10 Facts About Intermittent Fasting

This article is general information, not medical advice. Speak to your GP before trying intermittent fasting, especially if you have diabetes or another health condition, take regular medicine, or are pregnant or breastfeeding.

Intermittent fasting means switching between periods of eating and periods of eating very little or nothing at all. It has become one of the most popular ways of trying to lose weight. Here are 10 facts about intermittent fasting, including what the research says.

1It is about when you eat, as well as what you eat.

Most diets focus on which foods to eat. Intermittent fasting instead sets times or days when you eat and times when you do not, although what you eat during your eating periods still matters.

216:8 and 5:2 are the best-known types.

With 16:8, sometimes called time-restricted eating, you eat all your food within an eight-hour window each day and fast for the other 16 hours. With 5:2, you eat normally five days a week and cut right down, usually to around 500 to 600 calories, on two non-consecutive days.

3A BBC documentary made it famous.

The doctor and broadcaster Michael Mosley helped popularise the 5:2 diet with the 2012 BBC Horizon documentary Eat, Fast and Live Longer, followed by the bestselling book The Fast Diet, written with Mimi Spencer. Mosley died in June 2024 while on holiday in Greece.

4People have fasted for thousands of years.

Fasting is part of many religions, including during Ramadan in Islam, Lent in Christianity and Yom Kippur in Judaism. Fasting for health or weight loss is a much more recent idea.

5It mainly works by cutting calories.

Limiting when you eat tends to mean eating less overall. Trials comparing intermittent fasting with ordinary daily calorie cutting have generally found similar amounts of weight loss.

6Timing alone may not make much difference.

In a 2022 study published in the New England Journal of Medicine, people who ate within an eight-hour window and cut calories lost no more weight over a year than people who cut the same calories with no time limit.

7It suits some people better than others.

Some people find it easier to follow simple rules about when to eat than to count calories every day. Others find long gaps between meals lead to strong hunger, headaches, irritability or overeating later. The best diet is generally the one you can stick to.

8“Autophagy” is popular, but unproven as a benefit.

Fans of fasting often mention autophagy, the process cells use to recycle their own worn-out parts. Yoshinori Ohsumi won the 2016 Nobel Prize for discovering how autophagy works, but his research was in yeast, and there is little evidence yet about what intermittent fasting does to autophagy in people.

9It is not safe for everyone.

Intermittent fasting is not recommended for children and teenagers, pregnant or breastfeeding women, or people who have had an eating disorder. People with diabetes, especially those who take insulin or other medicines that lower blood sugar, should talk to their doctor first because fasting can make blood sugar fall too low.

10You can usually drink during a fast.

Water, and usually black tea or coffee without sugar, are allowed during fasting periods in most intermittent fasting plans. Drinking enough water helps prevent dehydration and headaches.

FAQs about intermittent fasting

What is the most popular type of intermittent fasting?

The 16:8 method, where you eat within an eight-hour window, for example between noon and 8pm, and the 5:2 diet are the most widely used.

Is intermittent fasting better than other diets?

Research so far suggests it leads to about the same weight loss as cutting calories in other ways. It may suit people who prefer a simple routine.

Is intermittent fasting safe?

It is generally considered safe for healthy adults, but it is not suitable for everyone. Speak to your GP first if you have a health condition or take medicines.

Sources and further reading

• New England Journal of Medicine: calorie restriction with or without time-restricted eating (2022)
• British Dietetic Association: intermittent fasting
• Nobel Prize in Physiology or Medicine 2016

Explore more: facts about weight loss and breakfast foods that can help with weight loss.

Do you know any fun facts about intermittent fasting? Share them in the comments below!