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introduction

Fasting is one of the oldest remedies in history, but it is not to be

confused with starvation, which is a notably unhealthy state. Starvation

is involuntary abstinence from food; it is neither deliberate nor con-

trolled. If you have not eaten for a while, and have no idea when you will

eat again, you are starving. By contrast, fasting is voluntary abstinence

from food for spiritual, health, or other reasons. You may fast as long as

you like, but you can always decide to eat again, if you like.

People often worry that if they don’t eat, they’ll have less energy

and they won’t be able to concentrate as well, but that’s simply not true.

Think about the last time you ate a huge meal—for example, at Thanks-

giving. Did you feel more energetic and mentally alert afterward? Or did

you feel sleepy and a little dopey? More likely the latter. Eating shunts

blood to your digestive system to cope with the huge influx of food, leav-

ing less blood for brain function. Fasting does the opposite, meaning

there’s more blood for your brain. The human body has adapted to func-

tion and thrive in the temporary absence of food.

Glucose and fat are our main sources of energy. When glucose is not

available, the body adjusts by using fat. Fat is simply our stored food

energy. That’s what it’s designed for. In times of food scarcity, stored food

(fat) is naturally released to fuel our bodies. That’s entirely normal. The

transition from the fed state to the fasted state occurs in several stages:

1. Feeding: During meals, insulin levels go up, allowing glucose uptake

by tissues such as the muscles or brain for direct use as energy. Excess

glucose is stored as glycogen in the liver.

2. The post-absorptive phase (six to twenty-four hours after fasting

starts)

: Insulin levels fall. The breakdown of liver glycogen releases

glucose for energy. Glycogen stores last for roughly twenty-four hours.

3. Gluconeogenesis (twenty-four hours to two days): The liver manu-

factures new glucose from amino acids and glycerol. In people who

do not have diabetes, glucose levels fall but stay within the normal

range.