Posted: March 4, 2005

Science of Sport: Pre-Race Carbs - Good For Performance?

By Owen Anderson, Ph. D. (copyright © 2003-2005)

Should you take in some carbohydrate during the hour before your workout or competition? Many athletes are fond of the practice, believing that last-minute carbs provide a bit of an edge during ensuing exertion. The carbohydrate, they argue, goes straight to their muscles and provides ample fuel for intense activity. Other athletes, especially those inclined toward prolonged efforts, contend that the pre-exercise carbs “cap off” muscle-glycogen levels, enhancing endurance. Since the carbohydrate is usually dissolved in a sports drink of some sort, it is usually well tolerated by the stomach and small intestine during exercise.

While these arguments have a certain logical appeal, research has not always been kind to the idea of taking in carbohydrate shortly before working out or competing. Basically, scientific investigations have revealed that carbohydrate ingested during the hour before vigorous exercise boosts blood-glucose and blood-insulin levels at the start of exertion. This combo of high glucose and insulin can then be followed by a rapid drop in blood glucose (a potential “rebound hypoglycemia”) during the first 30 minutes or so of exercise (1).

This fast fall of blood glucose, when it occurs, is probably caused by the rapid uptake of glucose by the muscles during exercise, spurred on by the high insulin concentrations. Key worries for the athlete are that the rebound hypoglycemia itself might be a fatigue inducer – and that the high insulin at the start of exercise might very well block the breakdown of fat during exertion (this is one of insulin’s characteristic effects). The blockade on fat oxidation might actually intensify glycogen utilization in the muscles; paradoxically, then, the athlete taking in carbs in hopes of “capping off” intramuscular glycogen might actually end up depleting glycogen at a faster rate than usual (2). Glycogen depletion, of course, is a significant cause of fatigue.

However, some studies have shown that pre-exercise carbs do not actually augment muscle-glycogen utilization, and many investigations have shown that the drop in blood glucose associated with taking in carbs before exercise does not actually hurt performance (3); indeed, one might argue that the appearance of a certain amount of hypoglycemia in the early stages of exercise is a good thing – a sign that muscles are taking in copious amounts of glucose from the blood. At any rate, the impact of consuming carbohydrate before exercise on actual performance has been unclear: Some studies have shown that pre-exercise carbs can heighten subsequent performance (4), but others have found no difference between carbs and placebo (5), and a few investigations have linked pre-activity carbohydrate with reduced performance (6).

One reason for these contradictory findings is that the effects of pre-exercise carbohydrate probably depend a lot on the nature of the exercise which follows. For example, an athlete with decent muscle-glycogen levels would be unlikely to get much benefit from pre-exercise carbohydrate if his/her workout involved a high-intensity, 40-minute effort; his/her intrinsic stores of carbohydrate could get him/her through the session quite nicely.

On the other hand, someone about to engage in 90 minutes of glycogen-depleting exertion might be able to sustain a high-quality intensity for a longer period of time within the workout because of the additional, pre-exercise carbs. It is also apparent that the quantity of carbs taken in before exercise could be important: Swallowing 25 grams of carbohydrate might make no difference to the muscles at all, while gulping down 200 grams could give muscles an instant, massive amount of “jet fuel” (or perhaps bury the athlete under the crushing load of rebound hypoglycemia, in a worst-case scenario).

To try to decrease the uncertainty surrounding pre-exercise carbohydrate intake, renowned sports-carb expert A. E. Jeukendrup and his colleagues from the Human Performance Laboratory at the University of Birmingham in England recently completed a study with nine well-trained male athletes (7). Average age of the cyclists was 30, mean body fat was 14%, maximal aerobic capacity was an impressive 64 ml/kg-min, and maximal power output (Wmax) was 360 Watts (determined from a test in which the athletes started cycling at 95 Watts for three minutes and then increased the intensity by 35 Watts every three minutes until they could no longer continue).

All of the athletes completed four different exercise trials, separated by at least three days; diet and activity patterns were standardized between trials, and each bout of exercise was completed at the same time of day. 45 minutes prior to the start of each exercise session, the athletes consumed one of four different drinks. One solution contained 25 grams of glucose (LOW), another had 75 glucose grams (MED), a third potable possessed 200 grams (HIGH), and the fourth carried only a glucose-free, artificially sweetened placebo (Saccharine®); these quantities of glucose or placebo were combined with distilled water to attain a volume of 500 ml (about 17 ounces). To make the drinks similar in taste, 100 ml of a calorie-free liquid orange flavor were added to each beverage.

The exercise itself (which commenced 45 minutes after the drink was consumed) consisted of 20 minutes of relatively easy exertion at an intensity of 65% Wmax (72% VO2max), followed by a 40-minute time trial in which the athletes attempted to cycle as powerfully as possible.

Somewhat surprisingly, plasma glucose concentrations were not significantly different at the beginning of the exercise periods, despite the sizable difference in carbohydrate intake 45 minutes earlier; blood glucose held steady at about 4.5 to 5.0 mmol/liter, exactly the concentration observed before the drinks were quaffed in the first place. However, for LOW, MED, and HIGH, plasma glucose fell rapidly to about 3.6 mmol/liter within the first 15 minutes of exercise (there was no such plummet with placebo). After that, glucose began a steady climb, until it reached – at the end of the time trial - a level a bit higher than the average concentration at rest for all three glucose-ingesting groups.

Plasma insulin concentrations, which were the same in all four groups prior to drink consumption, were highest in MED and HIGH when exercise began and remained significantly above LOW and placebo after 15 minutes of cycling (there was no difference between LOW and placebo, both of which basically did not change from the pre-drink stage of the experiment). However, insulin in the MED and HIGH groups then went on a steady downslide, and by the time the time trial began (after 20 minutes of 65%-Wmax cycling) insulin levels were actually the same in all four groups.

Blood-lactate levels climbed to about 7.5 mmol/liter during the 40-minute time trials, but there were no differences between groups (one might expect lactate to be greater in the HIGH group if the ample carb consumption increased carbohydrate breakdown, for example). In addition, the power outputs during the time trials were not different between groups. Consuming varying amount of carbohydrate 45 minutes prior to exercise was not associated with different levels of performance, nor was such carbohydrate consumption better than the ingestion of a carb-free placebo.

The lessons? Bear in mind that the following conclusions apply to the kind of exercise utilized in this study – exercise which is about 60 minutes in duration, with 40 minutes or so at high intensity (like a warm-up followed by a race of approximately 40 minutes, for example). First, when carbohydrate is ingested in varying amounts 45 minutes before exercise, it does lead to hypoglycemia during the first 15 minutes of exercise, but this hypoglycemia is steadily reversed as exercise proceeds and has no negative impact on performance. Second, ample amounts of pre-exercise carbohydrate (75 to 200 grams) do yank insulin levels up at the start of exercise, but exercise itself mutes this insulin response so that after 20 minutes or so of effort there is no discernible difference between individuals taking in varying amount of carbs – nor between carb-takers and athletes who consume no carbs at all. Finally, consumption of carbs 45 minutes prior to exertion seems to provide no performance boost during 40 minutes of intense exercise.

Why didn’t the carb-related hypoglycemia hurt performance in this study? For one thing, it was only present during the 20-minute, “warm-up” period, not during the actual time trial. For another, the fatigue-linked hypoglycemia which can occur during very prolonged exercise (during a marathon, for example, when a runner fails to use sports drinks properly) is usually associated with a steep drop in liver and muscle glycogen stores; in such a case, the leg muscles are simply running out of fuel, and fatigue is a natural consequence (8). In this study, the hypoglycemia was not linked with low muscle glycogen but was simply a consequence of high insulin and rapid uptake of glucose from the blood, factors which should not directly cause fatigue (rather, the rapid uptake should do the reverse). Remember, though, that some individuals seem to be quite sensitive to low blood-glucose levels; if that is true for you, you should probably avoid carbohydrate intake during the hour before you exercise.

Are there some instances in which carbohydrate taken during the hour before exercise might aid performance? If you have been following a carbohydrate-poor diet and your muscle-glycogen levels are low, last-minute carbs might provide your muscles with some helpful fuel and keep you going a bit longer. In addition, if your exercise is longer than an hour, the last-second carbs might simply be a way of getting more carbohydrate into your body before the exertion begins. However, such carbs would not have to be taken in 45 minutes before exercise; they could be consumed two to three hours before, for example, which would – for many athletes, and especially for runners – increase gastrointestinal comfort during exertion. Please remember that there is nothing magical about the carbs taken during the hour before exercise, except for one situation: Efforts lasting longer than 45 minutes or so, during which carbs taken in while exercising will boost performance. In such cases, it is optimal to have a small bolus of carb-containing drink in the stomach before exercise begins – to increase the gastric-emptying rate and therefore the absorption rate of carbs. This can be accomplished by imbibing about eight to 10 ounces of sports drink 10 minutes before exercise begins, a practice which will not cause potentially worrisome tidal waves of plasma insulin – or vanishing acts by blood glucose. ©

References

(1) “The Influence of Pre-Exercise Glucose Ingestion on Endurance Running Capacity,” British Journal of Sports Medicine, Vol. 28, pp. 105-109, 1994
(2) “Effects of Elevated Plasma FFA and Insulin on Muscle Glycogen Use during Exercise,” Journal of Applied Physiology, Vol. 43, pp. 695-699, 1977
(3) “Pre-Exercise Sugar Feeding Does Not Alter Prolonged Exercise Muscle Glycogen or Protein Catabolism,” Canadian Journal of Applied Physiology, Vol. 22, pp. 268-279, 1997
(4) “A Moderate Glycemic Meal before Endurance Exercise Can Enhance Performance,” Journal of Applied Physiology, Vol. 84, pp. 53-59, 1998
(5) “Pre-Exercise Carbohydrate Ingestion, Glucose Kinetics, and Muscle Glycogen Use: Effect of the Glycemic Index,” Journal of Applied Physiology, Vol. 89, pp. 1845-1851, 2000
(6) “Pre-Exercise Snacks May Decrease Exercise Performance,” Physician and Sports Medicine, Vol. 12, pp. 89-91, 1984
(7) “Effects of Pre-Exercise Ingestion of Differing Amounts of Carbohydrate on Subsequent Metabolism and Cycling Performance,” European Journal of Applied Physiology, Vol. 88, pp. 444-452, 2003
(8) “The Effect of Pre-Exercise Feeding on Endurance Exercise Performance,” New Zealand Journal of Sports Medicine, Vol. 26, pp. 34-37, 1998

To learn about Owen-Anderson's running camps in California, please send a note to Owen at owen@rrnews.com.

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