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The OverFeeding Paradox — The Body Fat Thermostat Part 6

It’s all just Calories In Calories Out, right? The surprising answer.

Dr. Jason Fung
7 min readFeb 22, 2024


Our body controls the amount of body fat we carry like a thermostat (see Part 1). Obesity is the state of having the body fat thermostat set too high. They are the same thing. What causes obesity? This is the same question as — what controls the body fat thermostat?

Most people think, incorrectly, that the answer is ‘Too many calories’.

To this way of thinking, all calories, regardless of type, are equally fattening. 100 calories of broccoli are just as fattening as 100 calories of candy. Many ‘experts’ endorse this position — branding weight gain as a simple energy balancing.

Calories In. Calories Out. You’ve heard it a million times. Perhaps you’ve wondered why cutting calories works once in a million times.

Easy Access to Food causes obesity?

Yes, different types of calories (eg. carbs vs protein vs fat) evoke vastly different hormonal responses from our body, but this is considered irrelevant to this Calorie-centric way of thinking. Only the physical energy contained within is considered important.

We’ve all heard the argument that our bodies are engineered to gain weight and now that food is easily available, we are doomed to universal obesity. We’ll all end up like the movie Wall-E. This argument is completely incorrect. And there is a mountain of evidence to prove it.

We want to scientifically test the hypothesis that simply eating more calories leads to long term weight gain. This is equivalent to resetting the body fat thermostat upward. It’s a simple experiment, really. Get some people. Make them eat a lot of calories. See if they gain weight, and keep it on, and then struggle the rest of their lives to lose it. The scientific studies are almost unanimous.

The answer is no.

The Overfeeding Paradox

Dr. Ethan Sims was one of the pioneering researchers in obesity medicine. I’ve previously reviewed his key findings. Briefly, Dr. Sims…



Dr. Jason Fung

Nephrologist. New York Times best selling author. Interest in type 2 diabetes reversal and intermittent fasting. Founder