Are Fasting Diets Safe?

Intermittent Fasting Diets — Are They Safe?

Even if you haven't heard about the fasting diet yet, chances are someone you know is doing it now. Like the recent French Dukan Diet craze, intermittent fasting — also called the Fast Diet or the 5:2 Diet, among other names — seems to be the next overseas diet fad.

What It Is
The fasting diet was first popularized in the UK by physician and BBC journalist Michael Mosley's documentary Eat, Fast, and Live Longer. Each diet varies slightly, but in general the concept involves eating a low-calorie diet one or two days a week, and not restricting calories for the rest of the week. The 5:2 Diet, for example, recommends eating around 500 calories a day (600 calories for men) on two nonconsecutive "fast" days a week; for the rest of the time, you eat as if you are not on a diet (called "feast" days). The diet's flexibility is one major reason why people are flocking to the fad diet. No need to cancel those dinner plans — just rearrange your schedule so you are feasting on your nonrestrictive day. The relative lack of dietary restriction can also help you stick with it, even when your stomach is grumbling; after all, you know a feast day is just around the corner tomorrow! But is fasting a day or two a week actually safe?

The Science
In general, intermittent fasting diet claims, do, in fact, have a basis in science. As Mosley's documentary touches on, quite a few studies have found that short-term fasting can lead to many benefits, like lowering diabetes and other disease risk, extending life spans, and improving memory, among other health benefits. While researchers haven't pinpointed the exact reason why intermittent fasting leads to health benefits, many believe that it's because fasting puts cells under mild stress, causing them to adapt, cope, and therefore resist disease.

As for weight loss, studies are limited but have shown that fasting can lead to weight loss. A recent small study in obese women showed that fasting one day a week along with a calorie-restricted diet (between 880 and 1,080 calories a day) for the rest of the days was an effective weight-loss regimen in obese women. While this diet may be more extreme than mainstream fasting diets, the act of eating fewer total calories per week can help you shed pounds; if you're only eating 500 calories for two days a week, then you could be saving almost a pound's worth of calories every week. The other way that fasting can help you lose weight is merely by affecting the foods you choose on your fast days. After all, when you only have 500 calories to last you through the day, you'd be foolish to waste them on a candy bar binge. Instead, fasting diet books tell you to focus on eating nutrient-rich meals that still help you fulfill what your body needs. You'll feel happier and fuller eating small meals made of low-calorie vegetables and lean proteins, and staying away from empty-calorie foods is one weight-loss strategy that everyone knows works.

Too good to be true? Read on for potential risks of intermittent fasting diets.

The Risks
Experts have highlighted certain risks from intermittent fasting, notably an increased risk of binge eating on "feast" days as well as other disordered eating habits (like obsessing over calories to an unhealthful extent). And even if you are losing weight, it could just be muscle mass, not fat; a study on body mass and fasting in people who observe Ramadan found that while the month of intermittent fasting led to weight loss in most subjects, women age 36 and older in the study did not experience significant fat loss at all. Another risk is that while many studies have been done, they've mostly been in animals; researchers say longer, more well-controlled studies need to be done in humans to see if intermittent fasting is a safe, long-term option.

Some experts also caution that this latest diet trend is just another — and possibly harder — alternative to the age-old, sensible advice to exercise and to control portions. "If you [don't] eat two days a week, and limit what you eat the other five days, you will lose weight. It's one approach to losing weight. I'm not sure it works any better than cutting down slightly seven days a week," says Dr. Stephen Freeland, who researches intermittent fasting at Duke University Medical Center, in the Canadian Medical Association Journal. Others caution that diet books make it seem like you can eat whatever you want on feast days, which actually could negate any benefits you see from restricting calories on other days.

Even so, the studies linking intermittent fasting and calorie restriction to long-term health benefits are promising, and many people have testified about the diet's success. If you're thinking of trying a fasting diet, then make sure you talk to your doctor to learn more about the pros and cons and see if it's right for you. If you've already tried it, then tell us what you think about the latest fad diet!

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