The science of KETO

Dr Michael Mosley is constantly searching for ways to improve our health. His new book combines intermittent fasting with a keto way of eating.

- Dr Michael Mosley

The Fast 800 Keto is a keto-based diet (the clue is in the name) but what exactly does that mean? Well, let's start with where you get your day-to-day energy from. Your body works like a hybrid car, able to run on two different fuels: sugar (glucose) and fat. Although protein can be converted into sugar and used as fuel, this is not your body's main priority. It prefers to use the protein in your diet to make hormones, muscles and other essential components to ensure good functioning. Your body's main go-to fuel is glucose. Glucose is easy for your body to access and provides almost instant energy. If you have to escape from danger, such as an attack by a lion (not very likely these days, but it would have been a bigger problem for our remote ancestors living and foraging on the plains of Africa), then your body will release glucose into your blood so you can run as far and as fast as your legs will carry you.

That said, your sugar stores are relatively limited. You have just 4g (a teaspoonful) of glucose circulating in your blood at any one time.

This amounts to only about 16 calories. And, since most of us need around 2000 calories a day to sustain us, that won't last long.

Fortunately, you have another 500g of sugar, stored as glycogen, in your liver and muscles. That represents about 2000 calories, so these glycogen stores could keep you going for a while, unless you are exercising heavily. In practice, though, you are unlikely to seriously deplete your glycogen stores because every few hours you probably eat something sugary or with carbs in it, which tops them up.