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It is a central tenet of most successful weight-loss programs: If you want to trim pounds, you need to burn more calories than you consume. But an intriguing study suggests that when you eat should also be taken into account.

The researchers, led by Marta Garaulet of the University of Murcia, observed 420 adults who took part in a 20-week weight-reduction program in Spain. And, in particular, they focused on the main meal of the day – which, in this Mediterranean country, happened to be lunch.

The study participants were divided into two groups: those who ate early (any time before 3 p.m.) and those who ate late (after 3 p.m.).

The findings revealed that the early eaters lost about 25 per cent more weight than the late diners, even though there was no significant difference in the caloric consumption and energy expenditure between the two groups. On average, those who ate their lunch early lost 22 pounds, while the late diners lost about 17 pounds.

How could this be? The researchers say they are not sure of the exact mechanism, but they think it may be related to the body's internal biological clock.

"The body deals differently with food depending on the time of day," explained one of the researchers, Frank Scheer, a neuroscientist at Brigham and Women's Hospital in Boston.

"For instance, glucose is cleared more easily from the bloodstream in the morning than in the evening," Scheer said. And if glucose is not going to the organs and tissues where it is needed for energy, it could end up being deposited as fat, he said.

He noted that this explanation is related to a burgeoning field of research known as chronotherapy, in which medical treatments are timed to correspond to the body's 24-hour circadian rhythm. Earlier trials have found that some blood-pressure drugs work better if they are taken at bedtime and the side effects of certain cancer medications can be reduced by giving them at specific times of day.

"This is the first large-scale study to demonstrate that the timing of meals is predictive of weight-loss effectiveness," Scheer said.

But what does this study mean for Canadians and Americans who tend to eat their biggest meal in the evening? If a late lunch undermined the dieting efforts of the Spaniards, couldn't a heavy meal at night be even worse?

Scheer is reluctant to speculate. "It might be worse, but until further studies are done to actually rigorously test it, we can't conclude that firmly."

Nonetheless, the Spanish study, published in the International Journal of Obesity, adds a new dimension to the debate about the best way to slim down. "It's not just what we eat but when we eat that we need to start considering," Scheer said.

Other weight-loss experts, who were not part of the study, think that the research has merit. Indeed, some dieters are already told to avoid big meals late at night.

"We recommend people to eat while they are active during the day," said David Lau, an obesity researcher and professor of medicine at the University of Calgary. "Typically, I tell my patients to eat breakfast like a king, lunch like a prince and supper like a pauper."

It has long been assumed that "you really don't need any extra calories when you are not really exerting yourself" at the end of the day, Lau explained. "We don't have a lot strong data to support what we have be saying. But this is maybe one of the studies that actually supports it."

Still, Lau noted that it just one study and other factors may account for the difference in weight loss between the two groups. "Further interventional studies should be done to provide insights into possible causality."