Everyone needs a break. But relax your diet and exercise efforts too much on the weekend and you could offset the hard work you put in Monday through Friday, report researchers from the Washington University School of Medicine in St. Louis.

Susan B. Racette, PhD, assistant professor of physical therapy and medicine at the university and the lead study author, documented this phenomenon by carefully tracking the weight, eating habits, and physical activity levels of 48 women and men before and during her 12-month diet and exercise study. Participants kept food diaries (they weighed and measured everything they ate), weighed themselves on laboratory-grade scales every morning, and wore monitors equipped with sensors to measure physical activity.

The results surprised Racette: Before the study began, volunteers were eating enough additional calories each weekend—compared with their weekday regimen—to result in a gain of 9 pounds in a year.

Once the study started, participants who were supposed to be following a reduced-calorie diet had a steady weight dip from Monday through Thursday. But beginning on Friday, they simply stopped losing weight. The study also had an exercise-only group, and these people actually gained weight.

The reason for the gain was that on Saturday and Sunday, nearly everyone cheated a little—eating an extra 100 to 300 calories a day, mostly from higher-fat foods. And while physical activity increased on Saturdays, it was lower than average on Sundays.

"We thought weekends would present a problem for some people, but it was actually a problem for almost everyone—and these were motivated people who knew they were being watched closely," Racette says.

The good news is that the gain each weekend was, on average, less than half a pound, the kind of small increase that only becomes a problem over months. Preventing the gain should be easy, Racette says, provided you plan for it. "Set aside some time for a walk on Sunday," she says. "Eat before you leave the house to run errands so you don't end up at the drive-through or the mall food court. Bring snacks if you're going to sit at a child's soccer game all day so you don't buy nachos from the snack stand."
