Dr. David Sinclair

It costs a lot of energy and having to remake body parts is energy expensive.

And our body tries to conserve energy as much as possible.

When you're fasting, what it needs to do is to use your body as fuel.

So it'll start breaking down proteins for fuel that you need.

So first of all, what will happen is in the first few hours, you use glycogen from your liver.

Your liver makes glucose.

You'll feel a little bit hungry, but you'll eventually be fine.

Then once you run out of glycogen, then you're going to start breaking down fat and making ketones.

That's when you start to get a bit of bad breath from that.

But you feel great.

When you're in between about 15 hours and 24 hours, that's when you get a lot of ketones.

And your brain uses those for fuel.

So you'll have a sharp mind.

You can remember things.

You can focus on work if you ever get there.

Beyond that, you need to break down fat, and that is when your body is starting to do that.

But ultimately what happens after three days is your body says, hey, I'm going to start breaking down protein as well.

And I wouldn't do that often because I don't want to break down a lot of protein, but your body will start to turn over old proteins preferentially.

And a little bit of that, that's why I do it maybe once a month,

has been shown, at least in animals, to be not just healthy, but life-extending.