Weight loss can shave minutes off your race times.
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People often run with the goal of getting leaner, but if you're a competitive runner you should consider getting leaner for the sake of your running. In general, losing weight will result in improved performance as a runner. But be careful, there's a limit to how much improvement you can see with weight loss.
Your maximum volume of oxygen -- commonly known as VO2 max -- is a key factor in how fast you can run. VO2 max is simply the amount of oxygen that your body can deliver to your muscles, per unit of body mass. Reducing your body mass means that your body can deliver oxygen to your muscles more efficiently, improving your VO2 max. This, in turn, leads to improved running efficiency -- meaning you can run a set distance at a faster pace.
According to exercise physiologist Susan Paul, M.S. losing 10 pounds will shave about 20 seconds per mile off your time. That means you can expect about two seconds per mile of improvement for every pound that you lose. For instance a runner who runs a 6:30 mile can expect to run a 6:20 mile by losing 5 pounds. The same runner could further reduce her time to a 6:00 mile by losing a total of 15 pounds.
Shaving seconds off your time can have a dramatic effect on your race times. By simply dropping 5 pounds and improving your pace by 10 seconds per mile, you can expect to shave 31 seconds off your 5K race time or 1:02 off your 10K time. In a half marathon this would result in a savings of 2:11. At the 26.2-mile marathon distance you could see an improvement of 4:22.
Leaner isn't always better. According to the University of Pennsylvania there is little evidence that there is any health benefit to dropping your body fat below 8 percent as a man or 14 percent as a woman. Going below these recommendations may actually have an adverse effect on your performance. Before starting a weight loss program, consult a physician to determine your safe goal weight and an appropriate course of action.