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The Body’s Fat Thermostat — Part 2 — The Science

Eating Less Lowers Fat Burning

Dr. Jason Fung
8 min readJan 25, 2024


My last article, The Body’s Fat Thermostat — Part 1, discussed the concept of homeostasis and how body fat is regulated like a thermostat rather than simply the end product of Calories In — Calories Out. The level set by the thermostat is determined by multiple hormones (like most everything else in our body), rather than the number of calories we eat. It is true, however, that the types of food and the amount of food will influence those hormones, but different foods, even with the same calories, have different hormonal effects. This is why calorie counting is such an unsuccessful weight loss strategy.

Is there scientific evidence to show the existence of this Body Set Weight (BSW)? Totally. Virtually every study done over the last 50 years shows this. It’s rather stunning that so very few people discuss the mechanisms of this fat thermostat because it has massive implications for weight loss. Massive.

How the Body Fat Thermostat Works

The Body Set Weight (BSW) is like a thermostat. Our body sets a certain level of body fat and then defends it according to the energy balance equation Body Fat = Calories In — Calories Out, which is always true.

If Body Fat goes too high, then our body brings it back into range by activating multiple mechanisms to:

· reduce appetite (so that we eat less)

· increase energy expenditure (so that we burn off some of the stored calories (body fat)

If Body Fat goes lower than the BSW (as we lose weight), then our body will also bring it back to its original state by activating mechanisms to

· Increase appetite (so that we eat more)

· Reduce energy expenditure (so that we store more calories (body fat))

This is CRITICAL for weight loss because it means that successful weight loss depends on fixing the Body Fat Thermostat that is set too high, not reducing Calories (Eat Less) because that will simply cause the body to increase appetite and reduce metabolic rate.

We will look at the evidence and mechanisms that hunger is adjusted in our next article. Today we will…



Dr. Jason Fung

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