Skip to main content

What is intermittent fasting — and is it safe?

A guide to intermittent fasting
(Image credit: Shutterstock)

Intermittent fasting is an eating plan that is focused not on how much you eat or what kind of food you eat — as is the case with most diets — but on when you eat. An intermittent-fasting plan involves going without food for a set number of hours each day, or greatly reducing what you eat on one or two days a week. It is one of the most popular diets in the US, though it’s worth noting that some proponents consider it an eating pattern, or even a lifestyle, rather than a diet.

Disclaimer: It's a good idea to check with your doctor or dietician before trying intermittent fasting or dramatically changing your diet. 

What are the benefits of intermittent fasting?

A study from the American Heart Association (opens in new tab) found “There is evidence that both alternate-day fasting and periodic fasting may be effective for weight loss, although there are no data that indicate whether the weight loss can be sustained long term”. Examining earlier research into alternate-day fasting and periodic-fasting regimes, the AHA noted that “body weight decreased significantly in all studies by 3% to 8% after 3 to 24 weeks of treatment”. 

An overview of studies by Harvard T.H. Chan School of Public Health (opens in new tab) found intermittent fasting was effective for weight loss but no more so than diets that involve continuous calorie restriction.

Other, smaller studies (opens in new tab) — many involving animals — suggest intermittent fasting can help to lower blood sugar, slow the development of some cancers, improve brain health and add years to your life. More research on humans is needed to asses if these wider benefits of intermittent fasting can be maintained in the long term.

How does intermittent fasting work?

Mark Mattson is a Johns Hopkins neuroscientist who has studied intermittent fasting for more than 25 years, and is the author of The Intermittent Fasting Revolution. He has said: “If someone is eating three meals a day, plus snacks, and they’re not exercising, then every time they eat, they’re running on those calories and not burning their fat stores.” Intermittent fasting breaks this cycle.

After several hours without food, your body runs out of glycogen (from carbs) to burn and begins to burn fat, instead. Mattson calls this “metabolic switching”. Intermittent fasting means this happens over the long term and your body also begins to find it easier to access your fat stores for energy. This is important because there are nine calories in a gram of fat (opens in new tab) and only four calories in a gram of carbohydrate, the body’s go-to source of energy.

How do I do it? 

Eat nothing for a specified period (see below), eat sensibly when you are not fasting, and allow yourself the very occasional treat (denying yourself entirely may only worsen your cravings (opens in new tab)). 

You can eat and drink as normal on non-fasting days, but try to ensure you get plenty of nutritious foods, such as fruit, vegetables, whole grains, healthy fats, and good sources of protein. Highly processed foods are often packed with sugar and salt, and won’t fill you up for long, which will make it far more difficult to stay on the straight and narrow during your fasting period.

With most plans, you can drink only water, and black coffee or tea, on fasting days. Aside from ensuring you stay hydrated, water can take the edge off your hunger and caffeine (opens in new tab) can help suppress your appetite, at least for a short time.

Try a plan for a few days and see how you feel, but bear in mind it may take weeks for your body to adapt to the new nutrition rules. If you experience any unpleasant side effects — hunger and irritability don’t count — consult your doctor. 

If you are the kind of person who thinks they can cram in as many calories as possible to offset the fasting period, look elsewhere for a weight-loss program. 

Is it safe?

Intermittent fasting is not inherently risky, as long as you are generally in good health, but talk to your doctor first. 

If you have diabetes, are pregnant or breastfeeding, or have special dietary needs, intermittent fasting is not for you. It is not advised for people who are prone to eating disorders and adolescents should avoid it, too, as they are in an active growth stage.

Is it for me? 

Some people find it easier to maintain than a calorie-controlled diet. It is more convenient, as you don’t have to think so much about meal portions, and your prep and cooking time will be reduced just by following the plan. Some research (opens in new tab) has shown that while intermittent fasting can benefit people who are overweight or obese, it may lead to mood changes, fatigue, tension and overeating on non-fasting days among people who are of normal weight.

The appeal for some is that intermittent fasting can become more a way of a life than a diet that has to be observed, and so it helps them maintain their healthy weight in the long term. On the other hand, you may find it too difficult to go regularly without food and it’s easy to cheat. In one study (opens in new tab), participants consumed 14% more food than usual when they knew their calorie intake was going to be severely restricted the following day.

Fasting plans 

This is not an exhaustive list but here are some of the most popular intermittent-fasting plans:

Fast for 12 hours
This is probably the easiest to maintain if you include your seven or eight hours of sleep in the fasting period, such as between 6 pm and 6 am.

16 hours

You restrict your eating to one eight-hour period in the day. Many people choose 7 am-3 pm or noon to 8 pm as their windows, but experiment to see what works best for you.

Eat Stop Eat

This version was developed by Canadian Brad Pilon when he was a graduate student. It involves fasting for 24 hours (over two days – noon to noon, for example) once or twice a week. A key part of this plan (opens in new tab) is that you eat at some point every day. He also says some resistance training is needed if any such plan is to work. “You must challenge your muscles at least twice a week,” he says on his website (opens in new tab)

5:2

This one involves restricting yourself to 500 calories (women) or 600 calories (men) two days a week. You choose which two, but it’s best to leave at least one day between fasting days, to ensure you keep up your energy levels and don’t get an insatiable desire for sugary, fatty fast food. Michael Mosley, the man behind 5:2, has now evolved the plan: The New 5:2 (opens in new tab) limits your daily intake to 800 calories twice a week, and a low-carb Mediterranean (opens in new tab)-style diet at all other times.

Whichever one you choose, be sensible. It may work for you, or it may be something you cannot maintain. Whichever plan you choose, you will need to eat well and get as much exercise as you can to get the greatest benefits.

John is a writer and editor based in London. He was worked for magazines such as Runner’s World, Men’s Health, Women’s Health and Cosmopolitan. A keen runner, what he lacks in ability he makes up for with enthusiasm and excuses.