Fasting and exercise: should I work out on an empty stomach?

We all know that our body works like a machine; it needs fuel to function! But what happens when we fast for long periods of time and then we exercise? Our Nutrition team breaks down everything you need to know regarding this popular topic.

How does our body obtain energy when fasting and immediately after a meal?

Glycogen is the main storage carbohydrate of our body and is found in the liver and muscles. During fasting, not enough glucose is circulated in the blood, so our insulin levels remain low and our stored glycogen in our liver starts acting as the fuel provider. As a result, liver glycogen stores are reduced to maintain circulating glucose levels. Muscle glycogen is used only as a source of energy for our muscles and can not be broken down into glucose and enter the bloodstream.

In contrast, when we consume food, glucose levels rise sharply, insulin is secreted by the pancreas and then returns to normal levels after some time. Carbohydrates replenish glycogen stores, and when there is an excess of them, the process of lipogenesis (synthesis of fat tissue) begins. 

Fasting and exercise

When we are fasting, the body releases glycogen which is converted to glucose to maintain stable blood sugar levels. Once glycogen levels are starting to be depleted, the stored fat is converted into fuel, creating ketones. Thus, in post-fasting training, the body relies on burning fat to obtain energy if there is no other available source of energy i.e. glucose or glycogen 1. However, for some, this can have negative effects, as they will feel intense hunger, which will prevent them from concentrating, exercising for a long time, and performing to the maximum.

What are the benefits of fasting prior to exercise?

Exercise on an empty stomach seems to have beneficial results in weight loss, when combined with a calorie deficit diet. The same has been suggested for improving insulin sensitivity and glycaemic control, which could be of particular clinical benefit to people with type 2 diabetes 2.

Who is it for?

Exercising on an empty stomach is more suitable for people who exercise early in the morning or for those who can not eat breakfast when they wake up. However, it is contraindicated for people suffering from chronic diseases, children, pregnant women, nursing mothers and the elderly. Fasting before exercise is not for everyone, and should be done after discussion with your doctor.

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