Is fasting good for you?

Is fasting good for you? We examine the research and speak to those in the know to find out

Is fasting good for you? Image shows plate with food
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With so many diets out there, you’d be forgiven for wondering, is fasting good for you? Some people are really enthusiastic about fasting diets, claiming they have changed their lives and helped them shed serious weight, but are they safe?

Intermittent fasting is a method of calorie restriction that requires you to go for extended periods of time without eating. There are different types of fasting programs - some recommend fasting for a few hours or a specific chunk of each day, some require fasting for a few days each week, and some involve fasting for several days each month.

Intermittent fasting has gained momentum because of a widespread belief that the regime can help you improve your fitness level, boost longevity, and get faster and longer-lasting weight loss results when used alongside the best exercise machines to lose weight.

Before we delve into everything you need to know about fasting, if you’re considering any major change to your diet, it’s important to check with your doctor before starting intermittent fasting, particularly if you have any pre-existing medical conditions.

What is fasting?

Registered dietician Beth Kennedy explains, “Fasting involves abstinence from all or some food and drink for a period of time. Despite several ‘modern’ fasting diets, the practice of fasting dates back centuries. Many diets focus on what to eat but fasting is all about when you eat.”

With intermittent fasting, you only eat during a specific time. Proponents of the diet say fasting for a certain number of hours each day or eating just one meal a couple of days a week can help your body burn fat.

Kennedy says how you choose to fast is completely up to you, “You can pick a daily approach, which restricts daily eating to one six- to eight-hour period each day. For instance, you may choose to try 16:8 fasting: eating for eight hours and fasting for 16. Another approach known as the 5:2 diet involves eating normally for five days of the week. For the other two days, you limit yourself to 500–600 calories.”

Longer fasts are not necessarily better for you and can be dangerous and counterproductive. Kennedy says, “Going too long without eating might kick your body into starvation mode and actually encourage your body to start storing more fat in response.”

Is fasting good for you? Image shows person holding a glass of water and a bowl of food

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What are the benefits of fasting?

Many dieters see fasting as a manageable approach to weight loss because it allows them to continue eating the foods they love such as on a keto diet. Other programs may require avoiding food groups altogether. While that is sometimes tolerable in the short term, many people have a hard time giving up familiar foods in the long term.

Studies show that controlling the times we eat or undertaking short-term fasts can aid weight reduction and fat loss. That’s not all. Other studies have shown fasting to increase the ability to switch metabolism to fat burning, preserve muscle mass and improve body composition in overweight people.

Some research has found that fasting can help decrease levels of inflammation and help promote better health. For example, one study of 50 healthy adults showed that intermittent fasting for one month significantly decreased levels of inflammatory markers. Another smaller study saw similar results when people fasted for 12 hours a day for one month.

Some research has supported the idea that fasting could enhance weight loss. For example, a review found that intermittent fasting over 3–12 weeks was as effective in inducing weight loss as continuous calorie restriction and decreased body weight and fat mass by up to 8% and 16%, respectively. In addition, fasting was found to be more effective than calorie restriction at increasing fat loss while simultaneously preserving muscle tissue.

Several studies support fasting as a means of improving blood sugar control and potentially reducing the risk of diabetes. 

What are the downsides to fasting?

Despite the long list of possible health benefits associated with fasting, it may not be right for everyone.

Nutritionist Aimee Brady explains who shouldn’t try fasting, “Fasting is not generally recommended without medical supervision for older adults, adolescents, or people who are underweight. There’s also concern that fasting for long periods may lead to disordered eating. The feast-or-famine approach to eating and starving yourself for limited periods may lead to overeating or binge-eating at other times.”

Intermittent fasting has been studied extensively for its potential weight loss benefits. However, many of the earlier studies were performed on rodents, with more studies needed on humans. As a result, scientists don’t really know the long-term impact of fasting, or if it is a safe or effective treatment of obesity and other conditions.

So far, the research studies evaluating intermittent fasting have been relatively short and have enrolled only a limited number of participants. A randomized control trial published in 2017 assigned 100 overweight people to one of three eating plans: a calorie-restricted diet, fasting on alternate days, and continuing with normal eating habits. At the end of the 12-month study, both diet groups had lost weight compared with the normal eaters. However, there wasn’t a difference between the fasters or the conventional calorie cutters.

The U.S. Department of Agriculture provides guidelines for the daily intake of certain foods (such as fruits and vegetables) and important nutrients (such as fiber, protein, and fat). However, certain types of intermittent fasting require that you avoid food except for water and clear liquids for prolonged periods. It would, therefore, be impossible to meet your nutritional guidelines on those days. Other variations of intermittent fasting would allow you to meet your nutritional needs, but only if you were very careful in your food choices.

Is fasting good for you? Image shows person looking in fridge

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Brady said that fasting diets can encourage unhealthy habits. “Someone following an intermittent fasting plan might meet their nutritional needs, but because these diets don’t focus on what you eat, but rather when you eat, there’s no encouragement to follow healthy eating guidelines.”

“Fasting may also lead to social isolation. If you’re not eating at the same time as your friends, family, or co-workers then you may feel left out of social occasions involving food.”

If you’re considering changing your diet, it’s always best to consult your doctor or a medical professional.

If you’re looking for other ways to up your fitness, don’t miss our feature on how to tone your stomach.

Catherine Renton

Catherine is a freelance journalist writing across titles such as Verywell Health, Healthline, The Daily Telegraph, Refinery29, Elle, and Vogue. She specializes in content covering health, fitness, wellness, and culture. A once reluctant runner, Catherine has competed in 30 running events in the past five years and looks forward to one day running the London Marathon.