TUESDAY, Jan. 29 (HealthDay News) -- People who like to eat lunch late in the afternoon may have more trouble shedding pounds than those who dine earlier, a new study suggests.

Researchers found that of 420 people in a weight-loss program, the late-lunch crowd lost about 25 percent less weight than those who usually lunched before 3 p.m.

The findings, reported Jan. 29 in the International Journal of Obesity, come with caveats. The researchers cannot be sure that a late lunch itself thwarts people's diets. And the study participants were from Spain, where lunch is the biggest meal of the day.

It's not clear if the findings would translate to a country like the United States, where most people eat a lighter lunch and save their main meal for dinner, said senior researcher Frank Scheer, an assistant professor of medicine at Harvard Medical School in Boston.

It is a common belief that it's better to have your big meal earlier in the day. Scheer pointed to the popular advice to eat breakfast like a king and dinner like a pauper. But there hasn't been much scientific evidence that the timing of your main meal matters in the battle of the bulge.

"This is the first large-scale, long-term study to show that it is an important factor in weight-loss success for overweight and obese individuals," Scheer said.

It's uncertain why a late lunch would be related to slower weight loss. One possibility, though, is that at least some late lunchers were going too long between meals, which might have effects on metabolism.

Some studies have suggested that evenly spaced meals -- eating every three to four hours -- are helpful in weight control, noted Connie Diekman, director of university nutrition at Washington University in St. Louis.

In this study, the late-lunch group was more likely to eat a light breakfast or skip breakfast altogether. Almost 7 percent of later lunchers did so, versus less than 3 percent of people who ate lunch earlier.

So the findings show a "potential connection between going too long between meals and weight gain," said Diekman, who was not involved in the study. "But given the study design, more studies are needed to determine if there is a cause-and-effect connection."

The problem is that people who hold off on lunch may differ from other dieters in many ways -- including ways that could hinder their weight loss.

Scheer's team did account for some of those possibilities. They found that the early- and late-lunch groups ate a similar number of calories and burned a similar amount (based on their reported activity levels). The two groups also averaged about the same amount of sleep each night, which is important because sleep loss has been linked to a higher risk of obesity and less weight-loss success.

Still, that's not enough to prove the late lunch caused the slower weight loss, Scheer and Diekman pointed out.

The findings are based on 420 overweight and obese Spanish adults who took part in a five-month weight-loss program. They were encouraged to follow a traditional Mediterranean diet, which includes plenty of fish, olive oil, vegetables and whole grains, but goes light on red meat and butter.

They got no advice, however, on the timing of their meals.

In the end, the half of the group that usually ate lunch after 3 p.m. lost an average of 17 pounds. That compared with 22 pounds in the early-lunch group.

As is typical in Spain, lunch was the biggest meal of the day. The dieters downed 40 percent of their daily calories at lunchtime, on average -- whether they ate early or late.

In contrast to the lunch findings, there was no evidence that the timing of people's breakfast or dinner affected their weight loss. (Half of the group ate their dinner after 9:30 p.m.)

So in a culture where the biggest meal of the day is dinner, would it matter if you ate it at 9 p.m. or 6 p.m.? "It's hard to say, based on these data," Scheer said. Further research is needed to answer that question, he added.

For now, the current findings are in line with animal research showing that meal timing seems to affect weight, Scheer said.

It may have to do with effects on the body's circadian rhythms, which influence a range of functions, including the sleep-wake cycle and metabolism. There is a "master clock" in the brain that coordinates those rhythms, but there are also "peripheral clocks" in tissue and cells throughout the body, Scheer explained.

In animals, unusual feeding times seem to disrupt some of those peripheral clocks and throw them out of sync with the master clock. In theory, that clock "decoupling" could affect weight control.

More research is still needed, though, to see whether the timing of a person's main meal directly influences weight -- and how important that influence really is, Scheer said.

"We need to know if this has clinical relevance," he said.

Diekman said the findings support the notion that meal timing matters, but she agreed that the ultimate importance to weight loss remains to be seen.

"As a registered dietitian, this study helps me feel comfortable with recommendations about the importance of meal spacing," she said. "But it does not give an answer to why or what impacts that might have on weight."

More information

Learn more about weight-loss dieting from the U.S. National Institutes of Health.