Intermittent Fasting for Weight Loss: What to Know

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Maybe you’ve tried every diet in the book: low-carb, low-fat, vegetarian, South Beach. Maybe you’ve simply read about some of the benefits of fasting and want to give it a shot. After all, Plato and Hippocrates fasted, and many religions say fasting is good for the soul.

There are plenty of versions of intermittent fasting for weight loss—the 5:2 diet, the 16/8 method, alternate-day fasting—but they all involve the same concept: reducing the amount of food you eat or stopping eating entirely over several hours up to a couple of days. But is intermittent fasting for weight loss effective, and—more importantly—is it healthy?

The Potential Benefits of Intermittent Fasting for Weight Loss

Proponents of intermittent fasting for weight loss say it’s not crash dieting, since you’re not cutting back drastically on calories for weeks on end—you’re just cutting calories for a day or two. This means your body won’t go into “starvation mode,” where your metabolism slows down because your body is trying to conserve calories. Instead, the brief fast makes your metabolism more efficient.

Like a low-carb diet, intermittent fasting initiates a process called ketosis, where the body burns fat instead of glucose for fuel, experts say. Fasting may also lower the hormones insulin and insulin-like growth factor (IGF-1). Researchers think this decrease may slow cell growth, and thereby the aging process and disease risk. Lastly, similar to exercise, fasting puts your body through temporary stress. Over time, this makes your cells better able to cope with stress, in theory protecting them against chronic disease.

Preliminary research has hinted at potential benefits to these fasting diets:

  • The 16/18 diet: With this fast, you eat all your calories for the day in a 6- to 8-hour time window and fast for the other 16 to 18 hours. It has been found to lower cancer risk and help maintain weight in both human and animal studies.

  • The 5:2 diet: Also known as the fast diet, this plan has you eat whatever you want (no calorie limit) for five days, then fast for two days. During fast days, women eat 500 calories and men 600. One study found that participants on the 5:2 diet lost 14 pounds more over a six-month period than those on a simple low-calorie diet. People on the 5:2 diet also maintained more muscle and had healthier blood sugar levels. Other research has found the 5:2 diet may reduce risk for some cancers linked to obesity, like breast cancer.

  • Alternate-day fasting: With this approach, you fast (eat around 500 to 600 calories) every other day, then eat whatever you want on non-fasting days. One 10-week trial of alternate-day fasting found that the diet helped people lose 13 pounds on average and lowered their LDL cholesterol, blood pressure, and insulin levels.

Several studies have found intermittent fasting may help reduce the risk of some chronic diseases at the same time it helps you to lose weight.

Potential Problems With Intermittent Fasting for Weight Loss

There’s not enough science yet to say for sure if intermittent fasting works. Most studies have been done in animals—and research in animals does not always (or even often) result in the same outcomes in humans.

In the U.K., the National Health Service has warned that much more research needs to be done before intermittent dieting can be approved for everyone. They say research has not yet proven what type of fasting is the most effective and safest in the short- or long-term, how many calories people should eat on fasting days, and how sustainable intermittent fasting is over time. They also note some people who have tried intermittent fasting have reported difficulty sleeping, irritability, anxiety, dehydration and bad breath (a side effect of a low-carb diet).

On days when you’re fasting, you’re likely to have less energy. That can mean you’ll move around less and, if you’re trying to get into a regular exercise routine, may not have the energy to work out. Some health experts have voiced concern that intermittent fasting encourages people on non-fast days to binge on unhealthy foods—fries, burgers and cake—rather than maintain a steady balanced diet.

Other experts say fasting is too unrealistic for most people and may even slow down your metabolism. And if you drop the diet and haven’t changed your eating habits for the better, you’re liable to gain back whatever weight you lost (and then some).

Bottom line: Fasting is not right for everyone. People who are underweight, have an eating disorder, have a serious chronic health condition (including diabetes), are pregnant, are sick, or are recovering from surgery should not try intermittent fasting. Talk to your doctor before starting any new diet or exercise program—especially one that involves skipping meals. Together, you and your doctor can discuss your goals and find a weight loss plan that’s healthy and effective for you.

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Medical Reviewer: William C. Lloyd III, MD, FACS
Last Review Date: 2021 May 8
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