When you eat, not just what you eat, may play a role in weight loss, finds a new study from Spain.
During the study, which investigated overweight women participating in a weight-loss program, those who ate lunch later in the day (after 3 p.m.) lost 25 percent less weight over a 20-week period than women who ate lunch earlier.
Researchers found this difference even though the two groups did not differ in the number of calories they ate each day, the amount of physical activity they engaged in, their levels of appetite hormones or their sleep duration, all factors known to influence weight regulation.
The findings agree with those of earlier animal studies that suggest meal timing affects weight gain. For instance, in a study published last year, mice that were allowed to eat whenever they wanted gained more weight than mice who had their meal times restricted, even though both groups consumed the same number of calories.
The results suggest that "eating late may impair the success of weight-loss therapy," the researchers write in the Jan. 29 issue of the International Journal of Obesity.
However, the researchers note the study only found an association and not a cause-effect link. Future studies of weight loss should randomly assign people to eat meals at certain times of day to confirm the findings, they said.
The study examined 420 overweight women enrolled in a weight-loss program. The women received nutritional education and recommendations on the number of portions to eat from each food group, but were not told what time of day to eat.
In this Mediterranean population, lunch is the main meal of the day, comprising 40 percent of a person's daily calories.
Women who ate lunch after 3 p.m. lost about 17 pounds during the study period, compared to 22 pounds in women who ate lunch before 3 p.m. The timing of other meals did not appear to play a role in weight loss, the researchers said.
Because the women reported their food intake and physical activity, it's possible inaccuracies in reporting could have influenced the results, the researchers said.
Exactly how meal timing affects weight gain, independent of calorie intake, is not known, said study researcher Frank Scheer, associate neuroscientist at Brigham and Women's Hospital in Boston. But previous research in animals suggests meal timing affects metabolism, Scheer said.
Each organ has its own clock, and eating at odd times may cause these clocks to get out of synch with the body's master clock in the brain, affecting the way the body uses and stores energy, Scheer said.
Some studies have also suggested that eating breakfast helps keep people full longer, reducing calorie intake for the rest of the day. [See Cake for Breakfast? Study Says Go for It.]
Pass it on: Eating meals later in the day may affect weight loss.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.