Multiple Transportable Carbohydrates: Effects on Gastrointestinal Tolerance and Performance
The assertion that carbohydrate intake influences exercise performance dates back to at least the 1920s (6), and the topic has been researched extensively since the 1980s. In the current scientific literature, there is an abundance of evidence that carbohydrate ingestion improves performance in endurance-based events, under certain exercise conditions. As a result, the consumption of carbohydrate in the form of sports beverages, gels, and other foods is a common practice among endurance athletes. The performance-enhancing effects of carbohydrate intake are most apparent during prolonged endurance events (> 2 hours) performed at moderate-vigorous intensities, in which the body’s own carbohydrate reserves are depleted over time. Ingested carbohydrate (aka: ‘exogenous carbohydrate’) provides a valuable supplemental source of energy to the muscles, helps maintain normal blood glucose levels, and preserves the body’s limited supplies of liver/muscle glycogen. As a result, athletes can sustain higher rates of carbohydrate utilization in the late stages of exercise, permitting increased power output and/or longer time to fatigue for athletes.
Our understanding of the effects of carbohydrate continues to grow. Recently, the potential advantages of multiple transportable carbohydrates (i.e. combining glucose and fructose) have been investigated. This paper will discuss the impact of glucose+fructose ingestion on carbohydrate oxidation, gastrointestinal tolerance, and exercise performance.
When carbohydrate is consumed during exercise, it is absorbed from the intestines, enters the bloodstream, and can ultimately be delivered to the working muscles to fuel energy demands during exercise. Performance gains are likely to be greater when larger amounts of supplemental carbohydrate can be utilized by the muscles (i.e. increased ‘exogenous carbohydrate oxidation’). For example, Smith and colleagues (9) examined the effects of varied glucose doses, using a trial consisting of two hours of sub-maximal cycling followed by a simulated 20 km time-trial. The investigators reported improvements in time-trial performance (versus a placebo) when cyclists consumed as little as 0.25 g/min of glucose during exercise (i.e. 15 g/hr, or ~250 ml/hr of commercially-available sports beverage). When glucose intake was increased to higher levels (0.5 g/min and 1.0 g/min), exogenous carbohydrate oxidation rates increased in proportion with the increased dosages. Furthermore, incremental improvements in cycling performance were also observed with the increased glucose dosages. Therefore, there is evidence that higher rates of carbohydrate ingestion are desirable to enhance endurance performance, at least up to a point.
Peak exogenous oxidation rates are not only dependent on the amount of carbohydrate consumed, but also the type of carbohydrate. For example, glucose has a higher peak oxidation rate than fructose, due to differences in intestinal absorption mechanics. Glucose is primarily absorbed from the intestines via the sodium-dependent glucose transporter 1 (SGLT1), which becomes saturated at ingestion rates of ~1.1 g/min. By contrast, fructose is primarily absorbed by a sodium-independent facilitated transport mechanism (GLUT5), which becomes saturated at ingestion rates of ~0.6 g/min. Therefore, if glucose is consumed at rates > 1.1 g/min (or fructose at rates >0.6 g/min), the ‘excess’ carbohydrate remains in the intestines, potentially contributing to gastrointestinal discomfort. This provides a rationale for the carbohydrate ingestion rates recommended by the American College of Sports Medicine (and other agencies), which advise that athletes consume 0.5-1.0 g/min of carbohydrate (30-60 g/hr) during endurance events (7). Conceptually, these values represent a dose that is high enough to promote meaningful increases in carbohydrate availability, while also minimizing the potential for gastrointestinal intolerance.
When glucose and fructose are ingested together, greater amounts of total carbohydrate can be absorbed into the bloodstream, because the two carbohydrates are transported from the intestines via separate transporters. Thus, multiple transportable carbohydrates could theoretically benefit endurance athletes in two ways: 1) increasing peak exogenous oxidation rates beyond those attainable by any single carbohydrate, and 2) reducing gastrointestinal discomfort associated with excess carbohydrate remaining in the gut. Jentjens and colleagues conducted a series of studies (i.e. 3-5) which investigated the effects of various carbohydrate combinations/amounts on oxidation rates, and showed that the consumption of high rates of glucose+fructose (1.8–2.4 g/min) produced peak oxidation rates that were up to 65% higher than those obtained with the same amount glucose alone.
The aforementioned findings led researchers to investigate whether glucose+fructose ingestion improved exercise performance to a greater extent than single carbohydrate sources. Currell and Jeukendrup (2) used an exercise protocol consisting of two hours of moderate-intensity cycling, followed by a time-trial lasting approximately one hour. Cyclists completed the time-trial 8% faster when consuming glucose+fructose (1.2 g/min glucose + 0.6 g/min fructose) than when they consumed an equal amount of calories from glucose alone (1.8 g/min). In addition, average time-trial performance was 19% faster with glucose+fructose versus a placebo. Similarly, Triplett and colleagues (10) reported that power output during a simulated 100 km cycling time-trial was 8% higher when cyclists consumed glucose+fructose (1.6 g/min + 0.8 g/min) versus glucose alone (2.4 g/min).
The studies above demonstrate that glucose+fructose ingestion may improve endurance performance substantially versus single carbohydrate types. However, a limitation of these studies was that neither included a moderate-dose glucose beverage for comparison. Because the glucose-only beverages exceeded the absorption rates of intestinal transporters, it is possible that gastrointestinal discomfort affected performance in the glucose trials, magnifying the perceived benefits of glucose+fructose. In support of this theory, Rowlands and colleagues (8) observed improvements in cycling performance with a glucose polymers+fructose beverage (versus a glucose/glucose polymers beverage), and demonstrated that these effects were partially due to increased gastrointestinal discomfort when consuming high rates of glucose. A recent study from our laboratory directly compared glucose+fructose (~1.0 g/min + 0.5 g/min) versus both a high-glucose beverage (~1.5 g/min) and a moderate glucose beverage (~1.0 g/min) (1). During a trial consisting of two hours of moderate-intensity cycling followed by a 30-km time-trial, the glucose+fructose beverage produced a 3% improvement in time-trial performance versus the high-glucose beverage (statistically classified as a ‘likely beneficial effect’), but only a 1.2% improvement versus the moderate-glucose beverage (an ‘unclear’ effect). Therefore, although the consumption of high rates of glucose+fructose (>1.0 g/min) appears to be a promising strategy during prolonged exercise, further research is warranted to quantify the potentially beneficial effects of this approach versus single-source carbohydrates consumed at traditionally recommended doses (0.5-1.0 g/min).
In summary, carbohydrate intake has been shown to improve exercise performance, particularly during prolonged endurance events. There is evidence that performance benefits are accentuated with higher carbohydrate doses, which is likely related to increased rates of exogenous carbohydrate oxidation. Peak rates of exogenous carbohydrate oxidation are limited by intestinal absorption, and thus the consumption of carbohydrate types that use separate intestinal uptake pathways (i.e. glucose+fructose) have been shown to optimize exogenous oxidation, while potentially reducing gastrointestinal discomfort. A few recent studies have reported that high doses of glucose+fructose can improve endurance performance to a significant degree versus high doses of glucose alone. However, it is not yet clear whether this strategy elicits meaningful performance benefits versus moderate levels of glucose. ‘Optimal’ doses of carbohydrate are difficult to recommend due to individual differences in gastrointestinal tolerance. Therefore, athletes are encouraged to experiment with various amounts/types of carbohydrate to determine the maximal dose that they can tolerate during exercise conditions that match their competitive event, and consider glucose+fructose ingestion as a method to maximize the beneficial dose of carbohydrate while minimizing the risks of gastrointestinal discomfort.
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