For years, we have been told that three square meals a day are essential to health. But popular dieting advice suggests that eating more often in small amounts may be a better approach. Some new diets advocate going the other way: consuming only one small meal a couple of days a week, followed by days of unrestricted eating.

These diets claim to help people manage the tricky business of taking in fewer calories. What does science say about their effectiveness?

Let us start with the idea that lots of little meals are better than three squares. While numerous studies have indicated a link between a snacking diet (four to six small meals or adding healthful snacks to the three squares) and a healthy weight, the research remains inconclusive.

A classic study published in the New England Journal of Medicine in 1989 compared “nibbling” — 17 snacks a day — to a traditional three meals that were equivalent in calories. Seven men ate each way for two weeks, and researchers found lower cholesterol and insulin levels with the nibbling diet — factors that lower risk for heart disease and metabolic diseases such as diabetes. And in large surveys of how people eat, researchers have found lower body weights in people who report eating more often than three times a day.

While this might suggest that nibbling (within reason) is good, surveys can find only associations, not cause and effect. In addition, though a recent review of clinical studies found that frequent meals make people feel less hungry, this eating pattern does not necessarily correlate with weight loss. That may seem confusing, but it is the current thinking, according to Heather Leidy, an assistant professor of nutrition and exercise physiology at the University of Missouri in Columbia who co-authored the review.

At the Johns Hopkins Weight Management Center in Baltimore, overweight people seeking to shed weight are generally advised to spread out small meals over the course of a day while keeping an eye on calories. The act of eating and digesting increases the resting metabolic rate — the calories your body burns to keep up physiologic functions without any physical exercise — so eating more frequently should favor weight loss, according to the center’s director, Lawrence Cheskin. Also, he says, “it keeps people satisfied — they’re less likely to do a big overeat.”

Unless — and this is a big unless — eating more frequently leads people to consume more calories. You can guess what happens then.

How about skipping breakfast? We have long been told to eat breakfast for health and attentiveness. Breakfast is thought to keep hunger at bay and prevent overeating.

A small recent study pokes holes in this conventional wisdom. Researchers at Cornell University either fed breakfast to or withheld breakfast from 18 student volunteers. Those who skipped breakfast reported being hungrier than those who ate breakfast; they also ate more at lunch. Still, they did not eat enough to fully compensate for the missed meal. In fact, those who had skipped breakfast took in 408 fewer calories over the course of the day than those who ate breakfast.

Study author David Levitsky, a professor of nutritional sciences at Cornell, says his work adds to a growing body of evidence that overturns a long-held belief: that people will compensate for missed calories. Rather, skipping a meal is “one small weapon” people can use to fight the battle of weight, he says. “If you skip two to three meals per week, you can decrease” your caloric intake, he said.

A more extreme version of this is the alternate-day fasting diet, a technique popularized by such books as “The Fast Diet” and “5:2.” Such diets advocate drastically limiting calories on certain days; on other days, a person can eat as he pleases. The approach is thought to address diet fatigue — the constant strain of counting calories and skipping dessert that can occur with dieting — by limiting willpower work to a few days per week.

Studies have shown mixed results with alternate-day fasting. A 2010 study showed the technique effective among a group of obese patients. The 16 participants ate only one meal — lunch — every other day. Caloric intake on these fasting days was limited to about 500 calories, or roughly 25 percent of their own usual daily intake. On the intervening days, they could eat whatever they wanted. Over eight weeks, the participants lost 5½ kg, on average.

Krista Varady, an assistant professor of kinesiology and nutrition at the University of Illinois, Chicago, who co-authored the study, said she was surprised by the results. She thought the participants might eat so much on their eating days that they would make up for those calories lost on the fasting days, but that was not the case. They only ate about 10 to 15 percent more on feeding days than their daily intake before the diet, Varady says, which, combined with the fasting-day savings, resulted in a whole lot of calories cut.

“There’s so much individual variability,” Cheskin said. But there’s only one bottom line: “You have to cut calories.”

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