Summer 2018

Fast Times

Are fasting diets effective and safe for losing weight?

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Photo: Ingimage

Sai Das, NG02, a scientist in the Energy Metabolism Laboratory at the Jean Mayer USDA Human Nutrition Research Center on Aging at Tufts and an assistant professor at the Friedman School, serves as our expert.

Q: Are fasting diets effective and safe for losing weight? Are they better than other diets?

A: Although traditional reduced-calorie diets are a science-based way to lose weight, intermittent fasting is a good alternative that studies suggest is just as beneficial.

With intermittent fasting diets, people aren’t necessarily avoiding all food for days on end. More often, they’re just significantly reducing their calorie intake or abstaining from eating for extended hours of the day or certain days of the week. They might restrict their eating to between noon and 8 p.m., for example, or on alternate days eat only half of their typical daily calorie needs, or choose two days a week when they only eat five hundred calories. In between those times, people eat as they normally would. I think one reason intermittent fasting is popular is that it gives people some structure to help them restrict calories, yet it doesn’t carry the mental strain of having to be on a diet every day.

Although some proponents of fasting claim it is better than a typical calorie-restricted diet at preserving muscle mass and reducing bad cholesterol and blood sugar, there isn’t enough data to confirm these claims.

Some people are better able to manage hunger with periods of fasting than others. So if the fasting regimen is still making your hunger worse after a couple weeks of adapting, it’s not right for you. Total calories still matter, so don’t take nonfasting days as an excuse to eat with abandon or double-up on desserts. And be sure to let your physician know if you start a fasting diet, particularly if you are prediabetic or diabetic, as fasting can affect blood glucose.

Send your questions for “Ask Tufts Nutrition” to Julie Flaherty, Tufts University Office of Publications, 80 George Street, Medford, MA 02155 or email

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