The Astonishing Overeating Paradox

By Jason Fung, MD

Influence of the Body’s ‘Fat Thermostat’ on Losing Weight

  • The body fat percentage is controlled like a thermostat, which will determine whether or not we lose weight
  • When we lose weight, our body tries to regain that weight by increasing hunger and decreasing basal metabolic rate
  • Similarly, when we gain weight, our body tries to lose it by decreasing hunger and increasing basal metabolic rate
By Jason Fung, M.D., Co-founder of The Fasting Method


Most people think that eating too much causes obesity and eating less will enable us to lose weight. If this is correct, when you overfeed experimental subjects, they should consistently gain weight.  Easy to prove, right? That’s what endocrinologist Dr. Ethan Sims also thought in the late 1960’s in his famous overfeeding experiments.

He recruited college students to participate in his overeating experiment. Everybody thought it would be a cinch. Pig out on food, no problem. But surprisingly, it wasn’t so easy to force these people to overeat and gain weight. For those of you who have ever tried to feed a screaming baby who is refusing (because you thought he/she might starve to death) – you know what I’m talking about. Or maybe think about the last time you stuffed yourself to the gills at the local buffet. Then try to imagine eating another 2 pork chops. Yeah, not so easy. One we get full, it’s not easy to force more food down. The mere thought makes you nauseous.

So Dr. Sims quickly abandoned that route and instead forced convicts at the local Vermont State Prison to be his experimental subjects. Physical activities could also be strictly controlled so they could not work off the excess calories. Initially convicts were fed 4000 calories/ day. They figured that was enough, but a funny thing happened. People started gaining weight initially but then the weight gain stabilized.

So, with a combination of scientific curiosity and loose ethical control (hello – informed consent?), some people were made to eat upwards of 10,000 calories/day. 10,000 calories per day! One man only gained 10 pounds with all that. However, most people did gain upwards of 20% of body weight. What happened to their total energy expenditure (TEE)? It increased by 50%. The body was trying to burn off all those extra calories.

That seems reasonable. Imagine that you have a fireplace in which you use 1 piece of wood every day. Now all of a sudden, you are getting 5 pieces of wood per day every day. Wouldn’t you start using a bit more wood? Of course. The body increases TEE and we feel great.

When the experiment ended, researchers were stunned by the rapidity with which body weight returned to normal. When left to their own choice, most people ate very little food, and their weight returned quickly to their normal weight. So, just eating more calories by itself does NOT, in fact, lead to long term weight gain. The body fat percentage acts like a thermostat. While body weight may temporarily go above the set weight, it quickly reduces things back to normal.

Let’s look at a more recent experiment: Metabolic response to experimental overfeeding in lean and overweight healthy volunteers Am J Clin Nutr Oct 1992;56(4): 641-55 Diaz EO

Subjects were overfed by 50% over 6 weeks and then monitored for another 6 weeks using a diet that was 46% carbohydrate. Weight increased over the first 6 weeks by up to 15 pounds. But over the next 6 weeks, the body weight quickly and automatically reverted to normal.

Changes in Total Energy Expenditure

Using doubly labeled water and indirect calorimetry to measure TEE, researchers found that from a baseline average TEE of 13.2 MJ/day, metabolic rates increased by 1.4 MJ/day (about 10%) to 15.0. In other words, the body is ramping up energy expenditure to burn off this excess of calories coming in. After the overfeeding period, TEE decreased back to 13.1 MJ/day. The weight came back down to normal and the TEE came back down to normal as well.

Experimental Overfeeding

From the paper “we conclude that there was evidence that a physiological sensor was sensitive to the fact that body weight had been perturbed and was attempting to reset it”.  In other words, it appears that there is a body weight set point (BSW). Focusing on calories alone is a losing strategy because the body resists changes to body fat. Instead, the important question is this – what controls the body set weight? The answer is hormones. We cannot simply restrict calories. We need to adjust this set weight.

The 5000 Calorie Experiment

In a modern twist to the classic overeating experiments, Sam Feltham decided to perform an absolutely fantastic experiment on himself. He would eat 5,794 calories per day of a low carbohydrate, high fat diet (LCHF) for 21 days with a macronutrient breakdown of Carbohydrate 10%, Fat 53%, and Protein 37%. According to standard calorie calculations, he should have gained 7.3kg of fat (yellow line in graph above). Actual weight gain, however, was only 1.3kg. What is more interesting is that his waist circumference dropped 3 cm, meaning that even as he gained some weight, much of that is lean mass.

An excess of calories alone is simply not sufficient to produce fat gain. Things are far, far, far more nuanced than a simple scale of Calories In vs Calories Out.

The Carb Challenge

Sam then switched his diet to a standard ‘American Diabetes Association’ style ‘balanced’ diet in the absolutely brilliant 21-day, 5,000 calorie Carb Challenge. Over 21 days, he would eat 5,793 calories/day of ‘fake’ foods – mostly refined grains and sugars. The macronutrient breakdown is 64% carbs, 22% fat, 14% protein. Interestingly this is not too much different than what most nutritional ‘authorities’ recommend for us. So, for the same amount of calories, would he gain or lose weight?

Following standard dietary recommendations, his weight gain almost exactly mirrors that predicted by the calorie formula – 7.1kg (15.6 lb) weight gain. Waist circumference increased 9.25 cm indicating that he is indeed gaining fat. For the same amount of total caloric increase, one diet – a low carb, high fat, (LCHF) natural foods diet – produced fat loss (waist size decrease). On the other hand, a standard high carb American Heart Association style diet produced a 15 pound weight gain!

The Body Set Weight

The only way we can successfully lose weight long-term is to lower the BSW – the body weight ‘thermostat’ instead of focusing only on calories. The BSW acts sort of like a body fat thermostat – acting to increase body fat if we lose weight, and decreasing body fat if we gain weight.

Consider an analogy. Suppose you set your thermostat in your house to 30C which is very hot and unbearable. In order to cool your house, you now bring in an air conditioner. This cools the house a bit, but then your thermostat turns on the furnace so that the A/C and the furnace are constantly fighting each other.

That is how we currently approach weight loss. We cut calories but we ignore the hormonally controlled BSW. You try to lose weight while your body tries to regain that weight. We get hungry and our metabolism starts to shut down.

Wouldn’t it be much, much easier to set the thermostat to 21C – a comfortable temperature, rather than constantly fighting with ourselves? The reason diets are so hard is that we are constantly fighting our own body. Understanding the hormonal underpinnings of weight gain will allow us to understand how to control the BSW and lose weight effectively.

For more, see The Obesity Code.

Learn more about Pique Fasting Tea.

For fasting education, support and community, go to The Fasting Method.

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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