Why the first law of thermodynamics is utterly irrelevant

There are many adherents to the Calories In/ Calories Out (CICO) theory that constantly bleat about “It all comes down to the First Law of Thermodynamics”. The First Law of Thermodynamics refers to a law of physics where energy cannot be created or destroyed in a closed system and is ALWAYS true.

However, in the complex world of human physiology, it is true but completely irrelevant. What the CICO people think it means is that if you reduce calories in, you will lose weight. Of course, it means nothing of the sort.

So, let’s see why. CICO1Here’s our representation of the human body. You have Calories In, Calories Out and Fat Storage.

This is the fatal flaw of CICO – there are two compartments where calories can go after being eaten, (Calories Out and Fat), not one. It is not a one compartment problem.

CICO adherents believe you take calories in, subtract calories out and whatever is left over is dumped into fat stores like a potato into a sack. So, they believe that fat stores are essentially unregulated. Every night, like a store manager closing its books, they imagine the body counts up calories in, calories out and deposits the rest into the fat ‘bank’. Of course, the body is far more complex than that.

How the body works

So here’s the way the body works. Every process is highly regulated. Whether we burn calories as energy or whether it goes towards fat storage is tightly controlled. As we eat, calories go in. Calories go out as basal metabolism (used for vital organs, heat production, etc) and physical activity. Fat can go into storage or it can go out of storage.

CICO3What controls this decision? We hopefully all agree that the main hormone involved is insulin. As we eat, insulin goes up. Notice that the body does not respond to calories equally. Some calories (white bread) will raise insulin a lot, and others (butter) will hardly raise insulin at all. This should have been the first clue that calories are not the common language of weight gain/loss. The body has no receptors for calories and has no way of measuring calories.

Consider two foods that are equal caloric values – a plate of cookies versus a salad with olive oil with salmon. As soon as you eat, the body’s metabolic response is completely different and easily measured. One will raise insulin a lot, and the other won’t. So why do we pretend like the body cares about calories?

That’s like saying that foods that are blue are the same – whether they are blueberries or blue raspberry Gatorade. The body doesn’t care about color, so why would I? In the same way, the body doesn’t care about calories, so why should we? However, the body DOES care a lot about the hormonal response to the foods we just ate.

Since we are eating more at that moment than can be used by the body, some of this food energy gets stored away, either as glycogen or fat. This is insulin’s role. It stores food energy through the processes of glycogen synthesis and de novo lipogenesis (making of new fat in the liver).

CICO5When we stop eating, insulin starts to fall. This is the signal to first stop storing food energy. As we continue to fast (say, during the night), we need to move some of this food energy back out from our stores to power our metabolism. Otherwise, we would die during our sleep, which obviously does not happen.

OK. So far, so good. Now let’s put some numbers on it. Let’s assume we are not gaining or losing weight, but have 100 pounds of fat we’d like to drop. Assume a daily average intake of 2000 calories. This is what it will look like.CICO2

Since Calories In and Calories out are balanced, and Fat is neither going up or down, everything is in balance. The body wants to burn 2000 calories to stay warm and feel good. So what happens when we decide to lose weight?

Weight loss the CICO way

The CICO model says that all you need to do is reduce your calories in. You don’t need to worry about what you are eating because ‘it all comes down to calories’. So, eating a calorie reduced, low fat, high carbohydrate diet, insulin levels stay high, but calories comes down. They do this on shows like ‘The Biggest Loser’, but this is the exact same strategies that all the universities, and governments use too.

CICO7What happens?

You reduce your intake to 1200 calories per day. Since insulin remains high, you cannot get much energy from fat stores. Why? Because the dietary strategy you are using (Caloric Reduction as Primary) only concerns itself with reducing calories, not insulin. Remember that the high insulin is telling the body to store energy as fat, or at a minimum, not burn fat (inhibits lipolysis).

So, as you reduce your caloric intake to 1200 calories in, the body is forced to reduce it’s metabolism to only 1200 calories. No energy is available anywhere else. This is precisely what happened on the Biggest Loser as seen in the study featured in the New York Times. This is also precisely what happens during any caloric reduction diet.

That is why these diets are doomed to fail. Studies of this strategy estimate failure rates at 99%. Notice that the First Law of Thermodynamics is not being broken in any way. It is irrelevant.

The lower metabolism means you feel cold, tired and hungry. Worse, the weight eventually plateaus and then as you decide that it’s not worth it, you start to eat more, say 1400 calories thinking that it’s still not as much as you used to eat. Hunger hormones are increased because the body wants to burn 2000 calories and you are only taking in 1200. So weight starts coming back. Sound familiar?

Weight loss by reducing insulin

Well, that was fun. What happens when you use dietary strategies that instead target insulin? Low carb High Fat (LCHF) diets, ketogenic diets, and the ultimate insulin-reducing strategy, fasting all target the reduction of insulin. CICO6What happens?

Since the point of these diets is to lower insulin, stored food energy (fat) can be broken down to power the body. Since the body wants to burn 2000 calories a day, it burns 1000 calories of fat and 1000 calories from food.

What we would predict is that basal metabolic rate remains the same, appetite is decreased and weight is steadily decreasing. Guess what? That’s exactly what is shown in studies. In Dr. David Ludwig’s study and Kevin Halls new study, ketogenic diets do not have this dreaded metabolic slowdown.

Anecdotally, hunger is also decreased with ketogenic diets. The effect is even more striking with fasting. I can only recount my experiences in the Intensive Dietary Management program. We’ve put over 1000 people on fasts of various durations. Many of them drag themselves since they have no energy. After fasting, their energy is massively increased. But despite this, they report that their appetite has shrunk to barely 1/3 of what it was previously. They often tell me they think their stomach has shrunk.

In a sense, it has. But if people are eating less because they are less hungry and then losing weight, that’s GREAT. Because we are now working with the body, instead of fighting it. With caloric reduction diets, people constantly fight their hunger and deny themselves food. Here, people are turning away food of their own volition. Because we lowered insulin.

The first law is right – but this isn’t physics

Notice once again, that the First Law of Thermodynamics is not being broken. There are no calories created out of thin air. It is simply irrelevant to human physiology.

I studied biochemistry in university and took a full year course on thermodynamics. At no point did we ever discuss the human body or weight gain/ loss. Because it has nothing to do with thermodynamics. If anybody mentions the ‘first law of thermodynamics’ regarding weight loss, I suspect they haven’t really thought about what thermodynamics actually is.

Nutritionists on the other hand, especially those who promote calorie counting, can’t seem to say enough about Thermodynamics. They want the quantitative and theoretical backing of hard science and therefore pretend that human physiology is like physics, with its hard rules and laws.

News flash, guys. Physiology is physiology and physics is physics. Don’t mess the two up.

FREGLYIn my personal opinion, those who promote CICO are like Fregley. He is the character in ‘Diary of a Wimpy Kid’ who is the unpopular kid who wants desperately to be liked. CICO people desperately want the approval of hard science that they overlook the fact that physiology is not physics.

You also can’t use the Heisenberg uncertainty principle for weight loss. The Bernouilli Effect doesn’t apply to the urine stream. Physics is physics. Physiology is physiology.

Fasting vs. calorie reduction

Sometimes I’m asked the question about the difference between fasting and calorie reduction. Doesn’t fasting reduce calories? Yes, but it also significantly reduces insulin. This allows you to release some of the stored fat energy so that you don’t need to or even want to eat so much.

What drives me crazy is this. The Biggest Loser study proved that cutting calories is a terrible, horrible, no good and very bad strategy, virtually guaranteed to fail. So, in all these articles talking about the Kevin Hall study, what do the ‘experts’ suggest instead? Cutting your calories!!

T2DEpidemic3What happens when you recommend a diet that is guaranteed to fail? Well, you might get a huge worldwide epidemic of type 2 diabetes and obesity.

Unfortunately, most of the nutritional authorities all belong to the same CICO mind frame, and we are all paying the price. You thought Scientology was bad. To me, CICO is even worse.

Let’s consider these simple facts. We’ve recommended cutting calories for weight loss for the last 40 years. During that time, we’ve had a huge obesity epidemic. All the science suggests that recommending caloric reduction as primary is doomed to fail. yet, senior researchers, academic physicians and virtually all health associations continue to recommend it.

One article interviewed ‘leading obesity experts’ and came up with these tips. Exercise regularly. Cut calories by avoiding high fat foods. Eat breakfast. Count calories. So, in other words, they would give the exact same advice that we’ve been giving for the last 40 years even as the obesity epidemic overwhelms our health care system. The 1980s called, they want their diet advice back.

In discussing the physiology of obesity, the First Law of Thermodynamics is not wrong – it’s irrelevant.

A better way

How to Lose Weight

Earlier by Dr. Fung

How to Fix Your Broken Metabolism by Doing the Exact Opposite

The Biggest Loser FAIL and That Ketogenic Study Success


The Cause of Obesity and Diabetes – Gary Taubes
The Top 5 Tips For Weight Loss
Weight Control – A Question of Calories or Insulin? – Dr. Andreas Eenfeldt
The Key to Obesity – Dr. Jason Fung
The Etiology of Obesity – Dr. Jason Fung

More with Dr. Fung

Dr. Fung has his own blog at intensivedietarymanagement.com. He is also active on Twitter.

His book The Obesity Code is available on Amazon.

The Obesity Code


  1. Helen
    Wow, I love the man's passion. I'm so happy & excited that my wonderful dietitian convinced me to finally embrace the wonderful world of LCHF. Thanks for great articles & information.
  2. Mahdi
    Great article, Dr. Fung
  3. Apicius
    I'd like a bone-head dietician convince me that they understand the laws of thermodynamics....while trying to light a wet log on fire in one vessel, while trying to light gasoline in another vessel. If in both cases, the same amount of calories is present in the fuels (wet log versus gasoline), I'd like to hear the brilliant testimony of the bone-head dietician explain to me how the burning action will be the same for both substrates, because of course, the laws of thermodynamics would say they should both burn equally.
  4. Tim
    I'm not sure I buy the alternative hypothesis. It fails to explain how people on Atkins eat 3000 calories and LOSE weight. I think there should be an arrow for "food your body can't use and goes out as waste". Am I wrong? The alternative hypothesis would have these folks bouncing off the walls.
    Reply: #15
  5. KarenNT
    I've been trying to explain this to people in laymens terms for some time now. So nice to hear it explained properly!
  6. David T Darby
    Tim, you obviously missed the point. It's about INSULIN REDUCTION. You eat food which cause insulin to be dumped into the blood system you get fat storage...no insulin input, no fat storage.
  7. Barbara
    Tim, not everyone on an Atkins diet eats 3000 calories either. Someone with 200 lbs to lose might need that much - but it would be high fat, moderate protein and low carbs. Someone with 20 lbs to lose would not need or WANT 3000 calories. What you have stated is one of the myths circulated about Atkins - just not true. The alternative to high carbs is high fat. That gets the body to burn its stored fuel. It DOES make sense whether you buy it or not.
  8. Rick
    I am a research nutritionist, and I agree that insulin is key to losing weight and cutting carbs works. But I see that you say the first law of thermodynamics is irrelevant, then show that decreasing insulin response is an effective way to decrease intake, and ACCORDING TO THE FIRST LAW with decreased intake and maintained metabolism you mobilize more fat and decrease body weight. So you are using the First Law but adding something else too. The First Law just says these things add up, which they do. I don't disagree with your conclusion, but as someone who believes in physics, I think the argument would be stronger not to suggest you are refuting physics.
  9. Thomas
    David, Tim’s point is right (think you misunderstood it?). When on a keto-type diet, you can over-eat quite a bit and still lose fat weight. This isn’t explained by Dr Fung’s theory.

    Look up an experiment by a guy called Sam Feltham, he ate 5.200 kcal per day over 21 days and lowered his fat % by about three units. He tried the same with a normal (UK) diet and gained a bunch too, felt awful apparently. Exactly in the same way we would feel awful on a high carb diet.

  10. Peter
    The first law is neither irrelevant nor is it wrong. The law applies to human beings as much as it applies to everything else. The only thing is that the simplistic view of the human body as used by the calories in/out brigade is too simplistic. Other things are going on that are affecting that energy balance. Factor everything in and I think you will find that not one scrap of energy has been created or destroyed.
  11. Tom Bentley
    Calories apply to the substance not the person.
    Calories are a measurement not a substance. You can’t ‘burn’ calories.
    The calorie was first defined by Nicolas Clément in 1824 as a unit of heat energy.
    He taught a course in industrial chemistry that emphasized the thermodynamics of steam in relation to powering steam engines.

    He developed methods for calculating the maximum amount of energy that could be obtained from a kg of coal. This was to identify what was the best source of fuel for the stationary steam engines of the day.

    The first and second law of thermodynamics refers to a closed or isolated system. The human metabolism, of 55 hormones, Is far from a closed/isolated system.

    Dr Zoe Harcombe has presented on calories a couple of times and explains that counting calories is far from an exact science and can become ridiculous

  12. Peter
    The human body is a closed system. There are a great deal of interactions, chemical processes, energy consuming activities including growth, generating heat, pumping blood around the body etc but it is still a closed system. I am not ignoring the effect of hormones nor am I ignoring the massively complex entity that is the human body but to conclude that the 2nd law is either irrelevant or somehow does not apply is naive.
  13. Loretta
    The laws of thermodynamics apply to the human body just as surely as the laws of gravity do. But neither of them are useful tools for understanding the path to weight loss.
  14. Francisco Navarro
    This is complete bullshit. If you eat at a calorie deficit even if it comes 100% sugar you will lose weight you probably won't be healthy but yoy will lose weight. The truth is that when it comes to weight loss total calorie intake is king. where is the energy going to come from if you dont provide the energy from the food you take? you can lose weight on a twinky diet or even a Mcdonalds diet as long as you track your total calorie intake. these are just extreme examples but I'm simply talking abour weight loss not healthy eating nor general health. Theoretically the low carb diet makes sense but studies that support this claim did not equate for total calorie intake. When you equate total calorie intake both low carb and moderate/high carb diets where found effective in weight loss proving that when it comes to weight loss, total calorie intake is king.
    Reply: #25
  15. Francisco Navarro
    People that succeed on an atkins diet is because they are eating at a calorie deficit. They would have to drink oil and eat a ton of bacon to be eating that much. those who lose weight is because they eat at a calorie deficit. There are also people who follow the IIFYM (if it fits your macros) aproach and they eat pop tarts and all kinds of junk and they always lose weight as long as they are eating at a calorie deficit. you just have to track your calories and macronutrients with something like my fitness pal or some other app.
  16. David Harralson
    One thing not mentioned is a person's efficiency in converting input calories to calories available to the body, for energy or fat storage. Simplistically, high glycemic foods are usually converted efficiently to available calories and, when in excess, are then stored as excess energy, usually as weight gain. If the body cannot convert the calories presented to it to energy, the body does not store excess calories and thus does not accumulate weight as calorie storage.

    So, if you eat foods that your body can convert to energy efficiently, then you need fewer raw calories to maintain a given weight. Conversely, if your body cannot convert the presented calories efficiently, then the body need more presented calories to obtain a certain amount of utilized calories.

    This is the basic theory in the back of some diet regimens. But, every person's body has differing efficiencies in converting presented calories in different formats. So some diets may produce weight loss in some individuals and produce weight gain in other individuals.

  17. Sambar
    I'm not sure who's right. Everyone has studies to back up what they are saying.

    We need a great debate of some kind.

    Only thing I'm 100% sure of is fasting has kept me warm and slender where as other diets (including keto without fasting) makes me cold and gain weight after a couples of years on it.

    The fasting is increases my metabolism. I'm not sure macros are behind that though.

    Either way I'm grateful to Dr Fung. Without his confidence I would never have fasted and I would still be morbidly obese with a likelihood of T2D.

    I'm not sure if calories are King but I'm sure Dr Fung is.

    Thank you from me and my children. ❤

  18. Teymur
    Brilliant! Just brilliant! So few people actually understand the real reasons behind fat gain (and other health-related problems that come with following conventional "food pyramids" and CICO advice) and yet so many are ready to defend the stupid notion of calorie management - this article is a ray of light in a sea of complete darkness. Thank you!
  19. Moses
    I just find it extremely entertaining that he believes physics has nothing to do with physiology. As an individual who has studied and currently do research in biophysics, all of this is written into computer simulations with variable initial conditions. Also, the crack about the Bernoulli principle is correct; biophysicist do not believes that to be the case. It sounds like you’ve only discussed introductory level topics in physics and have the gall to jab at a field you clearly do not understand, “Dr. Fung.” Mind you, the data found always matches simulations. That’s the point. If you have the physics correct, not the introductory level with idealized systems such as the ones you’ve presented, then all things follow the simulation.
  20. Robin
    The problem with low carb is that it gives me panic attacks. I never get them, otherwise. Other people have complained of this symptom, too.

    It seems to have something to do with serotonin/adrenal problems. What about focusing on cutting fructose (I have fructose malabsorption, anyway.) and a somewhat more moderate complex carb intake? Like enough to avoid panic attacks? I find that about 20 grams of carbohydrate per meal, or the equivalent of a slice of toast, works for me.

  21. EV
    - "I studied biochemistry in university and took a full year course on thermodynamics. At no point did we ever discuss the human body or weight gain/ loss. Because it has nothing to do with thermodynamics. If anybody mentions the ‘first law of thermodynamics’ regarding weight loss, you, too will know that they are just very smart. Or maybe they just haven’t really thought about what thermodynamics actually is."

    - "They desperately want the quantitative and theoretical backing of hard science and therefore pretend that human physiology is like physics, with its hard rules and laws."

    Sorry, but this is the craziest thing I've ever heard from someone who holds doctoral degree in natural science related subject. Either you didn't study it correctly, or your title is fake, Mr. Fung.

  22. EV
    And then funnily, you brought this interview:

    Isn't that (intermittent)-Fasting is cutting calories in?

  23. 1 comment removed
  24. Elektrolite
    An article with strong claims. However strong claims require strong evidence, and I don’t see a single reference in here. I would particularly like to see a meta study of your extraordinary claim that the body will reduce its metabolic rate to 1200 cals from 2000 if you eat the wrong foods.

    Without evidence all you have is pseudo science.

  25. Harianas
    "where is the energy going to come from if you dont provide the energy from the food you take? "

    Look at the low insulin graph condition above. You use energy from the body fat storage, HENCE consuming your body fat and that's how you lose weight on a reduction insulin method.

  26. 2 comments removed
  27. Terry Smalls
    This really fails to actually address the issue that you can't store calories in fat that you don't eat. Again, CICO is pretty accurate and I raise the same eyebrow at this "analysis" that I do against the three "scientists" who argue against anthropogenic climate change.

    You built a strawman and decided to strike it down....grats? Let's stick to the fundamental logic, however.

  28. 3 comments removed
  29. Ryan D
    If I go lift weights I am doing work and work requires energy. The energy has to come from somewhere. If I am not getting the energy from food where is my body going to get it? Your ass? It is going to turn to fat and muscle to get the energy. Point is there is an application for the first law.
  30. 3 comments removed
  31. K Monts
    Doc, I was following you well until you got to the "Weight Loss by Reducing Insulin" part. You say that "Since the body wants to burn 2000 calories a day, it burns 1000 calories of fat and 1000 calories from food". One, you only consumed 1000 calories of food which is reduced by half from the original 2000 calories, isn't that a CICO only thing? Two, if you still consumed 2000 calories of food since "calories don't matter" then does the body burn 1000 calories from fat, 1000 calories from food, and then it still stores the remaining 1000 into fat? Is that it? Hope you could give a clearer answer.
  32. Robert Cullum
    FFS. If anyone who's calorie intake is 2000 then drops to 1000 they will lose weight, you might feel like shit but you lose weight and if that person goes up to a 3000 calories a day he will gain weight. Simple. See all those anorexics, they're on a very low calorie diet. Reduce calories to lose weight. Works everytime.
  33. Niki
    There is so much wrong in this article I don't even know if it's worth arguing. I will just say this - I've had a personal trainer to help me gain weight (yes, the opposite of what this article is about) but 90% of the people in our group are people trying to lose weight. I constantly see their transformation pics in our facebook group, so the coaches' strategy must be working. And guess what - they are hardcore CICO and science guys. I have never ever heard them mentioning insulin.

    They have told me that the first thing they notice is that people underestimate or overestimate the calories in their food. Meaning they don't realize 100g of peanut butter is 600+ calories. They think they have been cutting calories, but they really weren't, hence the false impression that cutting calories didn't work for them.

    Last but not least, when we mention CICO, we are of course assuming healthy foods and not junk and sweets. I bet nobody that believes in CICO thinks that source of calories doesn't matter and you can lose weight only eating X number of cookies per day and nothing else. It's common sense, to me at least.

  34. Niki
    Anecdotal evidence, I'm not a doctor, it's just the experience I've had in my 1 year journey and the experiences of people around me.
  35. Kim
    No one here has addressed where the fat goes when you lose weight. Any physicist will dispel the theory that it is converted to energy. We need to stop thing of calories in/calories out and start thinking of atoms in/atoms out because humans and the food/beverages they consume are made up of atoms. We also breathe in oxygen. In 1772, Antoine-Laurent Lavoisier discovered that animal respiration is essentially a slow combustion inside the body, which consumes oxygen and converts organic matter (food) to carbon dioxide and water. If the atoms we consume + use = atoms we expel then our weight remains stable. However, if we lower the atoms (food) we consume, then the atoms we expel will be increase and we lose weight/fat. The elemental composition of human fat → 77% Carbon, 12% Hydrogen and 11% Oxygen.

    In summary, this is what happens when a human being is burning body fat: fat + oxygen → carbon dioxide + water. That's right, most of the fat is expelled through our mouths (and some through normal body excretions in the bathroom).

  36. James Watkinson
    Why does it never once mention the hormone glucagon? And that in the cast majority of studies cico diets actually does influence weight gain Vs weight loss the type of foods massively influence the efficiency and how and what type of activities we do..

    The obesity and type 2 diabetes epidemics around the world are primarily in developed areas with ease of access to fast food (fast calories) and sedentary lifestyles...

    Hormones play a big part in efficiency but I haven't seen any evidence to counter the point of calories in calories out.

    In the same way good quality fuel burns better in cars or machinery, than poor quality.

    Would be interested to see evidence (non anecdotal) of a hormone insulin based model that completely disproves cico

    The analogy of an accountant at the beginning is crude.. as the body is 24 machine constantly working

    Buy it is interesting that anecdotally :) all clients I have put in cico/macro based diets have either gained the weight they desired or lost the weight they desired and have felt and looked better for doing it... And the ones who DONT do as I've told them don't get the results they were after...

    Funny how most of the body budding industry works on cico/macro model as well

  37. Paul
    This pretty much sums up everything wrong with the fitness industry. A doctor tell you that the first law of thermodynamics is 'utterly irrelevant'...

    This is a website trying to make money off you becoming keto. Do your own research from people not trying to make money off you and you'll soon see that being in a calorie deficit is by far the most important thing when it comes to weight loss.

  38. David
    For all the people that believe it's just about less calories and not what those calories are go eat just doughnuts and that it for a month and get back to me
    Reply: #44
  39. Crystal Pullen Team Diet Doctor

    For all the people that believe it's just about less calories and not what those calories are go eat just doughnuts and that it for a month and get back to me

    Here is another article for you to check out. https://www.dietdoctor.com/low-carb/calories

  40. Youssef
    I think your missing the point here.
    CICO is simply a means of letting people understand the simple FACT that a calorie deficit needs to occur consistently to see weight loss results this has been proven countless times in studies now.
    CICO does NOT take away the fact that you should be eating low glycemic, high satiety, micronutrient dense foods WHILST in a caloric deficit to illicit the weight loss that you want. So I have no idea why you are saying CICO advocates are teaching simply that its ALL about calories in vs calories out.
    You are categorising CICO advocates (which is essentially anyone who has a science background in nutrition) as dimwits who don't understand how the body works... how is that even fair?
    You literally stated in your example that if you are on a 1000 kcal diet and your eating low glycemic foods that you would tap into 1000 kcals of body fat per day... Notice how you put numbers on it? Yeah... well that's all CICO is.
  41. Lee
    So you're saying CICO works better if you optimise some parameters?

    Seems to be the first law of thermodynamics is still entirely relevant.

  42. Fabien
    I actually disagree with those who say that Dr Fung is completely wrong on his understanding of the 1st law of thermodynamics for the human body.

    I think that by "irrelevant" what Dr Fung is hinting at is that CICO is not "just" an expression of the first law of thermodynamics but has many more implicit assumptions about how the body works and these assumptions are not grounded in our current understanding of human physiology. As a result the CICO doctrine becomes automatically unsound.

    Taken straight from physics textbooks the 1st law of thermodynamics states that the change dU in the energy content of a system follows the equation

    dU = Q + W + Sum udn

    Where Q is the heat received by the system from outside of it, W is the work received by the system from the outside and Sum udn is the energy change owing to variations in body composition.

    Is there any explicit reference to fats in this equation? No.

    Thermodynamics says nothing at all about how heat and work are going to be exchanged with the outside if we don't know the exact functioning of the system under investigation. Likewise the energy change owing to the system composition cannot be inferred from the first law alone. In addition the system composition term applied to a human body does include energy stored in stools and other wastes as well.

    What CICO says is that fat loss is governed by being in alleged caloric deficit. I say alleged because almost nobody knows even approximately their true BRM so inferring a caloric deficit is actually not possible for the majority of people.

    What Dr Fung says is that fat loss is governed by burning more fat than is being stored and that these mechanisms are controlled by hormones not by calories counters in our body. It could be that fat burning follows from a cascade of signaling requiring energy expenditure which would correspond to more "calories out" but these signaling cascades must be understood to truly understand fat loss.
    Furthermore, physically speaking burning more fat than is being stored usually does mean that the energy content of the body decreases but there physics doesn't provideis a directional arrow telling us which is the cause and which is the consequence.

    So, if I may rephrase Dr Fung's conclusion, CICO is no more grounded in thermodynamics than the insulin-governing-fat-loss hypothesis but the latter is physiologically more sound. At best CICO can serve as a naive substitute of the real thing going on. Taking CICO too seriously though can lead to major fat loss failures however.

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