July 25 (HealthDay News) -- Anyone who has avoided Monday morning weigh-ins knows this unalterable truth: Weekends are not a dieter's friend.

Now, researchers have some science to back up dieters' complaints about weekends being their undoing: Most people do eat more on the weekend, even when they're trying to lose weight.

"Weekend indulgences can wreak havoc on weight control, either causing our weight to increase or if we are following a diet to lose weight, can hinder our weight loss efforts," said study author Susan Racette, an assistant professor at Washington University in St. Louis. The research was published online recently in the journal Obesity.

Racette and her colleagues followed 48 men and women for a year, trying to determine the effects of weekends on weight loss efforts.

They assigned the participants, who ranged from being healthy weight to being nearly obese, to one of three groups: The controls did not change diet or activity levels; the calorie-restriction group reduced intake by 20 percent, and the physical activity group increased physical activity every day by 20 percent. Participants kept food diaries and wore devices to measure activity.

But even before the intervention started, Racette gathered data -- on daily weight, food intake and physical activity -- and found that the weekends were for indulging.

"At baseline, before they were supposed to be following a diet or exercise plan, we found on weekends, people gained weight," she said. During the week, the weight would decline. But the weekend effect was strong. "If you translate it out to a year, it could have increased weight by 9 pounds."

Before the intervention, participants ate an average of 2,257 calories on Saturday compared to just 2,021 during the week. But the average activity on weekends overall didn't differ much from average weekday activities. So, it was the food, not the lack of activity, that was to blame, Racette said.

Racette monitored the participants for a year after they started the intervention, and the weekend indulgences continued. The calorie restriction group stopped losing weight on weekends, while the physical activity group gained slightly (about .17 pounds). There were not significant weight changes in the controls on weekends.

Weekend indulgences help explain the slower-than-expected weight loss of many dieters, Racette said. "There is less structure on the weekend for a lot of people, and that can wreak havoc," she said. "A little indulgence turns into a big indulgence. Being vigilant on the weekends is really important for people either trying to lose weight or maintain a weight loss," she said.

SOURCES: Connie Diekman, M.Ed., R.D., director, university nutrition, Washington University, St. Louis, and past president, American Dietetic Association; Susan Racette, Ph.D., assistant professor, Washington University, St. Louis; June 12, 2008, Obesity, online