Running is a vigorous activity, so it burns many calories. If you don't compensate for the calories burned by eating more, running can help you lose weight. But if you eat more than usual after you run, due to an increased appetite, you may find yourself gaining weight. You also gain a little weight from the increased muscle mass you develop when you run.
Aerobic activity, such as running, may increase hunger in some people, according to a 2010 study published by Derek J. Laan and colleagues in "Applied Physiology, Nutrition, and Metabolism." In the study, a group of young, healthy adults were asked to engage in aerobic exercise for 35 minutes while another group remained sedentary, then consume a pasta meal afterward. Those who engaged in aerobic activity ate 14 to 18 percent more than those who were sedentary.
If your appetite increases after running, whether you gain weight will depend on how much you eat. Before you begin a running regimen, calculate how many calories you will need to consume per day to lose or maintain your weight. On days that you run, calculate how many calories your burned, and don't eat more calories than you burned. If a 150-lb. person runs 3 miles in 30 minutes, he will burn 342 calories, according to HealthStatus. An extra 2-cup helping of pasta at dinner would negate the calories he burned during his run.
Running builds muscle mass in your legs so they can tolerate increased work. If you feel sore after a run, a small amount of damage has occurred to your muscles. Your body responds to this injury by building muscle. Longer runs are especially likely to have this effect. The main muscles worked when you run are your quadriceps, hamstrings, soleus and gastrocnemius muscles. Weight gain from increased muscle mass should be minimal and shouldn't be a cause of concern.
Plan your meals carefully. A runner's diet should be at least 50 percent carbohydrates, but make sure they're whole grains. Know what you'll eat after a run to avoid turning to fast food or take-out. Prepare your meal ahead of time, since you'll be fatigued after your run. Have a small, high-carbohydrate snack, such as juice and pretzels, one to two hours before running. Stay properly hydrated, but watch for excess calories in sports drinks.