Studies have convincingly shown that carbohydrate delivery during prolonged exercise can help performance. It is also clear that it is an advantage to absorb more carbohydrate and leave less unabsorbed carbohydrate in the intestine. This has two advantages: 1. carbohydrate accumulating in the gut might cause gastro-intestinal problems and thus this risk would be reduced. 2. delivering more carbohydrate may mean that performance improves more. There are several lines of evidence for this:
First, a meta-analysis of the literature showed a dose-response relationship. Secondly, at least 2 carefully conducted experiments demonstrate a dose response relationship between carbohydrate intake and performance. Thirdly, studies with multiple transportable carbohydrates, ingested at high rates, show that an increased delivery and oxidation of carbohydrates is associated with improved performance. Furthermore, there is evidence from events like Ironman Hawaii that higher intake is correlated with faster times (read: Is more carbohydrate better? And how much is too much?).
It has also been shown that the main barrier to better carbohydrate delivery is absorption. From this, we can conclude that anything that improves carbohydrate absorption could potentially improve performance in endurance events. This is where caffeine may come in.
Many years ago I was intrigued by a study that suggested that caffeine could improve intestinal absorption of carbohydrates (1). The study used markers of intestinal integrity and suggested that caffeine improved carbohydrate absorption. We then designed a study to test this hypothesis. Cyclists rode for 2 hours at a moderate intensity on 3 occasions. They received water on one occasion, and glucose or glucose plus caffeine on the other occasions. The amount of carbohydrate ingested was a moderate amount (48 g/h) and the amount of caffeine was a very high dose (5 mg/kg per hour) (to read how much caffeine is in coffee, see How much caffeine is in coffee?). We observed a remarkable increase in carbohydrate use from the drink, suggesting that more carbohydrate was absorbed! The only problem? The dose of caffeine was so high (10 mg/kg in total) that it may not easily translate into practical application.
Therefore, we designed a second study. Dr Carl Hulston was the first author. Again, cyclists performed 3 rides: water, glucose and glucose plus caffeine. The amount of carbohydrate ingested was similar to the previous study but the amount of caffeine was about half. The cyclists rode 1:45h at a moderate intensity, followed by a time trial, so we could also measure the effects on performance. This time, we did not find any effect on carbohydrate use from the drink and thus caffeine did not seem to have increased absorption.
The studies may seem somewhat conflicting, but it is possible that a high caffeine dose is needed to see the effects on carbohydrate use. The second study did, however, show effects on performance with added caffeine. It improved time trial performance by 9% compared with water and more than 4% compared with glucose only.In other words: glucose improved performance but caffeine plus glucose improved performance even more.
So where does this leave us? It seems that in order to see the effects of caffeine on carbohydrate absorption, a large dose of caffeine is needed. Such high intakes are not recommended because of increased side effects. The lower dose is effective in improving performance but it does not seem to do so by enhancing carbohydrate use. With highly individual responses to caffeine, the recommendation is that athletes experiment with lower doses and work out what works best for them.
Van Nieuwenhoven MA, Brummer RM, Brouns F. Gastrointestinal function during exercise: comparison of water, sports drink, and sports drink with caffeine. J Appl Physiol 89(3):1079-85, 2000.
Yeo SE, Jentjens RL, Wallis GA, Jeukendrup AE. Caffeine increases exogenous carbohydrate oxidation during exercise. J Appl Physiol 99(3):844-50, 2005.
Hulston CJ, Jeukendrup AE. Substrate metabolism and exercise performance with caffeine and carbohydrate intake. Med Sci Sports Exerc. 40(12):2096-104, 2008.