Just how much exercise do you need to engage in to keep from gaining weight? A new study from the Harvard School of Public Health set out to answer this question that has puzzled experts for some time.

The issue is particularly important because Americans get conflicting advice from different sources about how much exercise is necessary to keep off the pounds. For instance, the study authors say that federal guidelines indicate that 150 minutes of exercise per week is sufficient, while The Institute of Medicine suggests that a much greater number – 420 minutes per week is necessary. Since, as the authors point out, this difference is almost three-fold, knowing what to tell the public about realistic activity levels is important.

The study, led by I-Min Lee, followed over 34,000 women for 15 years, tracking the kinds of activities they engaged in, the frequency of their exercise, and their body weight over the length of the study. Exercises included everything from higher-intensity activities like jogging, swimming, tennis, and aerobics to lower-intensity activities like yoga, stretching, and walking. The team analyzed the women’s energy expenditure and divided the women into three groups, based on how much moderate-level activity they partook in: the highest-level group averaged over 420 minutes of moderate exercise per week, the middle group between 150 and 420 minutes per week, and the lowest group exercised less than 150 minutes per week. All women were eating a normal diet throughout the course of the study.

The researchers found that women in the bottom two exercise groups both tended to gain at least five pounds within a three-year period (interestingly, there was no difference between these two groups in terms of weight gain). The group that exercised for at least 420 minutes per week (which is about an hour per day) was significantly less likely to gain the weight.

However, these results held true for normal-weight women only. The relationship between exercise and weight gain was not found for women who were overweight or obese.

To put this in perspective, doing at least an hour per day of moderate activity – like fast walking or riding a bicycle – kept off the pounds for women who were normal weight. But the researchers also said that doing just half an hour per day of more vigorous exercise – aerobics or running, perhaps – also had the same benefit.

More research may be needed to support these results before federal guidelines are changed, but in the meantime, it can’t hurt to add a little more exercise to your routine, if you can safely do so.