Carbohydrate intake during exercise

Carbohydrate feeding during a 1-hour effort allows athletes to exert themselves for longer periods of time and/or to produce more powerful bursts at the end of the effort. Coyle et al. showed that carbohydrate consumption during a cycling race at 70% V02max delayed fatigue by 30-60 min.

Coyle et al. compared the effects of carbohydrate feeding on fatigue onset and performance decline in cyclists. Carbohydrates allowed athletes to extend exercise time by an average of 33 min (152 min vs. 126 min) before fatigue was reached. This feeding maintained high blood glucose levels.

Coyle et al. also measured this long, strenuous cycling performance with and without carbohydrate feeding. During the carbohydrate-free ride, fatigue occurred after 3 h and was preceded by a drop in blood glucose. When cyclists were given carbohydrate with their meal, blood glucose levels were stable and they were able to ride for an additional 1 h before fatigue occurred. Both groups used muscle glycogen at similar rates, and endurance performance was enhanced by maintaining blood glucose rather than by preserving glycogen.

Carbohydrate nutrition maintains blood glucose levels at a time when muscle glycogen stores are depleted.

Running performance with and without carbohydrate feeding was also assessed. In a 40 km run in the heat, Millarg-Stafford et al. found that a carbohydrate feed (55 g-h) increased blood glucose levels and allowed runners to run the last 5 km significantly faster than in a carbohydrate-free run. In a treadmill run at 80% V02max, Wilbert and Moffatt found that running time was 23 min longer with carbohydrate (35 g-h) (115 min) than without carbohydrate (92 min).

Carbohydrate feeding may also improve performance in stop-and-go sports (soccer, basketball) that require intense, short-duration efforts. Davis et al. [26] assessed the effects of carbohydrate feeding on performance during an intense, rest-trained cycling test. Athletes performed repeated 1-min bursts at 120–130% V02max, separated by 3 min of rest, until fatigue occurred. Before and every 20 min during the race, athletes consumed a sedative drink or a 6% carbohydrate-electrolyte drink, yielding 47 g carbohydrate per hour. The mean time to fatigue was 89 min (21 bursts) with carbohydrate feeding compared to 58 min (14 bursts) with placebo. The results of this study indicate that the benefits of carbohydrate feeding are not limited to increasing the duration of intense exercise.

The improvements seen with carbohydrate feeding before exercise were complementary to those seen with carbohydrate feeding during exercise. Wright et al. [27] found that cyclists who consumed carbohydrate 3 h before and during exercise were able to exercise for a longer time (289 min) than those who consumed carbohydrate either before (236 min) or during exercise (266 min).

Combined nutrition yields higher values than carbohydrates alone. However, the increase in values with pre-load carbohydrate nutrition was lower than when small amounts of carbohydrates were consumed during the load.

The primary role of carbohydrates in water-replacing beverages is to maintain blood glucose levels and enhance carbohydrate oxidation. Carbohydrate feeding improves performance during exercise lasting 1 hour or longer, especially when muscle glycogen stores are low.

In fact, consuming carbohydrates and replacing water with beverages improves athletic performance.

Belois and Coyle evaluated the effects of fluid and carbohydrate alone and in combination during a 1-hour intense cycling test. In four tests, athletes received: 1330 ml of water, which replaced 79% of sweat; 1330 ml of fluid with 79 g of carbohydrate; 200 ml of water, which replaced 13% of sweat; and 200 ml of fluid with 79 g of carbohydrate. When either the high volume of fluid or 79 g of carbohydrate was given alone, each athlete improved performance by 6% compared with the placebo test. When the high volume of fluid and carbohydrate were given in combination, performance improved by 12%.

Coyle and Montain suggest that athletes should consume 30-60 g (120-240 kcal) of carbohydrates every hour to improve performance. This amount can be obtained from either food or carbohydrate-rich liquids.

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