Eat more carbs.
The American fast food diet – full of sodas and processed flour – has bestowed a bad name on carbohydrates and given rise to a slew of low-carb diets. Don't be fooled: Runners need carbs. "Low-carb diets may have certain health benefits for some people, but research has shown that decreasing the amount of carbohydrates in the diet reduces performance in prolonged exercise and the capacity to handle higher training loads," says Asker Jeukendrup, author of 'Sports Nutrition from Lab to Kitchen.' Unlike fat and protein, carbs are used almost exclusively to supply energy for activity, which means that the more you run, the more carbs you need. If you jog 10 miles per week, a little less than two grams of carbs per pound of body weight will do the trick (that's equivalent to about four servings each of whole wheat pasta and shredded wheat per week for a 170-pound man). Serious runners will need roughly twice that.