Research Update: How Much Carbohydrate Should You Consume During Exercise?
Feb 2013 24
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I still get a lot of questions regarding the optimal amount of carbohydrate to consume during exercise. Based on the totality of research, current recommendations are 30-60 grams of carbohydrate per hour of glycogen-depleting exercise. Because glycogen depletion in an athlete consuming a mixed diet typically starts kicking in around 45 minutes or so after clicking in and pedalling away, carbohydrate recommendations for the first hour of exercise are typically lower, in the range of 0-30 grams per hour.

Having said that, it should be pointed out that studies examining both Tour de France cyclists and Ironman competitors – athletes with massive calorie expenditures – have shown hourly carb intakes of 90g/hr are quite common.

While the 30-60g/hr range is a safe bet, the research is still unequivocal on the exact best dose – and given differences in event type and individual digestive capabilities, there may not be a single best dose that we can apply across the board.

However, researchers recently set out to find what would be the best dose for cyclists performing a 160 minute cycling test. The test involved hooking each subject’s personal bike to a Computrainer, having them pedal at around 70% of their Vo2Max for 2 hours, allowing them a brief break to empty their bladder if necessary, before jumping back on the bike within 2 minutes to complete a 20-km time trial as quickly as possible on a computer-simulated undulating course.

So in terms of real-life carryover, this test would be kinda like hanging with the peleton for 2 hours, briefly stopping by the roadside for a quick squirt, then jumping back on the bike and putting your head down, bum up, and not only catching the peleton but scooting past them as you make a sustained 20km breakaway.

Awright, snap out of it you lot…(because I know I just sent every cyclist reading this into a glorious breakaway daydream)…we have the rest of the study to cover here, and I need to get this article done with so I can get out in the sun.

Carbohydrate Dose

Carb dose is pretty much the focus of the study, so let’s take a look at that. Research using tracer techniques has demonstrated that when glucose alone is ingested, the body is able to use it at a rate of about 1 g/min, and about 1.5 g/min when multiple carb types (i.e., glucose and fructose) are consumed. The increase in oxidation rate with ingestion of multiple carb types may be the result of the ability to use both glucose and fructose intestinal transporters.

Therefore, the drinks in this study, regardless of carb dosage being tested, were all formulated using a ratio of 1:1:1 of glucose, maltodextrin (a.k.a glucose polymers), and fructose (a 2:1 ratio of glucose and fructose units).

The dosages tested ranged between 0 and 120 g/hr, in 10g increments. The study was conducted at 4 research centers, and because of the time commitment required to have all subjects consume all 13 dosages, the researchers had to make some compromises. Namely, each subject completed the cycling test on four occasions, once with the placebo, and the remaining three each with differing carbohydrate dosages so that, between the four centers, all dosages were eventually tested.

Statistical significance was set at p = 0.05, something we’ll discuss again later.

The Subjects

The participants were 51 recreationally trained, healthy male cyclists or triathletes (12–15 subjects at each laboratory). Mean age, height, body mass, and peak oxygen uptake were 28.4 years, 1.82 m, 77.7 kg, and 59.1 mL.kg/min, respectively.

During each 2-h ride, subjects ingested a total of 2000 mL of one of the 13 beverages, which in addition to varying levels of carbohydrate (0-120g/hr as per above), also contained 18 mmol/L of sodium, 3 mmol/L potassium, and 11 mmol/L chloride. The drinks were consumed in 250-mL allotments every 15 minutes, beginning at minute 15 and ending at minute 120, for an overall ingestion rate of 1 L/hr. Non-caloric sweeteners were used to ensure all drinks, including placebo, tasted similar.

sports-bars-and-gelsCompared to sports gels and bars, carb-rich drinks are an easier, cheaper, GI-friendly way for many folks to consume large dosages of carbohydrate during training and competition.

The Results

Because of the the way the study was conducted, the researchers were forced to perform two different types of analysis. The first was what is known as a mixed model analysis, which showed that in individual subjects, addition of carbohydrate had a statistically significant effect on time trial performance. But because each subject consumed different carb doses but none of the subjects consumed all the doses, this model was of little use in ascertaining the overall optimum dosage.

So what the researchers then did was pool the data, plot the mean results of each carbohydrate intake on a graph and use the resultant curves to estimate the optimal dose.

The dose they arrived at after doing this was 78 grams of carbohydrate per hour. However, this fell a whisker short of statistical significance (p=0.059); when the researchers loosened up the data, so to speak, in order to arrive at a statistically significant level, their analysis suggested that the optimal dose was somewhere in the 68-88g/hr range.

The predicted time to complete the simulated time trial for the placebo was 35.48 minutes; the curve minimum corresponding to the fastest time to complete the time trial was 33.80 minutes.

The researchers estimated incremental performance improvements of 1.0%, 2.0%, 3.0%, 4.0%, and 4.7% at 9, 19, 31, 48, and 78 g/hr, respectively. Increasing CHO beyond 78 g/hr did not produce any further performance improvement. Moreover, 4.0% and 3.3% performance improvements over the placebo condition were observed at 108 and 120 g/hr, respectively, suggesting a progressive decrement in performance beyond 78 g/hr.

Practical Considerations

One of the carb sources used in this study was fructose, which may cause gastrointestinal upset for some athletes. This author briefly tried switching to a well-known commercially available sucrose-containing formula (sucrose being a disaccharide comprised of glucose and fructose) some time ago, and promptly began experiencing GI symptoms during rides. Cessation of this product and resumption of a maltodextrin formula promptly resolved the issue.

Other Considerations

The study was sponsored by Gatorade, who also employed 2 of the researchers. It should be noted that Gatorade has sponsored a lot of sports nutrition research over the years and this author is not aware of any improprieties or documented bias in such research. In this study in particular, it would have benefited Gatorade if the results showed a linear increase in performance benefits with increasing carb dosages, but that wasn’t the case.

Obviously, another similar study in which all subjects consumed all carb dosages (perhaps leaving out some of the lower and redundant dosages to save time) would be real nice, and hopefully it will happen in the not-too-distant future.

In the meantime, this study suggests that real world carbohydrate dosages, well in excess of official recommendations, used by elite cyclists and triathlon competitors may have scientific merit and could provide further performance benefits.

Source:

Smith JW, et al. Curvilinear Dose–Response Relationship of Carbohydrate (0–120 g.h-1) and Performance. Medicine & Science in Sports & Exercise, 2013; 45 (2): 336–341.

Anthony Colpo is an independent researcher, physical conditioning specialist, and author of the groundbreaking books The Fat Loss Bible and The Great Cholesterol Con. For more information, visit TheFatLossBible.net or TheGreatCholesterolCon.com

Copyright © Anthony Colpo.

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