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Understanding Hunger — The Body Fat Thermostat Part 3

Hunger is a hormonal state — here’s the hormonal regulators

Dr. Jason Fung
7 min readFeb 1, 2024


In Part 1 of the Body Fat Thermostat, we discussed how body fat is highly regulated like a thermostat, and not merely left to the vagaries of how much we eat in a day. We considered the mechanisms for homeostasis, according to the Energy Balance Equation.

Body Fat = Calories In Minus Calories Out

Homeostasis of the Body Set Weight (BSW) means that if you lose weight, your body will try to regain that weight. In Part 2 of the Body Fat Thermostat, we discussed one mechanism- by reducing how many calories it burns (Calories Out). Eating fewer calories simply caused the body to burn fewer calories (mainly through the reduction of energy expenditure — body heat etc.) which meant that you lose a lot less weight than you expect.

In part 3, we’ll discuss how the BSW affects hunger. If you lose weight, your body will increase hunger to make you eat more, and regain that lost weight.

This understanding of the BSW is CRITICAL for weight loss, because it means that successful weight loss depends on adjusting the Fat Thermostat rather than adjusting Calories.

You Lose Weight, you get More Hungry

We’ve known for decades that losing weight makes you hungrier. It’s not some voodoo psychology, but the result of measurable, identifiable changes in the body’s hormones. The best example is the 2011 study published in the New England Journal of Medicine called “Long-Term Persistence of Hormonal Adaptations to Weight Loss”.

Weight Loss causes Hormonal Adaptations that cause regain

Patients lost weight on a 500 calorie/day diet consisting of 2 shakes and some low starch vegetables for 8 weeks with a target to lose 10% of body weight. After 10 weeks, those who lost the weight were monitored for another year while measuring the hormonal changes that occurred due to weight loss.

What happened? As predicted by the Body Set Weight, the hormones that increase hunger increased as you lost weight. This increased hunger would…



Dr. Jason Fung

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