Short fasts for weight loss vs. traditional diets

In an effort to make losing weight—and keeping it off—easier, researchers are studying what happens to the body when people eat next to nothing every few days.

Dieting books in the U.K. and elsewhere have used these studies as a springboard to tout the benefits of intermittent calorie restriction, such as the 5:2 Diet, which suggests five normal eating days and two restricted ones.

Some research shows that this more radical-sounding approach may be a struggle at first but ends up being easier to stick with compared with the typical route of cutting calories each day. Some animal studies suggest it also offers other health benefits, including cognitive improvements.

Many questions remain. For one, it isn't clear whether the very-low-calorie element of the diet confers health benefits in humans, or if the diet simply helps people eat less and lose weight temporarily, like with daily calorie restriction. The effects on metabolism and long-term effects on nutrition and health haven't yet been studied in humans.

It also isn't known how much people need to hold back on their restriction days, or how many days a week to restrict is optimal. Changing eating and exercise routines typically leads to an average weight loss of about 5 percent of initial body weight, and usually only temporarily, studies have found.

Animal research by the government's National Institute on Aging has shown the strategy of alternating days of eating regularly, known as intermittent fasting, appears better at improving cognitive functioning and maintaining muscle mass. Animals following a more typical reduced-calorie diet did not fare as well.

Preliminary evidence in humans suggests that a similar pattern of intermittent calorie restriction appears to lead to weight loss in the short-term. Eating much less on some days and normally on others is as or more effective than reducing one's calories to between 1,200 and 1,500 calories daily, though continued research is needed, scientists say. (Men and women between 31 and 50 respectively need about 2,200 and 1,600 daily calories, according to the U.S. Department of Agriculture.)

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