Running is an excellent way to lose weight, burning more calories per unit time than any other form of aerobic exercise. Beyond this basic consideration, however, many competitive runners are interested in knowing how much faster their race times might be given a specified drop in body mass. While this varies from person to person and depends on a host of factors, certain general guidelines apply.
One measure of running fitness is VO2 Max, or the maximum amount of oxygen a given runner can use per unit time per unit body mass. More simply, the lighter a runner is at a given fitness level, the higher her VO2 Max and--at least in theory--the faster she will be. Not all "excess" weight, however, is created equal; fat, for example, is merely dead weight, whereas muscle, at least to a point, can contribute to performance in distance running despite its high density.
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Estimates concerning the precise effect of a given weight loss on running pace vary. Longtime coach Frank Horwill reports that the ideal weight for a 6-foot-tall male distance runner is 151 lbs, or 15 percent below the median; on average, a number of world-record holders tip the scales at closer than 10 percent below average. While distance runners unquestionably perform better at weights significantly below average, there are clearly limits to how much weight loss is beneficial or healthy in general.
If you've already been running for a number of months or years and have found that your weight has plateaued at a figure higher than you believe to be ideal for performance, how do you go about shedding pounds? The answer has to be: Through some combination of eating fewer calories and exercising more. Some runners find that increasing their weekly mileage brings their weight down and has the added benefit of improving fitness independently, whereas others discover that paring foods such as fats and alcoholic beverages has a slimming effect.
So just how much can you expect to benefit from being lighter? Joe Henderson, the author of various books on running, has this to offer: "The loss of a single pound doesn't mean much for a single mile, but the effect multiplies nicely. Ten pounds equals 20 seconds per mile, which grows to a minute-plus in a 5K, more than two minutes in a 10K, nearly 4.5 minutes in a half-marathon and almost nine minutes in a marathon."
While the typical American may be out of shape and at a heavier than medically advisable weight, competitive runners are a psychological and physical breed among themselves and are notorious for trying to make too much of a good thing. Many successful runners have fallen prey to the trap of thinking that just a few extra lost pounds will catapult them to a new performance level. Many develop eating disorders, while even those who don't often experience various injures and illnesses have fertility problems or at a minimum, become slower instead of faster.