REVIEW ARTICLE Carbohydrate Intake During Exercise and Performance Asker E Jeukendrup PhD From the Human Performance Laboratory School of Sport and Exercise Sciences University of Birmingham Edgbaston Birmingham United Kingdom It is generally accepted that carbohydrate CHO feeding during exercise can improve endurance capacity time to exhaustion and exercise performance during prolonged exercise 2 h More recently studies have also shown ergogenic effects of CHO feeding during shorter exercise of high intensity 1 h at 75 of maximum oxygen consumption During prolonged exercise the mechanism behind this performance improvement is likely to be related to maintenance of high rates of CHO oxidation and the prevention of hypoglycemia Nevertheless other mechanisms may play a role depending on the type of exercise and the specific conditions The mechanism for performance improvements during higherintensity exercise is less clear but there is some evidence that CHO can have central effects In the past few years studies have investigated ways to optimize CHO delivery and bioavailability An analysis of all studies available shows that a single CHO ingested during exercise will be oxidized at rates up to about 1 g min even when large amounts of CHO are ingested Combinations of CHO that use different intestinal transporters for absorption e g glucose and fructose have been shown to result in higher oxidation rates and this seems to be a way to increase exogenous CHO oxidation rates by 20 to 50 The search will continue for ways to further improve CHO delivery and to improve the oxidation efficiency resulting in less accumulation of CHO in the gastrointestinal tract and potentially decreasing gastrointestinal problems during prolonged exercise Nutrition 2004 20 669 677 Elsevier Inc 2004 KEY WORDS carbohydrate feeding exercise performance exogenous oxidation carbohydrate absorption INTRODUCTION Whereas 100 y ago beef protein was believed to be the most important component of an athlete s diet nowadays it seems to be pasta carbohydrate CHO Athletes are often advised to eat a high CHO diet consume CHO before exercise ensure adequate CHO intake during exercise and replenish CHO stores as soon as possible after exercise In the most recent position statement of the International Olympic Committee IOC on nutrition for athletes it was stated A high carbohydrate diet in the days before competition will help enhance performance particularly when exercise lasts longer than about 60 min and Athletes should aim to achieve carbohydrate intakes that meet the fuel requirements of their training programs and also adequately replace their carbohydrate stores during recovery between training sessions and competition This can be achieved when athletes eat carbohydrate rich snacks and meals that also provide a good source of protein and other nutrients These recommendations have also been discussed in detail in reviews resulting from this IOC consensus meeting in 2003 1 2 CHO also played a central role in a joint position statement3 of the American College of Sports Medicine the American Dietetic Association and the Canadian Dietetic Association on nutrition for athletic performance and several recommendations were made specifically for CHO Research on the effects of CHO feeding before and during exercise has accumulated since the beginning of the 20th century Krogh and Lindhardt4 were probably the first to recognize the importance of CHO as a fuel source during exercise They reported that subjects found exercise easier if they had consumed a CHO Correspondence to Asker E Jeukendrup PhD Human Performance Laboratory School of Sport and Exercise Sciences University of Birmingham Edgbaston Birmingham B15 2TT UK E mail a e jeukendrup bham ac uk Nutrition 20 669 677 2004 Elsevier Inc 2004 Printed in the United States All rights reserved rich diet compared with a high fat diet and this was accompanied by higher respiratory exchange ratios during exercise Important observations were also made by Levine et al 5 who measured blood glucose in some of the participants after the 1923 Boston Marathon They found that most runners had reduced blood glucose concentrations after the race Levine et al 5 suggested that these low blood glucose levels were a cause of fatigue To test that hypothesis they encouraged several participants of the same marathon 1 y later to consume CHO during the race 5 This practice in combination with a high CHO diet before the race prevented hypoglycemia and significantly improved running performance i e time to complete the race In 1932 Christensen6 showed that with increasing exercise intensity the proportion of CHO utilized increased This work was expanded in the late 1960s with the reintroduction of the muscle biopsy technique by a group of Scandinavian scientists 7 8 These studies indicated for the first time the critical role of muscle glycogen The improved performance after a high CHO diet was linked with the higher muscle glycogen concentrations observed after such a diet A high CHO diet 70 of dietary energy from CHO and elevated muscle glycogen stores seemed to enhance endurance capacity compared with a normal 50 and a low 10 CHO diet In the late 1970s and early 1980s the effects of CHO feeding during exercise on exercise performance and metabolism was further investigated 9 11 In the following years more and more studies provided evidence of an ergogenic effect of CHO ingested during exercise and slowly this practice of consuming CHO during exercise became a habit in many sports especially endurance sports During the 1980s so called sports drinks became commercially available Now CHO drinks are deeply embedded in the culture of endurance sports Despite the general acceptance of the ergogenic effects of CHO supplementation during exercise there is a need to evaluate the existing evidence critically because some of the results may have been exaggerated by the choice of the experimental protocols 0899 9007 04 30 00 doi 10 1016 j nut 2004 04 017 670 Jeukendrup which were not always comparable to the situation of competition This review discusses the effects of CHO on endurance capacity and endurance performance when ingested during exercise and the underlying mechanisms for the observed performance effects The second part of this review discusses ways to improve the bioavailability of CHO and directions for future research CHO DURING EXERCISE AND PERFORMANCE Although early studies5 12 had suggested