Calories In/Calories Out? Science Says No

As the argument goes, the Law of Thermodynamics proves that if you burn more calories than you consume you will lose weight.  I blogged about this in a general, non-science-y way here.  Today I’ll give you the exciting science to prove my point.  Stick with me though, I promise to make this interesting.

That’s the argument that I hear most often when I tell people that I eat well and workout a lot and am still fat. They call me a liar  because the science behind the law of thermodynamics, they say, is simple, complete and irrefutable.

First things first, there isn’t a “Law of Thermodynamics”. There are four of them (the first one is actually called the zeroth law which you gotta love).   Since these people aren’t concerning themselves with the entropy of a crystal at absolute zero, I’m going to assume they are referring to the first law.  Actually I assume that they are phonetically parroting something they heard from someone else and that they wouldn’t know one of the laws of thermodynamics if it bit them in the ass but I digress.

The first law states that in a thermodynamic process the increment in the internal energy of a system is equal to the increment ofheat supplied to the system, minus the increment of work done by the system on its surroundings. It is often simplified to “energy can neither be created nor destroyed”.

I do not disagree with the law.  But, note the first four words “In a thermodynamic process..” What I disagree with the gross misrepresentation of the human body as a perfect thermodynamic process.  I have four issues with this:

Issue 1:  It assumes that there is no option for calories other than to be burned or stored

Carbohydrates work in basically that manner. Fats and proteins don’t because they have other things to do in the body.   Protein does everything from building muscle cells to repairing cell membranes.  Fat transports non-water-soluble nutrients around, insulates neurons, and can also be used to repair cell membranes.  Fat and protein can also be broken down and recombined into whatever the body needs.  Eat a steak and your body might use it to make a cell membrane. That delicious alfredo sauce might be used to produce insulate neurons in your brain.    I don’t know why I remember this, but my physics teacher told us that fat and protein are to the body what wood is to us.  You can burn it for fuel, but you can also build a house with it or make it into a piece of paper.  At any rate, the calories in the proteins and fats used in this manner are neither burned nor stored. So we have our first hole in the calories in/calories out equation.

Issue 2:  It asserts that Basal Metabolic Rate (BMR) is Easily Predictable and Stable

Your BMR is the number of calories that you would need to stay alive for 24 hours if you spent the day still and in bed – just what would be required for breathing, heart rate, etc.

It is typically calculated using the Harris-Benedict equation.  James Harris was a botanist who freely admitted that his equation failed to take into account large amounts of muscle mass or the additional calories provided by excess body fat and that it was thus much more effective for individuals at, or very close to, an ideal body weight.

It is well documented that BMR responds to decreased caloric intake by slowing down.  So if two friends have the exact same  Harris-Benedict score, but one has been dieting;  then the dieter will actually burn less calories at rest than his friend.  Said another way, if these two friends eat the same number of calories in excess of their BMR score, the dieter will gain more weight than the non-dieter. Again, in this case calories in/calories out just doesn’t hold up.

Issue 3:  It requires that the body be a perfectly efficient calorie burning machine

Not so much. The Second Law of Thermodynamics explains about entropy and how systems are never 100% efficient. [Edit:  As a commenter pointed out entropy only applies in a closed system which the human body is not for the reasons mentioned below.  What I meant to say and didn’t was that the first law also applies to a closed system and as such people who think that the first law applies to the human body would have a difficult time weaseling out of saying that the second law applies.]  Regardless, the human body is even more complicated because it’s ability to burn/store/use nutrients affected by things like genetics, environment, hormones, previous caloric restriction and subsequent reduction in BMR, hydration level, environment, chemicals etc.

Issue 4:  If we wanted long term weight loss using this theory then we’d have to eventually turn to starvation

If we wanted to use calories in/calories out effectively, knowing what we now know about entropy and how decrease in caloric intake causes decreased basal metabolic rate, we would just have to keep decreasing our calories and increasing our exercise until eventually we would be following disordered eating patterns.  It would be the only way to stay under our ever plummeting BMR and eat less than we burn. Perhaps this is why 95% of intentional weight loss efforts fail.  The body releases weight at first but then the damn science catches up to us and not matter how hard the people who’ve attended a physics amateur hour try to yell to the contrary, they will never be able to out-science the actual laws of thermodynamics and the complicated human body which is, I swear to god, not a lawnmower.

Bonus Issue:  Your friend who eats everything in sight and never gains weight.

Everyone knows somebody like this.  I have a number of friends who eat way more than I do, exercise less and stay rail thin. Why do these people get to credit their metabolism but I’m just fat lazy excuse-making slob if I suggest that my metabolism may be as slow as theirs is fast?

So next time somebody tells you that it’s just calories in/calories out, consider telling them that you’ll be happy to talk to them about it once they have recited the laws of thermodynamics, defined the Harris-Benedict Equation and discussed its specific limitations, and explained your friend who eats a ton and doesn’t gain weight.

55 thoughts on “Calories In/Calories Out? Science Says No

  1. Very good points, and something I’ve been trying to articulate myself for a while.

    In addition to your already excellent points, the first law of thermodynamics refers to a closed system, ie one that is completely isolated from the environment surrounding it. This is completely not the case for human beings; we are in no way closed systems. For starters we take in energy through food and expel waste, we take in and give out heat from our environment.

    If humans are not closed systems, th first law of thermodynamics cannot apply.

    Thanks for another great post 😀

  2. My father’s side of the family has the eat like hogs and remain average weight anyway genes. My mother’s side has the look at food and gain weight ones. I got my build from my mother’s side. Sadly, I’m still struggling very hard to accept myself and when things go wrong I tend to fall into disordered eating patterns. My father’s memorial service was the 31st of May, which would have been his 75th birthday. I starved myself for a couple of days previously until I got so sick I had to eat something.

  3. I follow the physics, and have tried on occasions to explain it to people but never as well as you have here.

    I will direct those who dont understand the science to your blog. Thank you

  4. Thanks for this! I too, am of the active, healthy, yet “still fat” category (and I currently breastfeed two babies…that burns 500 calories a day without even trying!) and I’ve had people accuse me of lying about my lifestyle, too. Yet, nobody accuses my slender friends of anything but looking great. Le sigh.

  5. Entropy doesn’t apply to the human body for the following reasons.

    1. It only applies in closed systems. Our bodies have multiple energy inputs
    2. We are a self-repairing mechanism, literally order from disorder- see closed system
    3. Entropy does apply- after we die, so there is that.

    Seriously? people lecture you on thermodynamic theory? Tell them that for 100 yrs every serious scientist was convinced that Phogiston was responsible for the combustibility of things and it was the only explanation.

    1. It is amazing what people think they can lecture you about if you are fat. Most of them dont have the first idea what they are talking about of course, but they still try.

    2. Yeah, fat people get lectured about all sorts of things- I actually had at some point posted the original ca = co post onto my fb….boy did that enrage some (thin) people. Wonder what will happen when I post this one too <.<

    3. Phlogiston may be a good comparison. As I understand it (insert disclaimer about fuzzy memory and having read this a while ago), the shift from phlogiston theory to the modern oxygen theory was not, as one might think, the result of scientists and experts analyzing the information and changing their minds. Instead, it was more a matter of the older scientists, who adhered to the phlogiston theory and refused to consider alternatives, dying off and being replaced by younger scientists who weren’t already mentally locked into “this is how it is and this is what explains the world”.

      1. This is the way in which Thomas Kuhn, author of Structure of Scientific Revolutions, explained the shift; however, his explanations are not uncontroversial, and many challenge both the history and the epistemology underlying his theories. He basically says that scientists have “paradigms” and that as paradigms shift from an old one to a new one, those who learn the new paradigm (literally) see the world a different way than the old ones, who can never learn to see it as the new ones do. The historical issues with Kuhn primarily lie in the fact that many scientists do change their views on the basis of evidence, and in fact, seem primarily to do just that. Just because a few old codgers hang on to their ideas dogmatically doesn’t justify the entire theory he constructs.

  6. I always thought the calories in/out thing was an insult to the incredible complexity of the human metabolism. Seriously, hundreds of biochemical pathways and they want to compare it to a bomb calorimeter? Epic science fail!

  7. thank you for finally explaining this! I’ve heard this argument quite a lot but never had a really elegant way to argue against it. I’ll be sharing!

  8. Love love love this! I’m coconuts for physics, and THANK YOU for explaining the raw science behind our bodies. The human body is SO beautifully complicated and diverse, and it frustrates me when people recommend a one-size-fits-all approach to health. By “people” I mean doctors/the First Lady/people who are part of the 5% that keep the weight off. By “health”, I mean weight loss. >:|

    I feel like printing this out and mailing it to the doctor who told me that I need to eat only 1200 calories a day for the rest of my life and avoid exercise, because I might “run the risk of eating” if exercise makes me hungry. Imgaine! The RISK of eating!! The worst part is that she’s a world-renowned doctor who was featured in Time Magazine late last year for her contributions to fertility treatments. I was seeing her for PCOS and never went back. World-renowned my fat ass. Scratch that, my very muscular which accounts for my high weight ass. BMI be damned.

  9. Great post! I’m filing this into my bookmarks under my ammo against people who try to use science against me. No matter how much I try to explain it, quite a few people won’t even hear me out unless it’s a ‘3rd party perspective’ Hopefully this will stop them in their tracks.

  10. Awesome post! In chemistry class we tried to simulate a bomb calorimeter by using a styrofoam coffee cup and a lid with a hole much bigger than the thermometer that went through it. Obviously there was lots of heat loss, etc, and it was nowhere near a bomb calorimeter. I think our bodies are more like those coffee cups.

  11. I think people misuse the term. All systems have mechanical and kinetic energy, and kinetic energy also is part of the “energy in, energy out”. Thus, when you run your motor (or your body) and 30% goes off as heat, that’s not in disagreement with any law of thermodynamics.

    Anyway, we all go to entropy someday!

  12. Yes, yes, YES!!!! Even if my lifestyle didn’t support the fact that you can be fat and healthy, and I didn’t care about the science behind it I would still be jumping for joy. As it stands, I love that the true science backs up what I know in my heart, that my body is beautifully and complicated made and over-simplifying things won’t work.

    Oh and once again I love being able to say that I am not a freaking lawnmower. 😀

  13. I want to print this out and shove it up my ex-doctor’s ass. Then maybe she would STFU about “it’s calories in/out and anyone can lose weight and keep it off if they just try hard enough.” But it probably wouldn’t convince her, she’s a fat-phobic bigot, which is why she’s no longer my doctor.

    1. There are still people who say that the world is flat. People aren’t required to learn grow and change and all we can do is surround ourselves with people who support us and our choices.


  14. Ragen,

    Could I link to this post on my blog? You say it all so beautifully!

    Oh, and I love it that finally someone has said that I am definitely NOT a lawnmower. I was worried.

    1. Hi!

      Thank you so much for the compliment and I’m glad that I could clear up that lawnmower thing! Of course you can link to it, that would be amazing: 🙂



  15. I am telling you, you are incorrect. I wanted to know if that was true, so I did absolutely nothing with my diet other than cutting it by 100 calories. I lost weight. I didn’t exercise more than usual, and it was fat, not lean body mass.

    Email me, so I can ask you why you’re so closed minded.

      1. Not “seem.”

        Like Karen said, you first need to wait 5 years to see if this weight loss is maintained to beat the current well-documented statistic that 95% of all intentional weight loss fails. Secondly, just because it “worked” (which shall remain to be seen) for you, does not mean everybody’s body is the same as yours and will react exactly as yours does. Finally, most diets (which you practiced to achieve your loss) will achieve weight loss at first; the actual problem is that they do not produced sustained (more than 5 years) weight loss.

        I’m unclear on how providing scientific evidence to support one’s point is being close minded. Close minded, to me, is someone reading all the scientific evidence and saying, “But it worked for me so it MUST be true!”

      2. “Kinda made me seem like a jackass…”

        Little bit.

        I think I’ll pass on the opportunity to e-mail you (as I feel pretty confident that you are confusing close-minded with well-researched) and just deal with this here.

        First of all, how much weight did you lose in what time frame? If you were maintaining your weight and then reduced 100 calories a day from your diet, based on the ci/co theory it would take you 35 days to lose one pound (as a pound is equal to 3500 calories). If you lost faster then that, congratulations you’ve disproved the theory of ci/co. If you lost at that rate and of course didn’t have any plateaus (since those are not accounted for in a ci/co theory) you lost 10.43 pounds in a year. (If you lost less than 10 pounds then your weight loss is not considered substantive and nobody cares about it – scientifically speaking. In many cases weight loss is not considered scientifically relevant unless it’s 10% of your original body weight).

        You would then have to maintain that weight loss for 5 more years to beat the statistical reality that 95% of people regain their weight.

        Next: How do you know that it was fat and lean body mass. Did you dexa scan? Unless you are a physical anomaly, when you start restricting calories the enzyme lipoprotein lipase will cause your body to start storing fat and burn muscle. For this and other reasons, it is considered physically impossible to lose only fat and not lean muscle mass through calorie restriction alone.

        Finally, even if you are in the magical 5% who maintain your weight loss after 5 years, and even if you are a physical anomaly who managed to lose pure fat and no lean mass, you are still only an n=1 experiment. You are not a statistically significant sample size and your experience cannot be extrapolated to populations who are not you. Said more simply – your experience (which you failed to quantify in any way, btw) is not everyone’s experience and if you are going to tell me I’m wrong, I would suggest that you come armed with something a little more substantial than your personal, completely unquantified, what-I-did-for-my-summer-vacation, weight loss story.

        Also, I’m not typically one to give unsolicited advice but may I suggest that if you’re going to start a comment by saying “I am telling you, you are incorrect” you might proofread your comment to make sure that it actually refutes anything that I said in the blog to which you are replying (read again and look for the sentence: “The body releases weight at first…”). As it stands you are a swing and a miss with this comment.


    1. I’m curious as to how long this weight loss has lasted? Unless you’re past the 5 year point then you have proved nothing, at least in my mind as well as those who have read the science. Just a thought. Take it or leave it.

  16. What I would like to know from Frank is how he knows that he lost fat and not lean muscle mass.

    And Frank, do you always eat exactly the same foods, measured to the gram? Otherwise, how do you know that you only cut 100 calories?

  17. It is precisely things like “Calories in, calories out” that make people think the human body is a machine in which equations can be plugged into and exact desired results can be produced. This is not how we should be teaching people to relate to their bodies.

  18. Just to add more evidence that the calorie in/out idea is basically flawed, there have been some recent scientific studies that show that what is eaten can cause calories to be stored instead of burned even with a calorie-neutral intake.

    One was a study done on monkeys a few years ago, where they showed that monkeys on a calorie-neutral diet (where there should be no gain or loss) who were fed trans-fat actually gained weight even though they weren’t taking in enough calories to do so. (The link only shows the beginning of the article, but it’s enough info to go on if you’re interested in the original research.)

    I believe something similar was shown with high-fructose corn syrup, but I can’t find the reference to link to.

  19. I’m ashamed to say that I’m someone who has spouted this nonsense–not at anyone in particular, but during random discussions of health and weight. As someone who never had a weight problem, it just seemed to make sense. Thank you for opening my eyes and preventing me–at least on this point–from making a raging ass of myself and offending people I love and respect.

    1. Thank you. I respect you for admitting that you had not understood previously that this doesnt work. Many people wouldnt have done that.

  20. Excellent explanation.

    I just usually simplify the argument and say, if it is just calories in/calories out why do dieters always plateau?

  21. Thank you thank you thank you thank you.

    Oh God, thank you for writing this.

    I am probably going to quote extensively from this post in the near future, as a certain foolish friend-of-a-friend decided to get all uppity about how “simple” weight loss is, and he *clearly* needs to have his insensitive, unscientific, biased ass handed to him on a platter.

    You are all kinds of wonderful, chica.
    Wish I were on a dance team with you.


  22. Slow metabolism, fast metabolism. I’m not entirely sure I believe that’s what accounts for different weights. It makes more sense to me to say that the body maintains a certain weight, a setpoint if you will. When you’re at that weight, your metabolism may be as fast or slow as someone half your size. My body sticks to a narrow weight range, and if it deviates from that (in either direction) by more than a handful of pounds, I can usually find a reason for it which has nothing to do with calories.
    Let me emphasize that I’m NOT saying everyone’s body works just this way; there is a tremendous amount of human variation.

  23. Very cool post, the first I’ve read on your blog and it has defintely made me want to see what else you’re writing. Science makes you think! 🙂

  24. I can’t pretend to have a real grasp on this, but Gary Taubes confirmed that the problem with ci/co is it ignores the relationship between those two (he doesn’t think the actual statement is false).

    The amount of calories you ingest in the main, directly affects your output of calories or energy. Lower one and you automatically tend to lower the other and vice versa.

    This seems even to me a fundamental disconnect in understanding laws which posit that all is energy in different forms.

    It also has to be said, that a lot of people claiming to either have degrees in or work in fields using physics, insist this themselves, so one really has to wonder how many people really have developed an instinctive understanding of thermodynamics.


  25. I am going to respectfully dispute the widely used 95% statistic that is liberally used in your blog although it probably won’t be posted. The 95% statistic mostly likely comes from Albert Stunkard, a researcher from the University of Pittsburgh. His study is the first place that this particular stat has been traced to. He put forth the 95% rate in 1959 based off of a study containing only 100 people. In the study, 100 people were put on a diet with no follow-up support. 95% of those dieters gained the weight back. This statistic has been applied liberally since then and no one seems to think it worth checking into the validity of that percentage.. Why is it that such an old statistic based off of such a poor study (a 100 person sample size!! come on!!) is used promote a fatalistic approach to all diets? I truly believe that a diet based on life style change has a higher rate of success than a diet with no life style change. It might still be doomed to failure for more than 50% of people, but I seriously doubt it is 95%. I find your frequent use of the 95% stat that might not be even valid disheartening to say the least. I know you are usually very meticulous in the use of your scientific facts so I wanted to point out this discrepancy. That being said, I think your blog is very well written and intelligent and I applaud your efforts to promote equality among all people.

    1. EAS, I’m curious about what you think of the studies quoted and deconstructed in
      Here is a quote from the National Weight Control Registry: “Recruitment for the Registry is ongoing. If you are at least 18 years of age and have maintained at least a 30 pound weight loss for one year or longer you may be eligible to join our research study”
      30 pounds for a single year doesn’t sound like a lot to ask. Yet, of all the millions of people who diet, there are only about 5000 members.

      It’s difficult to come up with a percentage unless you define precisely what it refers to. Can you come up with a better percentage based on more people and strict parameters? It’s even possible that the relevant percentage could be higher than 95%.

      It would be an excellent thing to promote a fatalistic approach to weight-loss diets. It would be so much more sensible to go directly for health, energy and stamina even if you stay fat. Perhaps others here can provide more current statistics based on larger sample sizes.

    2. Hi,

      Thanks for the comment. I do wish that you had checked your assumptions though, it’s not the Stunkard research that I’m using. I don’t think that 100 people is a statistically significant sample size. Regardless, I understand that this statistic is often argued. What is confusing me is that you are questioning the validity of my statistic, and your counter-argument is that you “truly believe” that the success rate is higher. We are both equally entitled to our opinions but if you are going to tell me that my statistic is invalid, I would expect you to direct me to a statistically significant study that shows a more than 5% success rate after five years, thus providing some proof that I’m wrong. Again, I respect whatever decision you make about dieting for yourself and I understand that you don’t believe that 95% statistic but in order for me to change my mind, or the statistic I use, I’m going to have to see something that disproves what my research shows. I’m working on a full blog about this now, I hope to have it up next week.



  26. I’m having a hard time seeing how these aren’t just variants of “calories out.”

    To extend your wood metaphor… If you cut down a tree in your neighbourhood and make a house out of it, it’s still in the neighbourhood. It’s not burned, it’s not stored… but it’s not “out” out of the neighbourhood. A calorie not burned = a calorie not “out”.

    An unstable BMR would mean that more calories are burned at some times, and fewer calories are burned at others. Seems pretty logical. If I fill my gas tank, more gas gets used when I drive in my Hummer in the city with the A/C on than when I drive my Mazda 3 on the highway with the windows open. Although the gas is being used at “unstable” rates, it’s still being used. Even in the same vehicle, different driving conditions result in different gas usage.

    And yes, I have a friend who can eat anything and still stay a size zero. Again, “calories out”. Her body naturally uses up more calories throughout the day than mine. I accept that, and do the best I can with the body, and metabolism, and ability to burn calories, that I’ve got.

  27. Linked to my blog over on Blogger. ^_^ I’m glad someone finally explained it! And someone who understands physics! If only my teachers in High School had thought to explain science in a way that was less “Let’s make water turn pink with acid,” (or the other way around can’t remember) and more, “Look this is a way in everyday life that this does or does not work!”

    1. That’d be “make the pink go away” with acid. Ah, phenophthalein, how I remember thee.
      And that’s a chemistry experiment not physics. Acid/base chemistry is actually very interesting and I’ve seen several studies postulating that our increased intake of acids is contributing to higher rates of osteoporosis as the bone gives up calcium (Ca 2+) to help balance out the bloodstream pH.

  28. LOL!

    Actually I assume that they are phonetically parroting something they heard from someone else and that they wouldn’t know one of the laws of thermodynamics if it bit them in the ass but I digress.

    This alone makes the post worth reading! Ouch, what the heck was that? 😉

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