(USA Today) -- Dieters have long been told not to eat too many calories late in the day.
Now, a new study suggests that dieters who eat lunch early lose more weight than those who eat a late lunch.
"We should start to consider meal timing in addition to calories and meal composition when thinking about weight loss," says the study's senior author Frank Scheer, an associate neuroscientist at Brigham and Women's Hospital and an assistant professor of medicine at Harvard Medical School.
To come to this conclusion, Scheer and researchers at the University of Murcia in Spain and Tufts University in Boston, studied 420 overweight and obese people who participated in a 20-week weight-loss program in Spain.
The mid-day meal is often the biggest one of the day in this Mediterranean culture. Participants consumed about 40% of their daily calories (roughly 550 to 570) at lunch.
Half of the participants were considered early eaters because they had lunch before 3 p.m. Half were classified as late eaters because they had their mid-day meal after 3 p.m.
Overall, participants consumed an average of about 1,400 calories a day during the weight-loss program. There was no significant difference in caloric intake or energy expenditure between late lunch and early lunch eaters.
Among the findings reported Tuesday in the International Journal of Obesity:
-- Those who ate lunch earlier in the day lost an average of 22 pounds in 20 weeks; those who ate lunch later lost about 17 pounds.
-- The late eaters consumed fewer calories during breakfast and were more likely to skip breakfast than early eaters. (Dieters are often advised to eat breakfast.)
-- The late lunch eaters had lower insulin sensitivity, which is a risk factor for diabetes.
"We cannot directly translate these findings to Americans, but we would expect that a later dinner -- the main meal for most Americans -- might similarly impair weight loss, " Scheer says. "But research is required to test that."
He says researchers don't know why weight loss was greater in the early eaters, but one hypothesis is that glucose (sugar) is processed differently depending on the time of day. Another theory is that the timing of meals can impact the circadian system (the body's clock) which may disrupt the proper function of the liver and fat cells, Scheer says.
Elizabeth Ward, a registered dietitian in Boston and author of MyPlate for Moms: How to Feed Yourself and Your Family Better, says this "an interesting study, but it's an observational study, so it doesn't prove cause and effect. If you cut back of your calories at any time of the day, you will lose weight."
Still, the research indicates that if you eat your calories earlier in the day, it may give you an edge with weight loss, Ward says.
She says there may be some truth to the old adage: "Eat breakfast like king, lunch like a prince and dinner like a pauper."