Carbohydrate during exercise can improve endurance performance. One important question is “how much carbohydrate do you need?” and “is more better?”. The only real answer to this question is “it depends”. It depends on your goals (some times you don’t want or need carbohydrate), it depends on the duration, it depends on your performance level.
Assume that the aim is optimal performance in a race or training session (workout) and assume your workout is lasting 3 hours or more. You would be looking at optimizing the delivery of carbohydrate, whilst minimizing gastro-intestinal discomfort.
Is more better?
So in this situation, is more better? Very few well controlled dose-response studies on carbohydrate ingestion during exercise and exercise performance have been published. Most of the older studies had serious methodological issues that made it difficult to establish a true dose-response relationship between the amount of carbohydrate ingested and performance. Initially it was concluded that the athlete needed a minimum amount of carbohydrate (probably about 20 g/h based on one study) but it was assumed that there was no dose response relationship so as long as you ingested some carbohydrate you were fine. The last few years evidence has accumulated for a dose response relationship between carbohydrate intake and endurance performance
Dr JohnEric Smith, now Assistant Professor in the Department of Kinesiology at Mississippi State University measured fuel use during prolonged exercise while ingesting glucose (15, 30, and 60 g/h) (1). Twelve subjects cycled for 2 h at 77%VO2 peak followed by a 20 km time trial. The results showed a relationship between the dose of glucose ingested and improvements in endurance performance. A higher intake was associated with greater use and better performance.
As a follow up Dr JohnEric Smith did another study, pretty unique in exercise science (2). The study involved several Universities and results were collated. In this study, across 4 research sites, 51 cyclists and triathletes completed four exercise sessions consisting of a 2-h constant load ride at a moderate to high intensity. Twelve different beverages (consisting of glucose: fructose in a 2:1 ratio) were compared, providing participants with 12 different carbohydrate doses ranging from 10 to 120 g carbohydrate/h during the constant load ride. At all four sites, a common placebo that was artificially sweetened, colored, and flavored and did not contain carbohydrate was provided. The order of the beverage treatments was randomized at
each site (3 at each site). Immediately following the constant load ride, participants completed a computer simulated 20 km time trial as quickly as possible. The ingestion of carbohydrate significantly improved performance in a dose dependent manner. Again a higher intake of carbohydrate seemed related to better performance. The authors concluded that the greatest performance enhancement was seen at an ingestion rate between 68 and 88 g carbohydrate/h. A meta-analysis using all studies in the literature came to the same conclusion (3).
When is carbohydrate intake too much? The only limitation is probably gastro-intestinal discomfort. If you feel carbohydrate is not leaving the stomach, intake is too much for that situation and it would be wise to reduce intake for a while and if possible dial back the intensity of exercise a little as well, to allow gastric emptying and absorption to continue.
So, from these studies discussed here there are a number of things we can conclude:
First, there is a dose response relationship, at least during prolonged exercise (>2h).
A higher carbohydrate intake will likely result in better performance than lower intake. This can also be turned around: a low carbohydrate is likely to compromise endurance performance.
We have seen in a previous blog that when more than 60 g/h is ingested this should come from carbohydrate blends (for more detail read this).
Of course in any case one should avoid gastrointestinal distress as a result of a high carbohydrate intake. It seems that this is trainable (as will be discussed in a future blog) and by regular training of a race nutrition plan that involves high intakes, stomach comfort is likely to improve.
Smith JW, Zachwieja JJ, Peronnet F, Passe DH, Massicotte D, Lavoie C, Pascoe DD. Fuel selection and cycling endurance performance with ingestion of [13C]glucose: Evidence for a carbohydrate dose response. J Appl Physiol 108: 1520-1529, 2010.
Smith JW, Pascoe DD, Passe DH, Ruby BC, Stewart LK, Baker LB, Zachwieja JJ. Curvilinear dose-response relationship of carbohydrate (0-120 g.h−1) and performance. Med Sci Sports Exerc 45: 336-341, 2013.