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Eat Now, Fast Later

Part-time fasting is a new diet trend — but does it work?

Mini fasting

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Can fasting for weight loss and longevity be right for you?

En español | Could fasting for a couple of days each week help you improve your health and live longer? That's the idea behind the recent trend in the weight-loss world for dramatically limiting calories, but only for short periods of time.

Called mini-fasting or intermittent fasting, these diets — including Leangains, the 5:2 diet and Eat Stop Eat — recommend a variety of strategies, from fasting every other day to restricting calories just two days a week to refraining from eating at least 13 hours between dinner and the next day's meal.

Proponents of such regimens say these diets may improve sleep and blood sugar control; reduce risk factors for chronic diseases such as cancer, heart disease and diabetes; and may even help you live longer. The appeal is that unlike typical daily diets, part-time dieting plans allow you to eat freely for a few days a week so you don't feel as deprived.

Consider these options.

5:2 diet. This weekly plan has you eating just 500 to 600 calories on two nonconsecutive days and consuming a normal diet the rest of the week. Mark Mattson, chief of the Laboratory of Neurosciences at the National Institute on Aging, who has published two studies comparing the 5:2 diet with daily calorie restriction, found that overweight women on the 5:2 regimen tended to lose more belly fat and less muscle mass than those simply cutting their calories. However, they didn't lose any more weight than normal dieters. Women following the diet also showed greater improvements in blood sugar regulation.

Every-other-day diet. On this plan, "feast days," when you can eat whatever you want, alternate with "diet days," during which you eat a maximum of 500 calories. Choosing high-fiber and high-protein foods for your fast-day meals helps to stave off hunger, according to Krista Varady, associate professor of nutrition at the University of Illinois at Chicago, who has conducted several studies of alternate-day fasting. Like those on the 5:2 diet, people on the every-other-day plan lose a higher percentage of fat and lower percentage of muscle than those on typical calorie restriction diets, Varady says. They also show significant reductions in LDL (bad) cholesterol, blood pressure, insulin and triglycerides (blood fats).

Prolonged nighttime fasting. Also called time-restricted feeding, participants refrain from eating for at least 13 hours overnight. A 2016 study of more than 2,400 women, published in JAMA Oncology, found that prolonged nighttime fasting may have protected those with early stage breast cancer against recurrence. Women on this plan who fasted 13 or more hours per night not only reduced their risk of developing new tumors by 36 percent, they also slept longer and had improved blood sugar regulation.

Is part-time fasting for you?

A 2015 review of intermittent fasting studies concluded that there were no known risks for healthy individuals, though experts recommend against it for women who are pregnant, those with diabetes, those prone to hyperglycemia (high blood-sugar levels) or elderly individuals who are very frail."If this eating pattern helps you sleep better, and there are all these signals that it may be beneficial, there is likely no harm in trying it," says lead author Ruth Patterson, a professor at the University of California, San Diego's Moores Cancer Center.

But critics say it's too soon to recommend these regimens for their health benefits, as most of the findings to date come from animal and limited, short-term human studies. That may soon change: In a study of 200 people, Patterson and her colleagues have teamed up with the Salk Institute to evaluate how time-restricted eating affects metabolism and the gut microbiome and other markers of metabolic health. And encouraged by animal findings suggesting that these minifasts may protect the brain against cognitive decline, Mattson and colleagues are looking at the effects of intermittent fasting on the brain in a large-scale study of obese, prediabetic 55- to 70-year-olds.

Meanwhile, what if losing weight is your main goal? There's no evidence that intermittent fasting works better than calorie counting, but some people may find such regimens easier to stick to. "I'm not much for extreme methods, but if that's what people have to do to cut calories, it's great for them," says Marion Nestle, a professor of nutrition, food studies and public health at New York University. "The best way to lose weight is to eat less — and intermittent fasting is one way to do it."