Review articleCarbohydrate intake during exercise and performance
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-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, 10, 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, 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.
Section snippets
CHO during exercise and performance
Although early studies5, 12 had suggested a role for hypoglycemia in the development of fatigue, when researchers started to study this in more detail in the early 1980s they initially could not confirm a role for hypoglycemia.10 There did not seem to be a clear relation between hypoglycemia and performance, and the effects of CHO feeding on perceptions of effort and general fatigue were inconsistent.11 These findings were also consistent with recent studies on rebound hypoglycemia.13, 14, 15,
The minimal amount of CHO needed
Mitchell et al.40 observed that 12 min of isokinetic time trial performance was enhanced at the end of 2 h of intermittent exercise. The improvements were similar with ingestion of 34, 39, or 50 g of CHO per hour compared with a water trial. Based on a study by Fielding et al.,17 it is usually believed that a minimum of 22 g of CHO per hour is required to observe a performance benefit. In that study subjects exercised for 4 h and performed a sprint at the end. Performance improvements were
Form of CHO
The form in which the CHO is provided during exercise (solid or liquid) does not seem to affect the ergogenic potential. Hargreaves et al.18 studied the effects of ingestion of a candy bar (43 g of CHO, 9 g of fat, and 3 g of protein) and observed a 46% improvement in sprint capacity after 4 h of exercise compared with placebo ingestion. Others confirmed these findings and reported that liquid and solid CHO feedings improved exercise performance to a similar degree.43, 44
More recently, Murdoch
Critical analysis
It must be noted that most of the early studies were performed after an overnight fast. This means that the subjects started the exercise with suboptimal glycogen stores, and it has been shown that after an overnight fast liver glycogen stores may be considerably reduced.46, 47 It seems obvious that exogenous CHO would have an effect in these conditions because it can provide an alternative substrate to compensate for the reduced endogenous CHO availability. Whether CHO feeding can also improve
Mechanism by which CHO feeding improves performance
There are several mechanisms by which CHO feeding during exercise may improve endurance performance. These include maintaining blood glucose and high levels of CHO oxidation, sparing endogenous glycogen, synthesizing glycogen during low-intensity exercise, or a central effect of CHO. The mechanisms may be different for relatively short-duration (∼1 h) high-intensity exercise (80% to 85% of Vo2max) than for long-duration (>2 h) low- to moderate-intensity exercise (60% to 75% of Vo2max).
Coyle et
Oxidation of ingested CHO
Several factors have been suggested to influence exogenous CHO oxidation including feeding schedule, type and amount of CHO ingested, and exercise intensity, and these have been intensively investigated (Figure 1). Some of these factors have only small effects and other factors have major effects on exogenous CHO oxidation. For example, the timing of CHO ingestion seems to have relatively little effect on exogenous CHO oxidation rates. Studies in which a large bolus (100 g) of a CHO in
Bioavailability of ingested CHO
The results of studies with different dosages of CHO suggest that with increasing intake the bioavailability does not necessarily increase. Several factors may reduce the bioavailability of ingested CHO, including gastric emptying and intestinal absorption. It has also been suggested that the liver plays an important role and that muscle glucose uptake could be a limiting factor.
There is, however, accumulating evidence that gastric emptying is not an important limitation to exogenous CHO
Importance of high exogenous CHO oxidation rates
A greater contribution of exogenous (external) fuel sources (CHO) will spare endogenous sources (liver and possibly muscle glycogen in some conditions), and it is tempting to believe that a greater contribution from exogenous sources will increase endurance capacity and/or exercise performance. Although many studies (including our own) are based on this assumption, the evidence for this is lacking. To our knowledge no studies have demonstrated that ingesting larger amounts of CHO that will
References (113)
- et al.
Fuel substrate kinetics of carbohydrate loading differs from that of carbohydrate ingestion during prolonged exercise
Metabolism
(1996) Fluid and fuel intake during exercise
J Sports Sci
(2004)- et al.
Pre-exercise carbohydrate and fat ingestioneffects on metabolism and performance
J Sports Sci
(2004) Joint position statement. Nutrition and athletic performance
Med Sci Sports Exerc
(2000)- et al.
The relative value of fat and carbohydrate as sources of muscular energy
Bioch J
(1920) - et al.
Some changes in chemical constituents of blood following a marathon race
JAMA
(1924) Der Stoffwechsel und die Respiratorischen Funktionen bei schwerer körperlicher Arbeit
Scand Arch Physiol
(1932)- et al.
Muscle glycogen synthesis after exercisean enhancing factor localized in muscle cells in man
Nature
(1966) - et al.
A study of glycogen metabolism during exercise in man
Scand J Clin Invest
(1967) - et al.
Glucose ingestion before and during intense exercise
J Appl Physiol
(1981)
Hypoglycemia during prolonged exercise in normal men
N Engl J Med
Influence of caffeine and carbohydrate feedings on endurance performance
Med Sci Sports
Arbeitsfähigkeit und ernährung
Scand Arch Physiol
Prevalence of hypoglycemia following pre-exercise carbohydrate ingestion is not accompanied by higher insulin sensitivity
Int J Sport Nutr Exerc Metab
Effects of pre-exercise ingestion of trehalose, galactose and glucose on subsequent metabolism and cycling performance
Eur J Appl Physiol
The effect of timing of pre-exercise carbohydrate feedings on metabolism and cycling performance
Med Sci Sports Exerc
Effect of carbohydrate feeding frequencies and dosage on muscle glycogen use during exercise
Med Sci Sports Exerc
Effect of carbohydrate feedings on muscle glycogen utilisation and exercise performance
Med Sci Sports Exerc
Effects of carbohydrate ingestion on gastric emptying and exercise performance
Med Sci Sports exerc
Improvements in exercise performanceeffects of carbohydrate feedings and diet
J Appl Physiol
Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate
J Appl Physiol
Carbohydrate feeding during prolonged strenuous exercise
J Appl Physiol
Endurance improved by ingestion of a glucose polymer supplement
Med Sci Sports Exerc
Influence of glucose and fructose ingestion on the capacity for long term exercise in well trained men
Clin Physiol
Carbohydrate feeding and exerciseeffect of beverage carbohydrate content
Eur J Appl Physiol
Effect of sucrose and caffeine ingestion on performance of prolonged strenuous running
Int J Sports Med
Influence of selected carbohydrate drinks on cycling performance and glycogen use
Med Sci Sports Exerc
Effects of glucose, glucose plus branched-chain amino acids, or placebo on bike performance over 100 km
J Appl Physiol
Carbohydrate-electrolyte feedings improve 1 h time trial cycling performance
Int J Sports Med
Effects of carbohydrate supplementation on performance during 1 h of high intensity exercise
Int J Sports Med
Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise
Med Sci Sports Exerc
Carbohydrate supplementation improves moderate and high-intensity exercise in the heat
Pflugers Arch
Carbohydrate ingestion improves endurance performance during a 1 h simulated time trial
J Sports Sci
Placebo effect of carbohydrate feedings during a 40-km cycling time trial (in process citation)
Med Sci Sports Exerc
Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise
J Appl Physiol
Fluid replacement drinks during high intensity exerciseeffects on minimizing exercise-induced disturbances in homeostasis
Eur J Appl Physiol
Carbohydrate ingestion immediately before exercise does not improve 20km time trial performance in well trained cyclists
Int J Sports Med
Effect of carbohydrate or carbohydrate plus medium-chain triglyceride ingestion on cycling time trial performance
J App Physiol
Reduced oxidation rates of orally ingested glucose during exercise after low CHO intake and low muscle glycogen
J Appl Physiol
Influence of carbohydrate dosage on exercise performance and glycogen use
J Appl Physiol
Effects of ingested fluids on exercise capacity and on cardiovascular and metabolic responses to prolonged exercise in man
Exp Physiol
Nutritional manipulations before and during endurance exerciseeffects on performance
Med Sci Sports Exerc
Metabolic responses when different forms of carbohydrate energy are consumed during cycling
Int J Sport Nutr
Effects of exercise and carbohydrate composition on gastric emptying
Med Sci Sports Exerc
Differences in the effects of carbohydrate food form on endurance performance to exhaustion
Int J Sports Nutrition
Liver glycogen in man: effects of different diets and muscular exercise
Liver glycogen in man; the effects of total starvation or a carbohydrate-poor diet followed by carbohydrate feeding
Scand J Clin Lab Invest
Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise
J Appl Physiol
Glucose kinetics during prolonged exercise in highly trained human subjectseffect of glucose ingestion
J Physiol (Lond)
Cited by (361)
Nutritional Considerations for the Vegan Athlete
2023, Advances in NutritionNutrition in Cycling
2022, Physical Medicine and Rehabilitation Clinics of North AmericaMind the gap: Limited knowledge of carbohydrate guidelines for competition in an international cohort of endurance athletes
2023, Journal of Nutritional ScienceThe Mediterranean dietary pattern for optimising health and performance in competitive athletes: A narrative review
2022, British Journal of Nutrition