Many athletes eat large helpings of carbs the night before a big race to make sure they have enough fuel for the next day. But research has found that loading up the night before may slow you down. Published in Medicine & Science in Sports & Exercise, the study put 21 experienced triathletes on a new training and diet programme for four days every week, for three weeks. The training portion of the programme consisted of two parts: in the afternoon, the athletes performed high-intensity interval training, designed to increase fitness and deplete their bodies’ carbohydrate stores; then, prior to breakfast the next morning, the athletes cycled moderately for an hour on stationary bikes. On each training day, the athletes all ate the same amount of carbs, but with one difference: half the athletes ate their carbs throughout the day, including during workouts (a "standard-sports diet"); the other half ate their carbs only during breakfast and lunch (a "sleep-low diet"), to limit their bodies’ ability to use carbs for energy. On non-programme days, athletes were allowed to eat whatever they wanted and to train at an easier pace. Researchers found that:

• Athletes eating the sleep-low diet finished a 10K run about 75 seconds faster at the end of the study than they did at the beginning of the study, while athletes eating the standard-sports diet showed no improvement.
• Athletes eating the sleep-low diet also showed greater improvements in muscle efficiency and performance during morning cycling over the course of the trial, and lost more body fat, compared with athletes eating the standard-sports diet.

These findings suggest timing is everything when it comes to carb loading. If you're getting ready for a race, it might be a good idea to double up on healthy carbs like those in whole wheat bread, oats, brown rice, barley, and quinoa in the morning and afternoon, and then to skip them in the evening.

Source: Medicine & Science in Sports & Exercise