None of the participants was obese when the study began, but some were classified overweight by their Body Mass Index, a ratio of height to weight.  The team, led by Susan Racette, assistant professor of therapy and medicine, divided the participants (all were part of CALERIE [Comprehensive Assessment of Long-Tern Effect of Reducing Intake of Energy]) into three groups. One lowered calorie intake by 20 percent, the second increased physical activity 20 percent and the third did nothing. All three were followed for the whole year while they kept food diaries, tracked exercise and were weighed at regular intervals.

Before the study, researchers recorded baselines for what the subjects were normally eating and how much they were exercising. Saturdays were their downfall, with participants consuming the most calories on that day. And they were generally bad calories: 36 percent of the total came from fat, compared to less than 35 percent the rest of the week. Even normal-weight participants would gain nine pounds a year if they continued that pattern.

After the study began, Saturdays were still a problem. The group that was cutting down on calories still took in more on Saturday and the group that was exercising more actually ate more on both Saturday and Sunday. The results were not good: The dieters stopped losing weight on weekends and the exercisers actually gained weight.

The researchers are still trying to figure out how to help dieters overcome this weekend slump, but one tip is to weigh yourself daily so that you're more aware of how much you are taking in. It's also important to plan ahead if you know you're going to a party or other event where fattening food will be around. Bring your own food or eat something healthy beforehand so you're not starving (and vulnerable to snacking) when you arrive.

Also, pay attention to the amount you're eating—especially portion sizes. Many restaurants, for example, offer meals that could easily feed two. As a guide, one portion of meat (about three ounces) is roughly the size of a pack of cards.

3. Work It Out
Many employers consider obesity to be a workplace problem because too many excess pounds puts employees at risk for heart disease, diabetes and perhaps even dementia. That affects the bottom line in the form of increased health insurance costs. According to the National Business Group on Health, more than 10 percent of employers' health costs are obesity-related. That's why a growing number of companies have entered the battle of the bulge.