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Willpower is NOT the Problem— The Body’s Fat Thermostat Part 4

“Obesity is a disorder which, like venereal disease, is blamed upon the patient” Astwood 1962

Dr. Jason Fung
6 min readFeb 7, 2024


The energy balance equation, which is always true says

If you don’t think about it too hard, it seems that to lose body fat, you only need to eat fewer calories or exercise more. That’s only been tried about a few million times and fails just about always.

The reason is that Body Fat is regulated like a thermostat — as we discussed in Part 1. If the fat thermostat is set too high, then your body will change the hormonal profile to either reduce energy expenditure (Calories Out — see Part 2) or increase hunger (Calories In — see part 3).

Thinking deeper upon the problem of excess calories, ask not “IF Calories In> Calories Out” but “WHY are Calories In> Calories Out?” The simplistic answer is that people choose to eat more or exercise less. But that’s completely wrong. Nobody who is obese chooses to eat more.

People eat more because they are hungry.

Hunger, which is a hormonal state and not a choice, drives eating behavior (as we discussed here in my article on What Ozempic teaches us about Weight Loss). If your hormones increase hunger, you’ll eat more. You can choose what you eat, but you can’t choose to be less hungry. That is, hunger is the root cause of the increase consumption, so merely telling somebody to eat less is not effective if you are not controlling the hunger.

Hunger drives eating behavior which drives Calories In

Drug Induced Weight Loss and Appetite

Decades of research shows that the body fights against weight loss below the Body Set Weight (BSW) by making you eat more and this is even true if you use drugs to lose weight. In the study “How strongly does appetite counter weight loss? Quantification of the feedback control of human energy intake”, researchers looked at type 2 diabetes patients taking the drug canaglifozin, which makes you pee out glucose and reliably causes a small amount of weight loss. At a dose of 300mg/ day…



Dr. Jason Fung

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