Many factors affect what sports scientists call your "running efficiency" — the amount of effort it requires for you to run. The better your running efficiency, the faster you will be able to run while expending the same amount of energy. In addition to factors such as your gait, running gear and the ability of your muscles to store energy, your body weight affects your running efficiency. So if you are looking to shave minutes off your half-marathon time, for instance, losing excess body weight can help.
Running and Body Weight
When you are running, your legs must both support your body weight and propel your body in a forward motion. It takes a substantial amount of energy to run, and most is used as your legs work in a spring-like motion to move your body from one location to another. So the more you weigh, the harder your body must work when you go for a run.
VO2 Max and Running
Your VO2 max is the maximum amount of oxygen your body is the maximum amount of oxygen your body is able to transport and use during exercise. The higher your VO2 max, the more physically fit you are. So anything you can do to improve your VO2 max can help you run faster while spending the same amount of energy.
How Body Weight Affects VO2 Max During Running
Sports scientists at the American College of Sports Medicine have developed a formula to calculate VO2 max in runners, determining that your body weight is inversely related to your VO2 max during running. In other words, the more you weigh, the more oxygen your body needs to run at the same speed. This is why losing excess weight is one of the best ways to improve your running speed.
How Much Faster Will You Run?
Shedding excess body weight can make it easier for you to run faster, but don't expect to move from the back to the front of your pack in your next race. The exact effect your weight loss will have on your running speed depends on many factors, but in general, you can expect to shave a few seconds off your 5-kilometer time when you lose excess weight. For example, research published in the "Journal of Exercise Physiology" estimated that a 225-pound man who loses 15 pounds of excess body fat can expect to run shave about 41 seconds off his 5K time.
Making Sure You Have Enough Fuel to Run
Losing some weight can help improve your running speed — but it is important to avoid going on a diet that is too sparse in calories, since that can backfire and cause your running performance to decline. For runners, food is fuel, and it is important to get the fuel you need to perform at your best. When you don't eat enough, your body is forced to use its fat and protein reserves for energy, which will cause you to run more slowly.