The End of the Calorie

For most of us, the calorie is just a number on the back of the packet or on the display at the gym. But what is it, exactly? And how did we end up with this one unit with which to measure our food? Is a calorie the same no matter what type of food it comes from? And is one calorie for you exactly the same as one calorie for me? To find out, we visit the special rooms scientists use to measure how many calories we burn, and the labs where researchers are discovering that the calorie is broken. And we pose the question: If not the calorie, then what?

The calorie seems to be an obvious measurement for food. But, in fact, it took hundreds of years and thousands of experiments to nail down what exactly the calorie means, both as a measurement of the food itself (that is, how much energy is contained in the form of a cheeseburger) and a measurement of consumption (how much energy we cheeseburger-eating humans burn as we go about our daily lives). With assistance from nutritionist Marion Nestle, author of Why Calories Count: From science to politics, we trace the calorie's earliest history, from an obsessive-compulsive Italian who invented a special chair with which to measure everything he ate, drank, and excreted over a thirty-year period, to the French chemist who put a guinea pig in a coffee urn to measure how much heat it gave off. Then we travel to the USDA laboratories in Beltsville, Maryland, to check out the way scientists measure calories today: by putting people in tricked-out walk-in coolers and freeze-drying their feces.

Santorio and Lavoisier
IMAGE: (Left) Santorio Sanctorius's weighing chair. (Right) Lavoisier and Laplace's Ice Calorimeter (Diucênio Rangel/Nature Education).

While we're in Beltsville, we also learn about intriguing new USDA studies that have found that, for some foods, the calorie counts on the label are off by double-digit percentages. Following this trail, we end up at Harvard, talking to anthropologist Richard Wrangham about the ways humans have learned to change the number of calories we get from our food by cooking it. And we dive into the differences in how people process calories by visiting microbiologist Peter Turnbaugh at UCSF, who points us to dramatic new research demonstrating the effect the tiny creatures in our gut have on the amount of calories we absorb. The University of Alberta's David Wishart offers us a glimpse of the future, in which truly personalized nutrition advice will evolve from the emerging science of how the chemicals in our bodies interact with all the different chemicals in the food we eat. And Susan Roberts, director of the energy metabolism laboratory at the Tufts USDA nutrition center, suggests an alternative unit as a replacement for the traditional calorie.

Bill explaining the calorimeter to Cynthia 2
IMAGE: David Baer, Bill Rumpler, and Cynthia inside the human calorimeter at the USDA in Beltsville, Maryland.

Listen for all this and more, and then tell us what you think of the calorie. Do you find calorie labels useful, or is counting calories the bane of your life? Let us know!

Episode Notes

Why Calories Count: From science to politics

Marion Nestle and Malden Nesheim's 2012 book is a great guide to the history and science of the calorie. Nestle's most recent book is Soda Politics: Taking on Big Soda (and Winning)

USDA Research

David Baer and Bill Rumpler both work at the Food Components and Health Laboratory at the USDA-ARS headquarters, in Beltsville, Maryland. Check out Baer and his colleagues' papers on the difference between the calories on the label and those our bodies can extract for almonds and walnuts.

Dinner tray from above
IMAGE: Dinner for research subjects at the USDA Food Components and Health Laboratory (Photo: Nicola Twilley).

Calorimeter routine
IMAGE: Human calorimeter research at the USDA Food Components and Health Laboratory (Photo: Nicola Twilley).

Richard Wrangham's Cooking Research

Richard Wrangham is the author of Catching Fire: How cooking made us human. His co-authored paper on the difference that cooking makes to the availability of calories in meat was published here.

Calories and the Gut Microbiome

Peter Turnbaugh's lab at the University of California, San Francisco, promises "better living through gut microbes." In our conversation with him, we discussed this study on the effects that transplanting gut microbes from lean and obese twins had on the weight of mice. Further examples of the impact of microbes on energy balance can be found in this paper on one woman's weight gain following a fecal transplant, and this paper on how risperidone is associated with altered gut microbiota and weight gain.

David Wishart and Metabolomics

David Wishart's research group is based at the University of Alberta. You can check out the Human Metabolome Project Database online here. And the Israeli study on personalized nutrition based on individual glycemic responses is available online here.

Calorie replacements?

Susan B. Roberts is the creator of the satiety-based "iDiet." She has also done extensive research into the accuracy of calorie counts on menu labels. David Ludwig's book, Always Hungry?, also proposes measuring foods based on their satiety score. Adam Drenowksi's Nutrient-Rich Food Index is explained here.

Why the Calorie is Broken

We wrote a feature article for Mosaic, the online publication of the Wellcome Trust, to accompany this episode. You can read it online here.

The Chemical Definition of the Calorie

In the episode, we say that a calorie is the amount of heat energy needed to raise the temperature of one gram of water by one degree centigrade, from 14.5º to 15.5º, at one unit of atmospheric pressure. This is accurate, but it is misleading, because throughout the rest of the episode, we are discussing a different kind of calorie—the kilocalorie, which is the amount of heat energy needed to raise the temperature of one kilogram of water by one degree centigrade. The kilocalorie is the number we see on our food labels and recommended daily allowances, but no one other than chemists actually calls it the kilocalorie. Instead, it has been shortened to "calorie" on labels and in everyday usage. Throughout our episode, we follow common practice by calling a kilocalorie a calorie, but then we mistakenly gave the definition of a true calorie without noting the difference. We apologize for any confusion!


  1. I’ve listened to your podcast ever since the first episode and have always found it interesting and entertaining.
    As a food scientist and somebody who struggled with weight issues most of my life, I think your last episode will do more harm than good by majoring in the minors.
    Thermodynamics cannot be denied, and as innacurate and oversimplified as CICO might sound, accurate portion control and food logging have been found to increase an overweight person’s chance of losing weight and keeping it off. Calories are, however, the best way we have so far of measuring daily energy intakes and expenditures.
    By stating that “the calorie is broken” you migh be enabling people who struggle with weight control. Please, be concient of the damage you can cause with the way you choose to transmit information to your listeners and readers.

    • Thanks so much for listening and for commenting. We are extremely grateful to have such a thoughtful and responsive audience, which is why we feel able to make episodes that cover complicated topics that require a great deal of nuance. We are quite clear in the episode that thermodynamics cannot be denied and that the calorie is still the best thing we have, and we are also clear about its shortfalls, many of which contribute to the difficulties individuals face when trying to lose weight. We both know and love people with weight control issues, but we also think that more knowledge of what calorie counts are (useful guides) and are not (completely accurate) is empowering, rather than dangerous.

    • While I agree that the calorie will likely be the best method of measuring energy intake for the foreseeable future, I think the argument that the ‘calorie is broken’ for weight management is an accurate one.

      Calorie intake is VERY important, but diet shouldn’t just depend on calorie intake. To properly diet for weight loss, a person should also consider the timing and frequency of their food consumption as well as the metabolism that is required to process different foods. A bowl of beans and a bowl of processed grains can have the same number of calories, but the metabolism will work differently for those foods. The beans will release glucose more slowly and be more difficult to metabolize because of fiber, whereas the processed grain quickly becomes glucose for the body to use. If not timed well with activity, glucose becomes stored as fat. It’s much more likely that processed grain will be stored as fat rather than beans.

  2. So much here, but I’ll try to be concise and focused. I won’t presume to know what Peter’s motivations are for his judgment on this episode, but after a first (and second, and third) listen I came to the same conclusions and I’d like the opportunity to share why.

    First, throughout this report there’s a subtle but extremely broad and clear muddling of the root question; namely whether we can trust that the number of calories in a given portion of food is truly indicative of the amount of energy that we absorb. There are multiple examples of individual variation in respect to caloric uptake, and even a nod to satiety.

    But the point is this. Bomb calorimeters aren’t designed to do what you’re representing that they should do, which is give precise numbers about the amount of energy that people will absorb from them. Rather, they establish an upper bound on the amount of energy that -could- be released from the portion of food in an ideal transfer, and this is a very very important point that you completely fail to mention.

    In other words, almost all the inaccuracies that you cite (with the exception of portion control variance for restaurant purchases, and let’s be honest… most people don’t read nutritional labels at restaurants) lead to LESS effective calories exposed to the the consumer than the label would suggest. You imply that because calories counts are imperfect, then they should be doubted in bulk, but this a dangerous game to play with facts when you present them to the public. In truth, it’s about upper bounds, and thermodynamics takes care of that just fine. You do say that thermodynamics are important, but you leave out the most crucial point.

    Luckily, it’s quite simple to explain. You can’t take in more calories than a pile of stuff has in it.

    This is the danger. By introducing doubt about the fundamental nature of our knowledge of caloric uptake without letting your audience know that we know how much it CAN occur (and that’s what we do), you misguide them into believing that the entire idea of calorie counting (and the calorie itself!) is broken, and that’s overly simplistic. The truth is that is that if we absorb fewer calories than we burn in a day, then we lose weight, and if we take nutritional labels as an upper bound for the total amount possible, we err on the side of correct.

    • Thanks for listening (three times!) and sharing your response. We appreciate it. I think it’s important to point out that we don’t claim that bomb calorimeters give precise numbers about the amount of energy that people will absorb from them, which is why we spend time explaining the Atwater Values, which do promise to do that.

      Your point that those Atwater Values are upward bounds is a good one, although it doesn’t take into account that emerging research seems to indicate that more efficient or less generous gut microbes can make more calories available than the Atwater Values suggest. (You’re right that you can never absorb more than the bomb calorimeter suggests, because that is the maximum possible number of calories, but, as we point out, companies actually tend to use the Atwater Values instead.)

      Nonetheless, we know and we explicitly say in the episode (as you acknowledge): you have to eat fewer calories than you burn to lose weight. The question then becomes: is the calorie a useful tool for achieving that weight loss? For many people, the answer is: not really. As Peter points out above, people do lose weight by reducing portion sizes and logging their food, both of which can be important tools, but that doesn’t necessarily mean they’re counting calories. The calorie is impossible to count accurately at home, so they know neither how much they have burned nor how much they have consumed, and it fails to include significant and perhaps more useful information about food, such as how full it will make you feel or how many nutrients it contains per unit of energy. We’re pretty confident that knowing the calorie’s limitations and understanding that it is just one of many ways to measure food is not dangerous; we hope instead that it might be empowering.

      • I disagree completely. I track my calories at home (using Noom), and it works. How do I know it works? Because I’ve lost weight, but also because when I gain weight, or don’t lose, I can directly tie it to my calorie consumption, at least over the long term (day-to-day weight fluctuates dramatically due to hydration status).

        Do I think that I’m perfectly estimating the calories in every meal? Of course not. This isn’t about perfectly understanding the calorie content of each meal, it’s about getting approximately right my daily calorie consumption. It’s about recognizing that 1500 calories of healthy food leaves me feeling a lot more full than 1500 calories of chocolate cake, and I tend to not meet my calorie limit on days when I eat chocolate cake (or have a few beers). It’s about recognizing that if I eat about 2000 calories a day, my weight will be static, and if I eat about 1500 calories I’ll lose weight.

        There is a difference between accuracy and precision, whether we’re biased in our estimates, or how approximately right we are. I don’t need to be perfectly correct in my calorie estimation, I need to get it approximately right, day-to-day. If I’m consistently over or under-estimating calories, then there may be a problem. However, in practice that is not even a problem. If you want to lose weight, you realize that stopping eating before you feel too full is important. Eating healthy snacks, and not “empty” calories, for example. Tracking calories helps you realize this quickly and simply. You establish a baseline that meets your daily calories needs, and it’s not about the precise thermodynamics of calorie consumption, it’s about the relative impact of high and low calorie foods on weight and hunger.

        • I’m glad you’re happy with your weight and your ability to control it, AJ, but I’m not sure exactly what you’re disagreeing with. As we say, you have to take in fewer calories than you burn to lose weight, the calorie can be a useful way to compare foods, and, yet, for many, counting calories does not lead to weight loss. For some, such as you, it does. Thanks for sharing your experience.

          • Anyone who actually eats at a caloric deficit will inevitably lose weight. Whether they can adhere consistently to a caloric deficit isn’t a problem with the model of the calorie, nor a violation of thermodynamics. It’s a problem with either math or motivation.

            I say this as someone who lost forty-five pounds five years ago and has kept it off consistently through calorie tracking.

  3. I’ve just discovered your podcast and I’m in love. I’ve been listening to the beginning episodes, but I had to listen to this new one. It’s fantastic! I also really appreciate the other questions on here and the thoughtful responses you’ve given. I find this all very fascinating. Thank you for all your work!

  4. Thanks for a great podcast.
    My comment to this is that I think too few people count (think of) calories or care at all. That’s the same way I feel about the dietary guidelines. People say they are wrong and misguided, my take is that too few people follow them. Only 4% of Americans get the recommended amount of fruit and vegetables.
    Most people know what they should eat more or less of by using common sense. You’re talking about both spinach and doughnuts in the podcast, we don’t need calorie-numbers to know what we should choose.
    Michael Pollan says it very simply: “Eat real food, not too much. Mostly plants”

  5. So glad to see that Gastropod is back for 2016!! I’ve been anxiously awaiting the January episode.

    Thank you for tackling such a polarizing topic. While I can understand the reason why a few other listeners are concerned that some may misinterpret the point of this episode, I have to say that I would never have jumped to that conclusion myself. I felt that the message was clear: calories may not be the most accurate way to evaluate how one’s body is absorbing and responding to certain foods, but they are still an effective tool to steer us in the right direction. My mother has wrestled with weight control for a long time, and often she feels that she is a failure because despite doing all the “right things” (eating reasonable portions of fresh fruits and veggies, and lean meats) she still struggles to maintain a weight that she feels comfortable with. I hope that the research regarding gut microbes/personalized nutrition may enlighten, give hope, and and ease the shame of people who believe that they are failing, despite their best efforts at weight control.

    Thanks for featuring such an interesting and relevant topic.

  6. As someone who sews, I got a chuckle out of the remark that those who get put into the human calorimeter were allowed to bring in things to pass the time. Except for those who sew, since that one woman made such a mess. I can relate.

    I just finished a big project that involved five shirts, three flowing jackets, several scarves and lots of silk cords that were dyed in various colors including ombre. If he thinks sewing is messy, wait until one of the human subjects brings in things that include white fabric/yard and dye. Though I doubt they would allow the use of the amount of water needed. I dread my next water bill.

    Now I am going to go finish reading a book, and then hopefully clean up my sewing area. Made easier since the sewing machine is in the shop getting its multi-million stitch routine maintenance.

    By the way, as an on subject remark: hubby and I have noticed we should not eat that much. Now when we go out to eat we share both a salad and an entree. We have noticed that restaurant meals have portions that are much too big (especially places that serve pasta dishes). There is really only one restaurant in our area that serves reasonable portions, and it turns out to be from north of the border, a Canadian company (they have locations in only two USA cities).

  7. I love all of your shows. In this episode,as in others, I have thought about how I would be so interested hear a show about the Aruvedic diet. The unique microbes that influence how our bodies proccess food seems related to ancient Aruvedic principals. In Aruveda different people have different doshas (body types, dispositions and personalities) and thus need different diets to stay healthy. Could doshas and our microbioms be connected?

    • That’s a fascinating idea, Claire (and thank you for the compliment). I can easily see an entire episode, or more, teasing out the history and science behind the way different, non-Western medical systems have understood the relationship between food and health. Something to add to the list!

    • Absolutely they could, Claire. In my opinion and personal experience, they are – which is of course an opinion, though a well-read one. I had the same thought. It’s fun and nice that western science is coming to similar conclusions about diets (one-diet-may-not-fit-all) that Ayurvedic medicine has said for a long time. (Also thinking of other “alternative” treatment methods out there like Traditional Chinese Medicine that take into account a larger number of specific, individualized symptoms than Western medicine has to date for say, a stuffy nose. “Everybody! Take this one particular decongestant!”)

      Researchers now say that the microbiome in our gut functions as our “second brain,” ( so affecting our disposition would fall under the umbrella of what it does.

  8. Slowly but surely working my way through Gastropod episodes, starting from the beginning, and loving it! While I’m not up-to-date yet (just got to The Whole Hog!) I wanted to let you guys know that I made a list on Goodreads for books related to the podcasts (books you’ve mentioned or whose author’s you’ve featured). I hope to read many of them, and maybe your listeners will go vote for their favorites if they’re ahead of me and have already started reading! Link below!

  9. I just started to listen to your show recently and find it really interesting! I think your balanced and well-informed way of approaching the subject of the calorie was really good. You mentioned that testing of FMT to see whether transplantation of gut microbiota of thin specimen into overweight specimen influences weight gain has only been conducted on mice. That is, however, not true: in the Netherlands, a study has been conducted to test this in humans: (Vrieze (2013) The role of gut microbiota in human metabolism. PhD thesis at AMC-UvA). Unfortunately, it did not show effects on weight loss after treatment. So, it’s not as simple as just picking microbiota of any thin person around. Too bad 😉

  10. Inflammation & satiety are the two key points of this episode for me, and the ones I base my diet around. Nutrient-dense food that keeps me full longer and keeps systemic inflammation down, bring ’em on.

    I wish Marion Nestle hadn’t made CICO sound like it was the real thing that counts when it all comes down to it. I know you said she’s right, but she’s also wrong, and I agree. One could of course lose weight eating nothing but chocolate bars and Doritos; one would just have to eat little enough of them so as to be fewer calories in than calories out. But how long one could keep up that diet so as to maintain the weight loss is questionable.

    Besides, nutrient deficiency is an issue, and is why I’m glad you made this episode. You were clear about the calorie not being completely irrelevant. My issue with the “only calories matter” and CICO idea is that not all calories are created equal. As one commenter aluded to above, a calorically equal amount of doughnuts and spinach do not nourish one’s body in an equal way. Nutrients matter. Calories follow later. It’s kind of too hard to binge on kale salad, or roasted carrots. Doritos is a whole other matter. I don’t know anybody worried about eating too many asparagus spears.

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