Dr. Jason Fung

If you're a predator and you're morbidly obese, you will never catch anything ever again, right?

You're just too slow.

If you're like a rabbit and you're morbidly obese, you will die because something is going to eat you.

If you have too little body fat, you will die because you won't survive the winter.

So how much body fat you carry is clearly dependent on these homeostatic mechanisms, which is critical because if your body fat is set at a certain temperature, you need to know why it's up or down.

Just like if you walk into a room and it's really, really hot, you don't say, oh, let me calculate all the sources of heat in minus heat out, right?

You say, why did somebody set the thermostat so high?

Same thing with your body fat thermostat.

If you're overweight, you need to know why is my body fat thermostat set this high?

Because if you lose weight, your body fat thermostat set really high, you lose weight.

The problem is your thermostat's up here.

So it's going to naturally defend that higher weight.

It's going to push you up.

How?

It's going to increase hunger.

That's the mechanism.

If you still don't eat, it will reduce your metabolic rate until you gain that weight back, right?

So what's important is to control the thermostat.

Some hormones push the thermostat up.

Some hormones push it down.