What happens in our bodies when we fast?

Food provides the cells in our bodies with their fuel: glucose. Our bodies release a certain amount of glucose into the blood and store the rest as glycogen, releasing it as needed. Once that supply is used up – after at least 12 hours without food – our fat stores are called upon.


Burning fats rather than glucose produces substances called ketones, and high levels of these can suppress hunger (which may explain why many fasters claim to be less hungry after several days of fasting). Researchers are looking at the effects of this metabolic state on the body. For instance, research dietician Dr Michelle Harvie at the University of Manchester has found that, by dampening down levels of certain hormones, fasting could help reduce the risk of breast cancer.

Is fasting an effective way to lose weight?

Fasting forces the body to start burning fat. So on a basic level, restricting your food intake can be an effective weight loss technique – but scientists are divided on which approach gives dieters the most health benefits. Intermittent fasting, such as the ‘5:2 diet’, recommends two consecutive, low-carbohydrate, 500-800 calorie days to safely lose weight.

However, you need longer periods (three to four days without carbohydrates) to put your body into ketosis, the state where your appetite will start to decrease. Experts don’t recommend trying this without medical backing, cautioning that the long-term consequences of high-intensity fasting are not fully understood.

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Are there any other unexpected health benefits to fasting?

Fasting seems to benefit the mind. Neuroscientist Dr Mark Mattson has shown that mice on calorie-restricted diets are sharper than their better-fed friends when it comes to memory tests, and in 2016, his work with humans suggested that fasting could help protect the brain from the amyloid proteins that build up during Alzheimer’s disease. Meanwhile, Prof Tim Spector at King’s College London has noticed that fasting also affects our gut bacteria: several species of bacteria found in people with good health appear in higher levels after a fast. So occasionally skipping breakfast might be enough of a fast to benefit your microbiome.

Marnie Chesterton is the presenter of Is Fasting Healthy?, an episode of CrowdScience. You can tune in to CrowdScience every Friday evening on BBC World Service, or catch up online at bbcworldservice.com/crowdscience

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