Why The First Law of Thermodynamics is Utterly Irrelevant

By Jason Fung, MD

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. It certainly holds true in the complex world of human physiology, but is completely irrelevant. What the CICO people think it means is that if you reduce calories intake, you must lose weight. Of course, it means nothing of the sort. So, let’s see why. The main reason is that the most important part of CICO is NOT the Calories In, but the Calories Out and the Insulin. Here’s our representation of the human body.

You have Calories In, Calories Out and Fat Storage. This is, of course 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, nothing is further from the truth.

Instead, every process in our bodies is highly regulated. Whether we burn calories as energy or whether it goes towards fat storage is tightly controlled by hormones. As we eat, calories go in. Calories go out as basal metabolism (used for vital organs, heat production, etc) and exercise. Fat can go into storage or it can go out of storage.

What controls this decision? We can all agree that the main hormone involved is insulin. As we eat, insulin goes up. Notice that insulin does not respond to calories equally. Some calories (white bread) will raise insulin a lot, and others (butter) will not 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 give two sh**s 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).

When 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.

Since Calories In and Calories out are balance, 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? The CICO people say 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.

What happens?

You reduce your intake to 1200 calories per day. Since insulin remains high, you cannot get any 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 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.  You are only taking in 1400 and burning 1200, so weight starts coming back. Sound familiar?

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. What 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.

Remember, the body actually wants to burn 2000 calories a day. If insulin is low, then the body can get its energy from fat stores as well as food. Anecdotally, hunger is decreased with ketogenic diets. Many people say they only want to eat 1000 calories because they are full. But they don’t feel the same metabolic shutdown because the body is getting the other 1000 calories from fat. Notice that no Laws of Thermodynamics are being broken.

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.

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, you, too will know that they are just not very smart. Or maybe they just haven’t really thought about what thermodynamics actually is.

Nutritionists on the other hand, especially the calorie counters, can’t seem to say enough about Thermodynamics. They have ‘science’ envy. 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.

New flash, guys. Physiology is physiology and physics is physics. Don’t mess the two up. The CICO people are 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 are willing to pretend that physiology is physics. Sorry buddy. Just because you have physics envy, doesn’t mean you get to make up stuff. …. (I was going to put in a very crass and crude joke about the Freudian concept of penis envy all for the sake of a few cheap laughs. Against my better judgement, I have removed it.)

You also can’t use the Heisenberg uncertainty principle for your bowel movements. The Bernouilli Effect doesn’t apply to the urine stream. Physics is physics. Physiology is physiology. Deal with it.

Sometimes I’m asked the question about the difference between fasting and calorie reduction. Doesn’t fasting reduce calories? Yes, but that’s not the point. Fasting is about reducing 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!! The only thing worse are those ‘experts’ who claim that that the key is not to define success by weight loss. Win the Diet Wars by not even trying. Buddy! People want to know how to lose weight. Success is defined as weight lost, not loving your body the way it is. As Justin Bieber would say – Go Love Yourself. I wanna know how to lose weight. That’s what The Obesity Code is all about. If you want to know how to lose weight, first understand what causes weight gain.

What happens when you recommend a diet that is guaranteed to fail? When you might get a huge worldwide epidemic of type 2 diabetes and obesity. Unfortunately, all the nutritional authorities all belong to the same CICO cult, and we are all paying the price for their stupidity. You thought Scientology was bad. 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 caloric reduction as primary is doomed to fail. Senior researchers, academic physicians and virtually all health associations continue to recommend it. They are sheep, constantly bleating. Count your calories! Cut your calories! It all comes down to calories! Anybody who believes otherwise doesn’t believe in the universal laws of nature! I have physics envy!

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. Hey, Julia Belluz, the 1980s called, they want their diet advice back.


The lunatics are running the asylum – and they all believe in the First Law of Thermodynamics. In discussing the physiology of obesity, the First Law of Thermodynamics is not wrong – it’s irrelevant.

By The Fasting Method

For many health reasons, losing weight is important. It can improve your blood sugars, blood pressure and metabolic health, lowering your risk of heart disease, stroke and cancer. But it’s not easy. That’s where we can help.

Jason Fung, MD

By Jason Fung, MD

Jason Fung, M.D., is a Toronto-based nephrologist (kidney specialist) and a world leading expert in intermittent fasting and low-carb diets.

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