Sleep loss might thwart dieters' fat loss

Getting too little sleep might prevent dieters from losing as much body fat as they otherwise would have, a small study suggests.

The findings, published Monday in the Annals of Internal Medicine, add to evidence that sleep habits play a role in weight regulation. They also suggest that people embarking on a weight-loss plan may want to make sure they are catching enough shut-eye each night, researchers say.

The study included 10 overweight men and women who lived in a sleep lab for two separate two-week periods. During both, they were kept on the same calorie-restricted diet; but for one period, the participants slept for 8.5 hours per night, while during the other, they got 5.5 hours.

Researchers found that the dieters lost the same amount of weight under both conditions - just under 7 pounds, on average. But during the sleep-restricted period, they mainly lost muscle rather than fat.

When participants got 8.5 hours of sleep, more than half of their weight loss came from shedding fat; when they got 5.5 of sleep, only one-quarter of their weight loss came from fat - translating to a 55 percent reduction in fat loss.

Instead, the majority of people's weight loss during the sleep-restricted period came from lean body tissue, which refers to muscle and any other body tissue that is not fat.

"So they lost the same amount of weight, but the composition was different," said senior researcher Dr. Plamen Penev, an assistant professor of medicine at the University of Chicago.

Successful dieters always shed a certain amount of muscle, Penev noted in an interview, but ideally one wants to limit that loss in favor of shedding excess body fat. Insufficient sleep, the current findings suggest, might interfere with that.

The study has a number of limitations. Besides its small size, it also looked only at short-term weight loss. More research is needed to see how sleep duration might affect dieters' body composition over time, Penev said.

It's also unclear how well these findings from a tightly controlled sleep-lab setting might translate to the "real world," according to Penev.

Still, the findings do add to a body of research linking sleep habits to body weight. A number of studies have found that self-described "short sleepers" - typically defined as those who get less than 6 hours of sleep each night - tend to weigh more or gain more weight over time than people who get seven to eight hours of sleep per night.

Those studies do not, however, prove that sleep differences are the reason for the weight differences. Small sleep-lab studies such as the current one help researchers zero in on the effects of sleep itself.

Lab studies have suggested, for example, that sleep loss may alter people's levels of the "hunger hormones" leptin and ghrelin. Leptin is secreted by fat cells; low blood levels of the hormone promote hunger, while increases tell the brain that body is full and encourage calorie burning. Ghrelin is secreted by the stomach to boost appetite.