Carbohydrate intake is beneficial in runs where your performance might be limited by your body’s ability to supply carbs to the working muscles. This limit comes into play in very long runs of moderate intensity, in long runs of moderately high intensity, and in moderately long runs with very high-intensity segments. Examples are an 18-mile endurance run done at a comfortable pace, a 10-mile run that includes 6 miles at half-marathon race pace, and a one-hour run featuring half a dozen two-minute bursts at 5K race pace. In each of these types of runs carbohydrate supplementation is likely to reduce your perceived effort and increase your performance. Shorter and slower runs don’t challenge the body enough for carbohydrate consumption to have any meaningful effect. The precise duration and intensity of running that are required to bring carbohydrate supply into play as a performance limiter depend on your individual fitness level. As a general rule, though, if a run is long or fast enough to leave you more than moderately fatigued at the end, then carbohydrate intake would make a difference.
Research indicates that you need 30 g of carbohydrate per hour to attain a measurable performance increase in runs that are hard enough for carbohydrate supply to be a limiting factor. Sixty g per hour is considered to be the minimal rate of carbohydrate intake you need to maximize your performance in the most challenging workouts and in longer races. Further increases in the rate of carbohydrate intake all the way up to 90 g per hour may bring you additional performance benefits in marathons and ultramarathons. Many runners are unable to tolerate such high rates of carbohydrate intake, however.