Fueling During Exercise

Asker Jeukendrup, PhD, FACSM
Professor of Exercise Metabolism
School of Sport and Exercise Sciences
University of Birmingham
United Kingdom

KEY POINTS

• Carbohydrate intake during exercise can delay the onset of fatigue, improve endurance capacity and exercise performance during prolonged (>2h) exercise.
• Carbohydrate feeding may also improve exercise performance during exercise of shorter duration and higher intensity (approximately 1 h) although the mechanisms are different.
• Small amounts of carbohydrate already have an effect on performance. It is not clear if there is a dose response relationship but emerging evidence suggests that a greater contribution of ingested carbohydrate may result in better performance.
• A single carbohydrate can only be oxidized at rates up to 60 grams per hour.
• New research at the University of Birmingham has shown that when a combination of carbohydrates is used (i.e. glucose and fructose) oxidation rates can increase by 75% up to 105 grams per hour if large amounts of carbohydrate are ingested.
• The ingestion of energy dense carbohydrate solutions will slow fluid delivery and can lead to GI discomfort in some individuals. Impairment of fluid delivery is reduced and GI distress less likely when combinations of carbohydrate are ingested.
• The amount of carbohydrate to be ingested for performance improvement is best individually determined and a balance should be struck between increasing carbohydrate availability during exercise and minimizing gastrointestinal distress.

Oxidation of ingested carbohydrate
Several factors have been suggested to influence exogenous carbohydrate oxidation including feeding schedule, type and amount of carbohydrate ingested and the exercise intensity and these have been intensively investigated. Some of these factors have only small effects other factors major effects on exogenous carbohydrate oxidation. For example, the timing of feeding, the training status of the subjects ⁴, gender ⁵ and the exercise intensity have relatively small effects. More important seems to be the type of carbohydrate and the amount ingested. Some types of carbohydrate are oxidized more readily than others ^6,7. Roughly they can be divided into two categories: carbohydrates that can be oxidized at rates up to 60 g/h (glucose, maltodextrin, sucrose, and maltose) and carbohydrates that can be oxidized at rates up to 30 g/h (fructose, galactose, amylose).

From a large number of studies in the literature it can be concluded that oxidation of orally ingested carbohydrate (from a single source) may already be optimal at ingestion rates around 60-70 g/h and will not exceed 60 g/h. Ingesting more than this will not further increase carbohydrate oxidation rates and is more likely to be associated with gastro-intestinal discomfort. This has therefore been the general guideline: ingest up to 60 g of carbohydrate per hour.

FIGURE 1. Oxidation of multiple transportable carbohydrates. This figure is compiled from a large number of studies investigating exogenous carbohydrate during exercise ^8-15. The grey bars represent the rate of carbohydrate intake in these studies (expressed in grams per minute). The colored bars represent the peak oxidation rates observed after 90-150 min of exercise. Glucose oxidation in these studies peaked at 0.83 g/min. However if multiple transportable carbohydrates were ingested at high rates (up to 2.4 g/min) very high rates of exogenous carbohydrate oxidation were achieved (up to 1.75 g/min).

Multiple transportable carbohydrates
The reason that exogenous carbohydrate oxidation is limited to approximately 60 grams per hour is most likely due to intestinal absorption (for review and detailed discussions see 7). It is suggested that by feeding a single carbohydrate source (for example glucose or maltodextrin) at high rates, the sodium dependent glucose transporters (SGLT1) become saturated. Once these transporters are saturated feeding more of that carbohydrate will not result in greater absorption and increased oxidation rates. By using an additional carbohydrate which uses a different transporter for absorption (i.e., fructose uses the glucose transporter GLUT5), the total amount of carbohydrate absorbed can be increased and fuel supply to the muscle is enhanced.

In one of the first studies we observed an increase in oxidation by 45% of a drink containing glucose:fructose (2:1) compared with a similar amount of glucose! In the following years we tried different combinations of carbohydrates and also different amounts in an attempt to see what the maximal contribution could be of exogenous carbohydrate ^8-15. We observed that very high oxidation rates were reached with combinations of glucose+fructose, maltodextrin+fructose and glucose+sucrose+fructose. The highest rates were observed with a mixture of glucose and fructose ingested at a rate of 144 g/h. With this feeding regimen exogenous CHO oxidation peaked at 105 g/h! This is 75% higher than what was previously though to be the absolute maximum! These high intake rates may not always be practical but the findings are convincing enough to tweak the old guidelines. The new guideline for carbohydrate intake is therefore: ingest a blend of carbohydrates and increase the intake up to 75-125 g/h.

Performance
The increased oxidation of ingested carbohydrate has been suggested to be beneficial for performance but concrete evidence for this has not yet been published. From a laboratory study in which subjects cycled for 5 hours with water, glucose or glucose+fructose there are some indications that drinks with multiple carbohydrates could improve performance ¹⁴. In this study carbohydrate was ingested at a rate of 90 g/h. The first indication of improved performance was that subjects’ rating of their perceived exertion tended to be lower with glucose+fructose compared with glucose, which in turn was lower than water placebo. In fact with water not all participants were able to complete the 5 h at 50%VO₂max. In addition, the self selected cadence dropped significantly with water, which is generally seen as an indication of developing fatigue. With glucose this was somewhat prevented but with glucose+fructose cadence was highest and remained almost unchanged from the beginning of exercise ¹⁴. We have since performed a study to confirm the beneficial effects of glucose+fructose drinks compared with glucose on prolonged exercise performance (Currell et al unpublished findings).

Gastro-intestinal discomfort during exercise
Drinks containing carbohydrates that use different transporters for intestinal absorption seem to result in a smaller amount of carbohydrate remaining in the intestine and therefore osmotic shifts and malabsorption may be reduced. This probably means that drinks with multiple transportable carbohydrates are less likely to cause gastro-intestinal distress. Interestingly, this is a consistent finding in studies that have attempted to register gastro-intestinal discomfort during exercise carbohydrate ^8-15. Subjects tended to feel less bloated with the glucose+fructose drinks versus glucose drinks. The tolerance of carbohydrate drinks and development of GI distress seems highly individual and therefore strategies for carbohydrate intake will always have to be developed on an individual basis.

Carbohydrate and fluid delivery
Often it is advised to avoid the intake of highly concentrated carbohydrate solutions because these solutions have been shown to delay gastric emptying and fluid absorption. Although there is a lot of evidence to support this there are also observations that impairment of fluid delivery is minimized when combinations of multiple transportable carbohydrate are ingested. Fluid delivery with a glucose+fructose solution has been shown to be greater than fluid delivery from a glucose solution ¹³.

SUMMARY
In summary, carbohydrate intake during exercise can improve exercise performance especially during prolonged exercise. An increased contribution of the ingested carbohydrate to the energy supply is generally believed to be beneficial. The maximal contribution of a single carbohydrate is approximately 60 grams per hour. New research at the University of Birmingham, however, has shown that when a combination of carbohydrates is used (i.e. glucose and fructose) oxidation rates can increase by 75% up to 105 grams per hour if large amounts of carbohydrate are ingested. These mixtures also seem to help fluid delivery and reduce GI distress compared with a single carbohydrate. The amount of carbohydrate to be ingested for performance improvement is best individually determined and a balance should be struck between increasing carbohydrate availability during exercise and minimizing gastro-intestinal distress.

REFERENCES

1. Carter, J.M., Jeukendrup, A.E., Mann, C.H. & Jones, D.A. The effect of glucose infusion on glucose kinetics during a 1-h time trial. Med Sci Sports Exerc 36, 1543-1550 (2004).
2. Carter, J.M., Jeukendrup, A.E. & Jones, D.A. The effect of carbohydrate mouth rinse on 1-h cycle time trial performance. Med Sci Sports Exerc 36, 2107-2111 (2004).
3. Jeukendrup, A.E., Brouns, F., Wagenmakers, A.J.M. & Saris, W.H.M. Carbohydrate feedings improve 1 h time trial cycling performance. Int J Sports Med 18, 125-129 (1997).
4. Jeukendrup, A.E., Mensink, M., Saris, W.H.M. & Wagenmakers, A.J.M. Exogenous glucose oxidation during exercise in endurance-trained and untrained subjects. J Appl Physiol 82, 835-840 (1997).
5. Wallis, G.A., Dawson, R., Achten, J., Webber, J. & Jeukendrup, A.E. Metabolic response to carbohydrate ingestion during exercise in males and females. Am J Physiol Endocrinol Metab 290, E708-715 (2006).
6. Jeukendrup, A.E. & Jentjens, R. Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med 29, 407-424. (2000).
7. Jeukendrup, A.E. Carbohydrate intake during exercise and performance. Nutrition 20, 669-677 (2004).
8. Jentjens, R.L., Venables, M.C. & Jeukendrup, A.E. Oxidation of exogenous glucose, sucrose, and maltose during prolonged cycling exercise. J Appl Physiol 96, 1285-1291 (2004).
9. Jentjens, R.L., Moseley, L., Waring, R.H., Harding, L.K. & Jeukendrup, A.E. Oxidation of combined ingestion of glucose and fructose during exercise. J Appl Physiol 96, 1277-1284 (2004).
10. Jentjens, R.L., Achten, J. & Jeukendrup, A.E. High oxidation rates from combined carbohydrates ingested during exercise. Med Sci Sports Exerc 36, 1551-1558 (2004).
11. Jentjens, R.L. & Jeukendrup, A.E. High rates of exogenous carbohydrate oxidation from a mixture of glucose and fructose ingested during prolonged cycling exercise. Br J Nutr 93, 485-492 (2005).
12. Jentjens, R.L., et al. Oxidation of combined ingestion of glucose and sucrose during exercise. Metabolism 54, 610-618 (2005).
13. Jentjens, R.L., et al. Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat. J Appl Physiol 100, 807-816 (2006).
14. Jeukendrup, A.E., et al. Exogenous carbohydrate oxidation during ultraendurance exercise. J Appl Physiol 100,1134-1141 (2006).
15. Wallis, G.A., Rowlands, D.S., Shaw, C., Jentjens, R.L. & Jeukendrup, A.E. Oxidation of combined ingestion of maltodextrins and fructose during exercise. Med Sci Sports Exerc 37, 426-432 (2005).

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