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You may have noticed a disagreement among advertisers as to the role of exercise in weight management. One commercial promises a beach-ready body if you only eat their food, while another shows that the latest technology in gym equipment is required to slim down.

And it isn't just advertisers that disagree. In 2008, federal guidelines called for 150 minutes of moderate exercise per week to provide substantial health benefits. However, the Institute of Medicine still holds to their 2002 recommendation that at least 420 minutes (one hour per day) of moderate exercise is the minimum to prevent becoming overweight or obese.

So, is physical activity really that important in either losing weight or maintaining your trim figure? And if so, how much exercise do we need? A recent, massive study provides some interesting answers.

I-Min Lee, associate professor at the Harvard School of Public Health, recently led a team of researchers to review the weight changes of over 34,000 women over 15 years at several levels of physical activity with a standard diet. They divided up the women into three groups: those who worked out for less than 150 minutes per week(the Federal minimum), those who worked out between 150 and 420 minutes per week, and those who exercised for more than 420 minutes per week (the IOM recommendation.

The researchers found two interesting results. First, the rather obvious conclusion that the women in the two less active groups gained significantly more weight over the trial than the most active group. However, the difference between the two less active groups was insignificant, meaning exercising for more than 150 minutes but less than the 420 minutes the IOM recommends did not bring results any better than exercising for less than 150 minutes each week.

Second, only women who had a baseline Body Mass Index (BMI) of less than 25 gained less weight when they worked for a longer time. (BMI can be calculated based on one's height in inches and one's weight in pounds; a BMI of more than 25 is considered to overweight and a BMI of more than 30 is considered to be obese.) In other words, for women with BMIs in the overweight or obese range, increased workouts did not help them lose weight.

"Once overweight, it may be too late because physical activityat least, at levels carried out by study participants was not associated with less weight gain," according to the researchers. "These data suggest that the 2008 federal recommendation for 150 minutes per week, while clearly sufficient to lower the risks of chronic diseases, is insufficient for weight gain prevention absent caloric restriction."

As with most health issues, the exact answer depends on you and your situation. Of course, a good diet and plenty of moving around can only help.

Dan Peterson writes about sports science at Sports Are 80 Percent Mental.