much less pleasant version of the inside of a fig newton—it was apple and walnut flavor, I will never eat it again, and the other was like a super dense chocolate Snackwell cookie. Also not pleasant.

TWILLEY: And together they were way more than your daily requirement for fiber. But this is not at all how I normally get my fiber—whole grains, veggies. This inulin stuff is extracted from a plant called chicory, and added to processed food as a supplement. So that seemed weird to me. Do our bodies really respond to the fiber in this one industrially extracted supplement the same way they do to the fiber that’s naturally in veggies and grains?

SPECTOR: You’re absolutely right. I mean, I’m a big believer that food components as normal food probably make your body react differently when it’s in a pill or a supplement or liquid form. But the reason for giving these fiber bars is to standardize it. It’s very hard to give people several pounds of spinach and kale, and they’ve got to boil it in the exact amount of water, and you know.

GRABER: This is part of why it’s tough to do nutrition studies—to do good science, everything needs to all be standardized, but once you create something standard, it’s not necessarily really like food, or like how we eat normally.

TWILLEY: Another weird standardized thing was this pure glucose drink they had us have for breakfast for two days. It tasted like flat, super-sweet Sprite, and you had to drink it and then sit still for two hours. I just went back to bed.

GRABER: These are used to test if someone is diabetic, to see how their blood sugar levels respond to this huge rush of glucose. I was super nauseous and light headed for about an hour after I drank it.

SPECTOR: And I found that the hardest thing of the whole study to have to drink, was the second one you know because I knew the first one was horrible.

TWILLEY: Yeah, it made me feel completely terrible. Shaky and loopy. I hated it. Cynthia and I called each other the next morning to commiserate.

TWILLEY: Here we are, slamming the muffins, but I was happy to see them this morning because…

GRABER: They weren’t glucose.

TWILLEY: They weren’t a glucose drink. LAUGHTER

GRABER: The final weird breakfast drink was this large glass of milk with an equally large package of protein powder.

TWILLEY: I got my husband Geoff to hold my mic while I prepped that particular treat.

TWILLEY: Here we go. Okay. And shake. LIQUID SHAKING SOUNDS. Well. It’s bubbly.

GEOFF: It’s like breakfast foam.

TWILLEY: The milkshake made me burp—big rancid cheesy burps all day.

GRABER: I don’t know exactly how much dairy was in this shake, but I’m a little sensitive to lactose and I was writhing in stomach-clenching agony for a couple of hours.

TWILLEY: So all in all, you can imagine our joy when we reached the end of the study! Which Cynthia reached three hours ahead of me because she lives on the East Coast.

GRABER: OK. It is Tuesday night, February 26th, 9:30 p.m. And you know what that means.

TWILLEY: That means I’m about to have the last meal that I have to weigh. But it also means that you are done.

GRABER: I am so done! I am so so done.

TWILLEY: So done.

TWILLEY: The set of scales Shannon gave me stopped working on the very last dinner. Which felt symbolic.

GRABER: The whole thing was so hard—there were times I thought I wouldn’t make it. So maybe my parents had a point—why bother?

SPECTOR: The idea is that the more individual data we can gather, the more we can break down the complexity of food, and break down the complexity of our individual responses so that everyone will benefit.

TWILLEY: Tim believes that this research—all the painful blood pricks and food logging and muffins—it’s going to lead to a huge breakthrough in nutrition and public health. He thinks the guidelines that we’re given in the developed world—these food pyramids and healthy plates and recommendations about how much of our diet should be carbs and protein and fiber—he thinks they’re wrong.

GRABER: And not just wrong, but that they’re harmful. He pointed out to us that they were originally developed after World War II to help people who didn’t have enough food to eat, to tell them what they should try to eat.

SPECTOR: We’ve been misled into believing that these guidelines which was to save people from massive malnutrition, should be then used to tell people what to eat in a time when there’s no malnutrition… i.e., reversed—has set everything back and stopped us believing in this individuality and tried to lump everyone together to say that everyone should have exactly this percentage of carbohydrates, this amount of calories, this amount of protein. And that’s all that matters. And you know, how you get it doesn’t matter. This is the ideal American standard and that everyone, if you do this you’ll be fine. And of course what’s happened is America has got three times fatter whilst following these standard guidelines that don’t fit anybody and were never really designed for that purpose.

GRABER: We’ll be getting back to whether these guidelines are entirely to blame for that. But in general, part of the problem scientists have in reassessing these guidelines, or updating them, isn’t just that we’re all different, it’s also that they use those food history questionnaires Nicky and I had to try to answer. And those just aren’t detailed enough to get super accurate information about what we’re eating, and then match that to how our bodies are responding and how healthy we are.

TWILLEY: So what we put ourselves through with this experiment and the muffins—how can that translate to new nutrition advice? What’s Tim’s plan for turning all that data we generated into something that helps us decide what to eat?

SPECTOR: Everything is going to come together to be put into this algorithm that will match one person’s responses. And it will come back in an app that will tell them: This is what I should be eating. This is what I should be avoiding or having more rarely. These are alternatives. The first predictions we’re going to give people are going to be reasonably basic, but hopefully give you insights into what sort of categories are good and bad, whether you’re someone who really deals with a high fat diet well, or a high carb diet well.

GRABER: Eventually, once hundreds of thousands of people have used it, TIm says the app will use machine learning to go through all that data and give much better, more accurate advice about which foods are better or worse for you. And then eventually it will even predict the future—it’ll tell you how your body will likely react to something you’ve never even eaten.

SPECTOR: The reason we can do this now is we’ve suddenly got the technology to have a wearable glucose monitor. We’ve got genetic testing for microbes. So this is why this is such a new and exciting field.

TWILLEY: They collected stuff they aren’t going to analyze or factor in for months, even years—saliva samples to look at our oral microbiome, for example. That stuff is just going to sit in a freezer until science has advanced enough to make it useful.

GRABER: They’re also sequencing the DNA of the gut microbe bacteria that we collected for them in our poop samples. And they’re looking at chemicals in our blood that are formed when we metabolize foods.

TWILLEY: And then they’re correlating all of that with the nutritional analysis of the food we logged. It’s kind of incredible.

SPECTOR: So all the information that comes in through the app gets matched up with the database so that every meal gets broken down into the amount of fats, amount of carbs, amount of protein. Total calories. And the timing of it.

TWILLEY: It’s a huge data analysis challenge. So they’re starting simple by looking at the standardized meals—the muffins. And the milkshakes and the fiber bars.

GRABER: So what in the world did they find out? About us, and about all the other people in the study?

TWILLEY: We’ve got the first look—and I mean really, the first look in the world, we’ve got the scoop on the preliminary results of this whole experiment.


SPECTOR: Okay. So who wants to go first.

TWILLEY: Three months after we took part in the experiment, Mass Gen sent us our very preliminary results, and we called Tim. We gave him permission to look at them. And then we asked him to tell us what he thought.

GRABER: I always like to go first and get it over with. You can do me.

TWILLEY: There are only three pieces of data that Tim and his colleagues have analyzed at this point: Blood glucose, insulin, and triglyceride levels over time, in response to the different standard muffin meals.

GRABER: They’ve begun looking at the more complex meals that we weighed out and carefully put in all those details for, but the ones included in this first report were mostly the standardized ones, and some where a lot of people have eaten something similar—mac and cheese, a tangerine, a handful of nuts.

TWILLEY: So we’re seeing results on only a tiny percentage of all the data they have. Still, Tim says it’s enough to start giving people advice.

GRABER: Okay, so how’d I do?

GRABER: You are slightly worse than average in terms of your blood glucose response. And your fat response is similarly worse than average. Okay. That’s the good news.

GRABER: Thanks. Great.

SPECTOR: But you’re not particularly worse than me which is… LAUGHTER The good news is your insulin is pretty good at the moment so you’re not stressing your insulin too much.

GRABER: Wonderful.

SPECTOR: Okay. So you may be producing a lot of glucose, but it’s not like your pancreas is desperately trying to keep up.

GRABER: Great.

TWILLEY: At this point, the tension was building. I was pretty convinced my glucose and insulin responses would be a dumpster fire, because there’s diabetes in my family.

SPECTOR: Your blood glucose response is actually slightly worse than average but not desperately so. But you have an amazingly good fat response. You really are one of our star fat responders.

TWILLEY: Hurray! I’ve found my true talent in life! Digesting fat!

SPECTOR: So you can see how you come down differently to Cynthia—you go up, but you come down really fast, so you just eliminate it.

GRABER: I am basically the opposite of a star fat responder.

SPECTOR: So this means that fat is hanging around in your bloodstream for long periods of time, which means these micro particles of fat are around and these are the ones we believe cause inflammation long term.

GRABER: Inflammation and, yes, heart disease. But does this also have anything to do with weight gain?

SPECTOR: The current theory is that the three things: the insulin, the glucose, and the triglycerides are all interacting to make more of the energy go into the fat cell. And so this sort of Holy Trinity, ideally you want to have all three working well and low and not hanging around. Then your body will sort of burn off that energy rather than storing it.

TWILLEY: Awesome. So our next question was—why is this? Why am I better at processing fat than Cynthia?

GRABER: Like Tim said, it could be because of differences in our DNA. In the twins, around a third of the difference was caused by genetics.

SPECTOR: But it could be, we’re seeing associations with microbes with fat responses. And so it could be that you’re lacking certain microbes that are involved in the way that enzymes clear your fats from your body. So there’s all kinds of complicated mechanisms—there’s, you know at least 20 different ones to explain it.

TWILLEY: This is it for now—that’s all Tim can explain at this point. He can tell us the differences between us, but not what’s causing them. And overall, what PREDICT can tell us so far is that in a big group of people, there is a LOT of variation in how we respond to the same muffin meals. A lot. And that’s news. We might have suspected it before but we didn’t know it.

GRABER: Even though he can’t tease out all the nuances of our response yet, in part because he hasn’t analyzed all the data—he told us that yeah, we could already make some changes in how we eat.

SPECTOR: In terms of general recommendations, Nicky responds to fat extremely well. Nicky seems to be able to have as much fat as she likes at the moment without causing her any problems. So you should just be eating chocolates, not tangerines.

GRABER: Tim suggestions to me weren’t quite as fun to hear.

SPECTOR: A bit more worried about your response to fat. I think you’re definitely not someone who should be on a high fat, low carb diet for example. But neither of you were sort of perfect with the carbs—you’ve both got to be careful of carbs. Nicky can substitute a lot of fats for carbs. It’s not clear that Cynthia can do that. You really perhaps need to cut back on both. As I said this is just done in a few minutes, and a snapshot really of what we’re going to hope to be achieving as we get more and more of this data back.

GRABER: So I have to say, I went into this whole experiment being fairly skeptical that these individual differences can and should dictate how we each eat. And so I was surprised at how badly I totally freaked out after we got off the phone with Tim. I first thought about the fact that someone in my family went on meds for having high triglycerides in their blood, though I should say no doctor has said I have a triglyceride problem. Also, and maybe even more importantly—and I’d say this isn’t unusual in America—I’ve never felt skinny, and my weight is definitely something I try to stay on top of and that I struggle with. And so suddenly I felt like I couldn’t—like I didn’t know how to eat well. Like maybe I should stop enjoying cheese at dinner. Or the olive oil I pour on my greens and white beans. Or my partner Tim’s homemade bread. And so I seriously spiraled into: I have to just stop eating all these foods I love, and that I usually think are part of my happy and healthy diet.

TWILLEY: I didn’t freak out the same way, but I have found myself deciding against fruit as a snack in favor of cheese, or justifying getting ice cream because the fat means I’ll handle the sugar okay. Which is a little bit of a weird way to think. But these sorts of changes are kind of Tim’s goal: the idea is that now I know that I have to be careful with anything that’s just high sugar, I can use that knowledge to improve my health.

SPECTOR: And so if you can just swap some foods around so that you have exactly the same calories and enjoyment or whatever it is, but for something that doesn’t have as big a peak either in glucose or in lipids, then you’re going to put on less weight and be healthier long term.

GRABER: But this is actually still a question. This is emerging research, and nobody has yet tried to find out whether making these changes will actually make a difference to our health.

SPECTOR: No one’s done these studies. What we need to long term prove is that people who switch their diets to reduce their sugar or fat responses are able to lose more weight, or not gain weight, have better appetite control, have less heart problems, diabetes etcetera. At the moment we’re doing based on the current science what we think is right and what should work.

GRABER: So I didn’t tell Tim how badly I freaked out, but I know from what he told us and from how optimistic he is about how this information can be helpful—he’d be surprised that I reacted the way I did.

TWILLEY: After all, the way he sees it, his results with the twins show that half or even less of their response was down to their genes. It’s not like oh, you have the genetic mutation for Huntington’s, you will get Huntington’s. Lots of other things influence your food response—your DNA is not destiny.

SPECTOR: You know, I can only see it as being an encouragement for people to get healthier rather than it being a negative.

TWILLEY: The idea is, that we can make changes that make us healthier. Like changing what we eat, substituting chocolate for tangerines. And then there are other things that Tim thinks we’re going to understand how to modify in the future, like our gut microbes.

GRABER: We do know the gut microbiome plays an important role in how we digest food—but of course we don’t understand how big a role it plays, we don’t really know how to alter it, and we don’t know what any alterations would do to our overall health. All we know now is that having a diverse gut microbiome seems to be a good idea…. But we don’t even know why.

TWILLEY: Then there are other things going on in our lives that we know affect how, say, our blood sugar spikes. Although again, experts like Jennie Brand Miller say that we don’t know exactly why for the most part.

BRAND MILLER: So on one day say you only had five hours sleep, you will give a bigger blood glucose response on that day than if you’d had seven hours or eight hours sleep. So sleep deprivation is challenging our ability to dispose of a glucose load. But it also varies for many other reasons that we’re still trying to work out on a day to day basis.

TWILLEY: One of those other things that can cause variation is stress. Another is exercise.

SPECTOR: Exercise has quite a big effect on how you metabolize food. And we saw this: One of the Zoe team did the same breakfast every day and he did five different 10k runs at different times. And nearly all of them reduced the sugar peak.

TWILLEY: Interestingly, it seems as though the timing of your exercise relative to when you eat makes a difference, but that’s still being teased out. Like everything else.

GRABER: But overall, Jennie says that these other factors might explain differences that show up in my glucose response to a food versus yours, Nicky, it might not be that we ALWAYS respond to that particular food differently.

BRAND MILLER: You know, on that day when you saw the higher response it may be that Cynthia had a bad night. She didn’t sleep as well. Or it may be that Nicola went for a run the day before, a good jog for half an hour or more. That would make her more insulin sensitive and her body would respond by showing a lower rise and fall in blood glucose on that day. But you know, teasing out exactly why on any given day, Cynthia had a higher level than Nicola would be very hard.

TWILLEY: It’s hard because as we keep saying we don’t know how exactly exercise and sleep deprivation and stress are causing those differences in how our body responds to food. And we don’t know if there are other things that affect that response that we don’t even know about.

SPECTOR: So again, we’re experimenting. This is a research project, it’s an ongoing science project. We don’t have the idea that we’ve worked everything out. And I think the more we do, the more we realize we don’t know.

GRABER: But Tim did still tell me that it seems as though I’m really bad at processing fat and somewhat bad at processing carbs. Even though, and this is the conversation I had with myself as a way of talking myself down—Tim doesn’t really have all the answers yet. I kept telling myself what I’m going to say again here, he didn’t test my response to olive oil with my dinner, just how I processed that super oily muffin.

TWILLEY: Still, Tim thinks he already knows enough about our responses to food to tell us something about how we should eat. He also thinks that as he learns more, this kind of personalized nutrition advice is going to completely replace the current standard nutrition guidelines.

GRABER: Jennie does agree that personalizing advice makes sense. She knows from her research, people who are predisposed to glucose and insulin spikes should eat differently to keep their blood sugar down.

BRAND MILLER: I think the one-size-fits-all nutrition guideline is antiquated. It’s no longer something we need to push. We can individualize dietary guidelines. No doubt about it.

TWILLEY: But the underlying point Tim’s making—that we should throw our nutrition guidelines out in favor of individual advice. I mean most nutritionists agree there have been problems with dietary guidelines in the past, like when they encouraged people to eat low fat and higher carb. But are today’s dietary guidelines actually wrong?

GRABER: We asked that question to Tim Caulfield, yes, another Tim. He’s a professor of health policy and law at the University of Alberta as well as host of a Netflix show called “A User’s Guide to Cheating Death.” He says we know overall what a healthy diet looks like.

TIM CAULFIELD: And it’s you know it’s pretty straightforward. It’s lots of fruits and vegetables, it’s whole grains, it’s healthy proteins. You know, there really is no magic.

TWILLEY: Tim Caulfield was skeptical about the idea that our nutritional guidelines need a complete rewrite.

GRABER: He says the problem isn’t with the guidelines. The problem is that we’re not following them.

CAULFIELD: When you look at the numbers, it’s grim. Clearly, a very large sector of the population is not eating enough fruits and vegetables. They’re getting too much of their calories from unhealthy overly processed sources.

TWILLEY: And that’s got nothing to do with guidelines. It’s got a lot to do with all the systemic problems in our society and our food system that make it hard to eat healthily.

CAULFIELD: Why we have the obesity epidemic is a fantastically complex issue. I mean it has to do with our built environment, it has to do with socioeconomics, it has to do with the food environment more broadly. It has to do with marketing. It has to do, yes, with our activity levels. It has to do with so many things. To actually pin it on one thing is both, I think, inaccurate but also can be distracting in a bad way, right. Because it distracts us from the things that we can be doing as a society to deal with this complex problem.

GRABER: On top of all that, the people who are probably going to want to buy the PREDICT app and use it to tweak their diet in the future? Are these the people who really need it?

CAULFIELD: Yeah, you know, it’s very funny because all of these quantified self approaches, which is really what this is, right. Whether you’re talking about the Fitbit, whether you’re talking about direct to consumer genetic tests, or whether you’re talking about these kinds of personalized nutrition things—when they become commercial products, who are we kidding, who’s going to buy them? It’s probably the last person that needs this, right? You know, they’re people that are generally probably interested in health anyway: high socioeconomic status, perhaps you know, motivated for other reasons to exercise et cetera. They’re probably the last sector of society. We need to get at the broader community, right. We need to get to communities that perhaps are struggling to eat in a healthy manner, perhaps don’t have access to healthy food, perhaps need more education on this front.

TWILLEY: But of course, it’s harder to tackle systemic problems. Which is why this idea of individual optimization—it’s so compelling. And it always has been.

SPECTOR: Well I think it does have a long history, you know—the sort of 19th century physicians who used to taste people’s urine and then say, Okay I know what you should have, I’m going to make the right port or brandy for you, and you have this with this herbal mix.

GRABER: In the past couple of decades, another trend has been the blood type diet, the idea an O positive blood type will be eating a different diet than someone with B type blood.

CAULFIELD: There’s always been this push for personalized nutrition. And I think that the concern about obesity has intensified this.

GRABER: But of course we do know that people are different, and maybe for some people, what Tim Spector discovers in his huge nutrition study will help them make changes. Tim told us that he stopped eating as much store bought bread after seeing that it sent his glucose levels spiking. Just by doing the process you start to think more about what you eat, and you start to see food as not just something tasty and nutritious but how it has an affect on your whole body. And I think just by taking part, all the people so far have really said it has changed how they think about food. And I think if we can start to do that, that’s a pretty good revolution to get going.

TWILLEY: People in the study have said that they have changed what they eat. And Tim Spector says he’s changed. But Tim Caulfield says that on average, people don’t change. At least not over the long term. Behavior change is hard.

GRABER: Okay, so it’s not clear that personalized nutrition is going to help in general, or even help the people who need it most, but what’s the harm? It’s a really interesting study, so why not do this research and suggest that people put it to use?

CAULFIELD: I am kind of torn because as I said before, I am kind of a science geek so I love that this kind of stuff comes out and I’ll be one of the first people to look at the data. Right.

TWILLEY: But. As much as he loves the science, Tim Caulfield says there are actual downsides to focusing on personalized nutrition—not doing the research, but publicizing the results, and commercializing them in an app.

CAULFIELD: First of all, it can distract the public when you come up with this kind of detailed advice about how you’re supposed to eat. It makes it sound like eating well and eating in a healthy manner is hard. And it’s not.

GRABER: There are plenty of very real reasons that it’s hard to eat a healthy diet. Time, money, stress, access, all sorts of general societal reasons. But Tim says there’s data to show making it seem like you have to tweak your meals to match your body, that can distract from the fact that we do already generally know what a healthy diet is.

CAULFIELD: Number two, I think it also, there’s concern that it puts a lot of the blame on the individual. You are giving information to an individual saying that you have these predispositions, you should eat in a certain way, it is your responsibility to do that. And if you don’t, the health consequences are kind of your fault, not society’s fault.

TWILLEY: Putting all that responsibility on an individual to eat healthily—it isn’t just ignoring all the structural reasons that make that hard—it also doesn’t work particularly well.

CAULFIELD: And there’s actually research to back that up. There’s been studies that have shown that the public health interventions that are the least effective are the ones that require a lot of individual agency.

GRABER: The most effective ones are the ones that deal with the societal problems Tim talked about, that make it easier to walk places, that make it easier and more affordable to buy healthy food. Or big black stop signs on highly processed foods and dramatic reductions in marketing to kids, the kinds of things that Chile is doing, that we told you about in our soda wars episode. These policy changes, built environment changes—they are the ones that have a real public health impact.

TWILLEY: There’s another risk built into this kind of research—the kind that looks at food as the sum of its chemical parts, rather than holistically. If you get obsessed with glycemic indices and triglycerides, it can take you away from eating real food and toward supplements and packaged foods that are marketed as healthy. And that’s a mistake.

GRABER: We experienced this temporary obsession with the chemical parts of foods ourselves, and we are people who think about food in general in a really nuanced and I’d say healthy way. Plus, frankly, we know that we’re both also fortunate to be in good health. But breaking food down into parts like Tim did when he talked to us about our responses—it very quickly made us start to obsess over fat and insulin in a way we never had before, and it did make us start to worry about eating foods that we love and that we feel good about.

TWILLEY: And that’s a problem. I ended up thinking that it’s actually a little naive of Tim Spector to think he can make these kinds of statements—that nutrition guidelines are useless and individuals vary on whether they should or shouldn’t regularly eat rice or pasta or whatever—and imagine that people are going to use that information to make healthy decisions. It’s just really hard to make sure people don’t respond in a weird way. Like we did.

GRABER: Right. It’s kind of ignoring the fact that we all have complicated relationships to food, and many of us, particularly but not only women, we also constantly battle really complicated feelings about our bodies. I have that complicated relationship with my body, and hearing those results made me feel really disordered about my eating for about a day or so, and I had to talk myself out of it.

TWILLEY: So there are real risks to how you publicize this kind of research. But of course nobody is saying that there isn’t benefit to doing it.

GRABER: Maybe one benefit is that people who take a close look at their blood sugar spikes, some of them will learn that they spike unusually high and frequently, and that they have the potential to develop diabetes. A third of people have that risk.

TWILLEY: Jennie says there is good evidence to show that if people who are predisposed to diabetes and who find out that they have those big sugar spikes—if they change their diet, then likely only one in six of them would develop the disease.

GRABER: Which is huge. And Tim Spector told us that if someone has the genetic predisposition to a high insulin spike after a meal, they’re about 13 pounds heavier than people without that predisposition.

TWILLEY: So if those people changed their diets to lower those insulin spikes, that would likely make a big difference too.

GRABER: But really, the most exciting thing about this research is that it’s totally new and really rigorous. The team is learning things about our bodies and our food that we don’t even know that we don’t know.

SPECTOR: I think this subtlety of what we haven’t noticed before is going to open up whole new areas of studying foods.

TWILLEY: And with that, a whole new understanding of how food affects our health. Questions we don’t even know to ask yet.

GRABER: Tim Caulfield pointed out that big studies like this one, where they didn’t know exactly what they might find, that’s how scientists figured out that cigarettes cause lung cancer.

TWILLEY: And so it’s likely that PREDICT and personalized nutrition research could lead to these kinds of breakthroughs—like a better understanding of how stress and sleep affect our blood sugar response, or a clearer picture of how exercise interacts with fat metabolism.

GRABER: Tim Spector is sure that this research will be more and more concrete and useful the more people take part in the study. But that also means that he and his colleagues have to convince people to pay to use the beta version of the app—those folks will basically do what Nicky and I did and monitor their eating for a couple of weeks, and they’ll get some preliminary information about their own results, and Tim will use their results to make the app more accurate.

SPECTOR: Well I think it’s already pretty useful. but it’s important not to oversell it at the moment. In a year’s time when we’ve got another 10,000 people in the database, it’s going to be twice as good. In two years time it’s going to be 10 times as good. It’s a journey.

TWILLEY: Tim doesn’t want to oversell it. But he has to sell it—otherwise he won’t get people to use it, which means he won’t get the data he needs.

GRABER: And if the app does end up overselling the benefits of personalized nutrition, beyond what the approach can actually deliver, there is the potential that it could erode public trust in nutrition research. As Tim Caulfield told us, that trust already isn’t so high.

CAULFIELD: I think that there is some evidence that this confused messaging has led the public to stop listening and not take nutrition advice seriously.

GRABER: And if we stop listening to the experts who do have a good idea about how we can be healthier, then we’re more likely to fall for someone like Gwyneth Paltrow who’s selling vitamin packs she says will solve all your problems. Or Tom Brady and his magical cookbook.

TWILLEY: Which you don’t need us to tell you is a gigantic crock of shit. So. So far, Tim Caulfield has been kind of the voice of doom this episode. Or at least the skeptic.

CAULFIELD: I want to step back a little bit and say I’m not a total pessimist. I think this research is fascinating.

GRABER: As we’ve said, PREDICT is an incredibly rigorous, well-designed experiment. There hasn’t ever been anything like this, and frankly, it’s really important. As we keep pointing out, nutrition science is really hard to do.

CAULFIELD: And the mere fact that this is so challenging, it’s worth exploring, you know, what can be done to help people eat better, what can be done about the obesity challenge that we face.

TWILLEY: But it’s important to not confuse good, interesting research with a complete nutrition revolution.

CAULFIELD: You know, I’ve been fortunate to work with research teams on a whole bunch of different scientific revolutions, right. Whether it was the stem cell revolution, or the nanotech revolution or the genomic revolution. Now we have the microbiome revolution. So maybe it’s made me a little cynical, right. Because I see these promises and they never pan out, right. So if history tells us anything, it tells us that it’s unlikely that it’s going to revolutionize nutrition, that it’s going to revolutionize public health.

GRABER: So while the science is really exciting, and while I’m really glad I participated in it, and if you listeners are interested in participating, we have details in the credits—still, I’m going to work hard to not use the information we got back to take me away from enjoying what I’m pretty sure is a fairly healthy and certainly a varied diet.

TWILLEY: And really, that’s Tim Spector’s ultimate goal, too.

SPECTOR: We’re not trying to take the fun out of eating. Hopefully this will introduce people to discovering a greater diversity of foods. Because you know, what we learned from a study with the American and British gut projects was that the number one factor that determined your gut health was the amount of different plants you ate in a week. And so everything is pointing towards getting a diverse diet.

GRABER: That is definitely nutrition advice I can live with!

TWILLEY: A huge thanks this episode to the Alfred P. Sloan Foundation for the Public Understanding of Science, Technology, and Economics, as well as the Burroughs Wellcome Fund for funding our coverage of biomedical research.

GRABER: Thanks also to Tim Spector, for both allowing us to participate in the study and for spending hours discussing it with us. Thanks to all the folks at Mass General Hospital, who were super patient and made special arrangements so that we could tape the entire day at the hospital. They really went out of their way to make that work, and we’re incredibly grateful.

TWILLEY: Thanks to Jennie Brand Miller at the University of Sydney and Tim Caulfield at the University of Alberta. As usual we have links to their books and research and TV shows on our website, gastropod dot com.

GRABER: If you are interested in the study, you too, can try those delicious muffins, there are links to getting involved with PREDICT on our website as well at gastropod dot com.

TWILLEY: We wrote a little piece to go with this story for The New York Times—the link for that is on our website too.

GRABER: We’ll be back in just one week with our final episode of the season, a battle for the perfect, original version of one of the most delicious foods around—a deep fried stick of potato!