I've read that it helps with water retention, too!
A PMS Workout
Regular exercise is central to relieving PMS, says Prior, whose studies have shown that women who began or accelerated a running program reported a decrease in premenstrual fluid-related symptoms--such as abdominal bloating--as well as a reduction in depression and anxiety. She advises women to do some form of aerobic activity that raises their heart rate above 120 beats per minute, for 20 to 30 minutes, three to four times a week. (Check with your doctor before starting any new exercise regimen.) This is also likely to help women lose weight, notes Prior, who says excess body fat may exacerbate PMS.
Some of the relief exercise affords may stem from the mood-enhancing effect of physical activity, the so-called "runner's high," experienced by both men and women who exercise regularly. A well documented stress reducer, exercise provides a "play break" away from the daily grind. But Prior also theorizes that "exercise alters something in the hypothalamus, probably the core body temperature, that signals the pituitary gland and ovaries to bring a woman's system more into balance."
http://health.ivillage.com/gynoperio...4mc3-2,00.html 
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