Elsevier

Nutrition

Volume 20, Issues 7–8, July–August 2004, Pages 669-677
Nutrition

Review article
Carbohydrate intake during exercise and performance

https://doi.org/10.1016/j.nut.2004.04.017Get rights and content

Abstract

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 higher-intensity 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.

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)

  • A.N. Bosch et al.

    Fuel substrate kinetics of carbohydrate loading differs from that of carbohydrate ingestion during prolonged exercise

    Metabolism

    (1996)
  • E.F. Coyle

    Fluid and fuel intake during exercise

    J Sports Sci

    (2004)
  • M. Hargreaves 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)
  • A. Krogh et al.

    The relative value of fat and carbohydrate as sources of muscular energy

    Bioch J

    (1920)
  • S.A. Levine et al.

    Some changes in chemical constituents of blood following a marathon race

    JAMA

    (1924)
  • E.H. Christensen

    Der Stoffwechsel und die Respiratorischen Funktionen bei schwerer körperlicher Arbeit

    Scand Arch Physiol

    (1932)
  • J. Bergstrom et al.

    Muscle glycogen synthesis after exercisean enhancing factor localized in muscle cells in man

    Nature

    (1966)
  • J. Bergstrom et al.

    A study of glycogen metabolism during exercise in man

    Scand J Clin Invest

    (1967)
  • A. Bonen et al.

    Glucose ingestion before and during intense exercise

    J Appl Physiol

    (1981)
  • P. Felig et al.

    Hypoglycemia during prolonged exercise in normal men

    N Engl J Med

    (1982)
  • J.L. Ivy et al.

    Influence of caffeine and carbohydrate feedings on endurance performance

    Med Sci Sports

    (1979)
  • E.H. Christensen et al.

    Arbeitsfähigkeit und ernährung

    Scand Arch Physiol

    (1939)
  • Jentjens RL, Venables MC, Jeukendrup AE. Oxidation of exogenous glucose, sucrose and maltose during prolonged cycling...
  • R.L. Jentjens et al.

    Prevalence of hypoglycemia following pre-exercise carbohydrate ingestion is not accompanied by higher insulin sensitivity

    Int J Sport Nutr Exerc Metab

    (2002)
  • R.L. Jentjens et al.

    Effects of pre-exercise ingestion of trehalose, galactose and glucose on subsequent metabolism and cycling performance

    Eur J Appl Physiol

    (2003)
  • L. Moseley et al.

    The effect of timing of pre-exercise carbohydrate feedings on metabolism and cycling performance

    Med Sci Sports Exerc

    (2001)
  • R.A. Fielding et al.

    Effect of carbohydrate feeding frequencies and dosage on muscle glycogen use during exercise

    Med Sci Sports Exerc

    (1985)
  • M. Hargreaves et al.

    Effect of carbohydrate feedings on muscle glycogen utilisation and exercise performance

    Med Sci Sports Exerc

    (1984)
  • J.B. Mitchell et al.

    Effects of carbohydrate ingestion on gastric emptying and exercise performance

    Med Sci Sports exerc

    (1988)
  • P.D. Neufer et al.

    Improvements in exercise performanceeffects of carbohydrate feedings and diet

    J Appl Physiol

    (1987)
  • E.F. Coyle et al.

    Muscle glycogen utilization during prolonged strenuous exercise when fed carbohydrate

    J Appl Physiol

    (1986)
  • E.F. Coyle et al.

    Carbohydrate feeding during prolonged strenuous exercise

    J Appl Physiol

    (1983)
  • J.L. Ivy et al.

    Endurance improved by ingestion of a glucose polymer supplement

    Med Sci Sports Exerc

    (1983)
  • O. Bjorkman et al.

    Influence of glucose and fructose ingestion on the capacity for long term exercise in well trained men

    Clin Physiol

    (1984)
  • R. Murray et al.

    Carbohydrate feeding and exerciseeffect of beverage carbohydrate content

    Eur J Appl Physiol

    (1989)
  • H. Sasaki et al.

    Effect of sucrose and caffeine ingestion on performance of prolonged strenuous running

    Int J Sports Med

    (1987)
  • M.G. Flynn et al.

    Influence of selected carbohydrate drinks on cycling performance and glycogen use

    Med Sci Sports Exerc

    (1987)
  • K. Madsen et al.

    Effects of glucose, glucose plus branched-chain amino acids, or placebo on bike performance over 100 km

    J Appl Physiol

    (1996)
  • A. Jeukendrup et al.

    Carbohydrate-electrolyte feedings improve 1 h time trial cycling performance

    Int J Sports Med

    (1997)
  • R. Anantaraman et al.

    Effects of carbohydrate supplementation on performance during 1 h of high intensity exercise

    Int J Sports Med

    (1995)
  • P.R. Below et al.

    Fluid and carbohydrate ingestion independently improve performance during 1 h of intense exercise

    Med Sci Sports Exerc

    (1995)
  • J. Carter et al.

    Carbohydrate supplementation improves moderate and high-intensity exercise in the heat

    Pflugers Arch

    (2003)
  • M.S. el-Sayed et al.

    Carbohydrate ingestion improves endurance performance during a 1 h simulated time trial

    J Sports Sci

    (1997)
  • V.R. Clark et al.

    Placebo effect of carbohydrate feedings during a 40-km cycling time trial (in process citation)

    Med Sci Sports Exerc

    (2000)
  • G.K. McConell et al.

    Effect of carbohydrate ingestion on glucose kinetics and muscle metabolism during intense endurance exercise

    J Appl Physiol

    (2000)
  • S.K. Powers et al.

    Fluid replacement drinks during high intensity exerciseeffects on minimizing exercise-induced disturbances in homeostasis

    Eur J Appl Physiol

    (1990)
  • G.S. Palmer et al.

    Carbohydrate ingestion immediately before exercise does not improve 20km time trial performance in well trained cyclists

    Int J Sports Med

    (1998)
  • D.J. Angus et al.

    Effect of carbohydrate or carbohydrate plus medium-chain triglyceride ingestion on cycling time trial performance

    J App Physiol

    (2000)
  • A.E. Jeukendrup et al.

    Reduced oxidation rates of orally ingested glucose during exercise after low CHO intake and low muscle glycogen

    J Appl Physiol

    (1996)
  • J.B. Mitchell et al.

    Influence of carbohydrate dosage on exercise performance and glycogen use

    J Appl Physiol

    (1989)
  • R.J. Maughan et al.

    Effects of ingested fluids on exercise capacity and on cardiovascular and metabolic responses to prolonged exercise in man

    Exp Physiol

    (1996)
  • A.R. Coggan et al.

    Nutritional manipulations before and during endurance exerciseeffects on performance

    Med Sci Sports Exerc

    (1992)
  • M. Lugo et al.

    Metabolic responses when different forms of carbohydrate energy are consumed during cycling

    Int J Sport Nutr

    (1993)
  • P.D. Neufer et al.

    Effects of exercise and carbohydrate composition on gastric emptying

    Med Sci Sports Exerc

    (1986)
  • S.D. Murdoch et al.

    Differences in the effects of carbohydrate food form on endurance performance to exhaustion

    Int J Sports Nutrition

    (1993)
  • E. Hultman et al.

    Liver glycogen in man: effects of different diets and muscular exercise

  • L.H. Nilsson et al.

    Liver glycogen in man; the effects of total starvation or a carbohydrate-poor diet followed by carbohydrate feeding

    Scand J Clin Lab Invest

    (1973)
  • A.N. Bosch et al.

    Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise

    J Appl Physiol

    (1994)
  • A.E. Jeukendrup et al.

    Glucose kinetics during prolonged exercise in highly trained human subjectseffect of glucose ingestion

    J Physiol (Lond)

    (1999)
  • Cited by (359)

    • Nutrition in Cycling

      2022, Physical Medicine and Rehabilitation Clinics of North America
    View all citing articles on Scopus
    View full text