When you eat matters, a Spanish study found. (Image credit: Getty Images)

In dieting, like comedy, timing is everything. That's the conclusion of a new Spanish study that suggests that when you eat might be just as important as what you eat.

During the first few weeks of the 20-week study, run by researchers from Brigham and Women's Hospital in collaboration with Tufts University and the University of Murcia, all 420 subjects lost weight at about the same rate. But starting around week five, weight loss for dieters who ate their main meal after 3 p.m. began to stall and remained sluggish for the duration of the study. In the end, they lost 22 percent less weight than dieters who ate the bulk of their calories earlier in the day.

The results left researchers scratching their heads. All the subjects ate and burned off about the same number of calories. They all followed a Mediterranean-style diet consisting of lean proteins, fruits and vegetables and healthy fats such as olive oil, and consumed about 40 percent of their daily calories at lunch. They all slept approximately the same number of hours each night and, when tested, their appetite and hunger hormone levels were comparable. Even their genetics were similar.

The late-in-the-day eaters did tend to be breakfast-skippers, and they showed a higher level of insulin resistance. But according to the researchers, these differences alone didn't explain the variability in weight loss between late and early eaters - and neither of these factors was correlated with the amount of an individual's weight loss.

The researchers' best guess is that that eating later in the day messes with the body's internal clock system, known as circadian rhythms, and this might somehow have an adverse effect on metabolism.

Frank Sheer, one of the study's coauthors and an assistant professor of medicine at Harvard Medical School, explained it this way:

"The circadian system is made up of a master clock in the brain and peripheral clocks in most cells throughout the body. Normally, the master clock synchronizes all peripheral clocks, similar to the conductor of an orchestra. When meal timing is abnormal this leads to de-synchronization between these different clocks, resulting in a cacophony."

Over the years, many a popular fad diet has sung the virtues of early-in-the-day eating patterns. But David Just, co-director of Cornell University's Center for Behavioral Economics in Child Nutrition, said the results of this latest study notwithstanding, such advice may not apply to Americans.

"Spaniards tend to eat their largest meal midday," he said. "Starting toward the end of the eighteenth century, as more people in this country began taking factory jobs that didn't allow them to pop home for lunch, Americans began shifting their main meal toward the end of the day, which is how most of us still tend to manage our eating."

Just said he suspects that Americans who eat less in the first part of the day simply eat more later on to compensate. His own studies with school-age children show that kids who eat an early lunch tend to skip breakfast and have large afternoon snacks. He's also found that kids who eat a late lunch often enjoy a light afternoon snack but make up the calories by having an enormous dinner. The kids who ate the fewest calories in his studies regularly had their lunch right around noon.

Sheer also said he believed it's too soon to relate his study's outcomes to the American lifestyle. But he does think the timing of eating could be an important part of weight loss.

"These new findings may help in developing new strategies to further optimize weight-loss therapies in the battle against obesity," he said.