Training for a half-marathon requires a runner to invest weeks of preparation and hours of running on roads, trails or treadmills. In the days leading up to a half-marathon, a runner should taper his training and focus on getting some extra rest, but eating the wrong foods during the week of the race could derail a runner's performance in a matter of minutes.
No runner wants to make a mid-race pit stop at a bathroom or porta-potty, but eating lots of foods high in fiber before a race could force a runner into such a detour. Because fiber is indigestible and settles low in the gastrointestinal tract, or GI, it promotes regularity and cleanses the body of toxins. However, high-fiber foods such as beans, broccoli, green peas, raspberries, pears, prunes or bran cereal, can trigger gas, cramping or unexpected bowel movements. Making matters worse, the constant pounding and motion of distance running can intensify GI pain.
Lactose, a natural sugar found in dairy products such as milk, cheese and ice cream, can be difficult for some runner's to digest, and can cause an uncomfortable upset stomach for runners suffering from lactose intolerance. When a runners are unable to break down and digest lactose, she may experience gas, bloating or diarrhea. In the days leading up to a half-marathon, runners should avoid large or uncommon servings of dairy products and may substitute typical dairy products for more easily digestible foods, such as swapping ice cream for yogurt -- which is easily digested due to its live and active cultures -- or trading cow's milk for soy milk.
Drinking a diet soda, sugar-free snack or sugar-free candy instead of its higher-calorie counterpart may seem like a sensible solution for cutting calories before a half-marathon, but the artificial sweeteners in many "diet" foods and drinks can cause GI problems. Artificial sweeteners such as sorbitol and mannitol are difficult for many runners to digest and can cause an upset stomach. Fructose, a sweetener commonly found in sports drinks and energy gels, can also sit in a runner's stomach and break down slowly, so runners should lay off these products leading up to race day.
Drinking a morning cup of coffee may not hamper your performance in a half-marathon, but consuming higher than normal amounts of caffeine -- a stimulant found in coffee, tea and chocolate -- can result in some serious side effects. Large doses of caffeine, especially in the late afternoon or evening, can disrupt a runner's sleep pattern and prevent him from getting the rest he needs prior to a race. A lack of sleep, compounded by the temporary spike in blood pressure and heart rate which caffeine can produce, can lead to excess stress, leaving a runner's mind and body feeling fatigued -- rather than refreshed -- on race day.