By Katherine Beals, PhD, RD, FACSM, CSSD
For the endurance athlete looking to eat during exercise, there are shelves worth of sports drinks, gels, and chews promising to boost performance. But, are they beneficial and, if so, which ones are the best? This month’s column will examine why, when, and what to eat during training and competition.
One of the most common causes of fatigue during endurance training and competition (particularly of a moderate-to-high intensity) is depletion of that all-important muscle fuel–glucose. Thus, the rationale behind eating during exercise is to provide glucose for the muscles so they don’t run out, not unlike stopping at the gas station during a cross-country road trip to fill up your car’s tank with gas. However, just as stopping at a gas station is only necessary if there is a possibility the car will run out of gas, eating during exercise will only be beneficial if there is a chance that the muscles will run out of glycogen (i.e., stored glucose) during the training bout or competition.
Whether the muscles will run out of glucose (and, thus, the athlete will benefit from eating something during training or competition) depends on what was consumed before exercise as well as how intense and how long the exercise will be. The International Olympic Committee (IOC), American College of Sports Medicine (ACSM) and the Academy for Nutrition and Dietetics (AND) all agree that if the training bout or competition is less than 45 minutes and the athlete has eaten appropriately before hand (thus, replenishing glycogen stores), there is no performance benefit from eating something during exercise. When it comes to exercise lasting longer than 45 min there is some discrepancy between the various sports organizations. But, the bulk of the research indicates that, eating during low-to-moderate intensity exercise lasting between 45 min -75 min is of little to no benefit from a performance standpoint. Up the intensity and a small amount of carbohydrate (~30 grams) may be beneficial.
For exercise of moderate-to-high intensity lasting between 1-2.5 hours, it is recommended that athletes consume approximately 30-60 grams of carbohydrate per hour. For anything longer than 2.5- 3 hours, the research suggests that consuming upwards to 90 grams per hour may be beneficial, although for some athletes, particularly those engaged in a high impact sports (e.g., running), such large doses can result in gastrointestinal distress. Thus, it is always recommended that you experiment with carbohydrate replacement protocols during training so that you have it dialed in before any/all important competitions.
Since it is glucose that the muscles need, then it is carbohydrate that the athlete should consume. But, what type of carbohydrate? Ideally you want a carbohydrate that will be rapidly digested and absorbed as well as maximally oxidized (i.e., used for energy) by the muscles. Research has shown that carbohydrates can be roughly divided into two categories: (1) carbohydrates that are oxidized rapidly (approximately 1 gram per minute or up to 60 grams per hour) and (2) carbohydrates that are oxidized relatively slowly (approximately 0.5 grams per minute or 30 grams per hour). Rapidly oxidized carbohydrates include glucose, maltose, sucrose and maltodextrin while fructose, galactose, lactose and isomaltulose tend to be more slowly oxidized. It should be noted, however, that these oxidation rates are based on the consumption of the carbohydrates individually (that is, by themselves). Recent research has shown that consuming glucose and fructose in a ratio of roughly 2:1 can increase oxidation rates to approximately 1.5 grams per minute or upwards of 90 grams per hour. And, what’s more, these higher oxidation rates can lead to better performance.
In one of the first studies demonstrating the performance enhancing benefits of consuming glucose + fructose, cyclists exercised for 2 hours on a cycle ergometer at a moderate intensity during which they ingested either (1) a pure glucose drink, (2) a glucose:fructose drink (at a 2:1 ratio) or (3) a placebo drink (flavored water). At the end of the 2-hour ride the cyclists then performed a simulated time trial that lasted approximately 60 minutes. The results showed that ingesting the pure glucose drink produced an increase in average power of 9% over the placebo drink while ingesting the glucose:fructose drink produced an additional 8% improvement in average power output over and above that of the glucose drink (and 17% greater than the placebo drink). These results have been confirmed by other studies, which have shown improvements in 100k time trial performance, simulated mountain bike races, and high intensity laboratory cycling.
The mechanism underlying the benefits of glucose + fructose ingestion center on the differences in intestinal absorption between the two monosaccharides. Because glucose and fructose use different intestinal transporters for absorption, carbohydrate delivery to the muscle and subsequent oxidation can be increased, thereby enhancing performance.
Which Products are the Best?
Ironically, many of the products designed for use during exercise are similarly formulated, so the best choice is something that you like and will actually consume (because if you don’t like it you probably won’t eat it and, thus, it will be of no benefit!). In general, look for products that contain a combination of glucose and fructose or glucose and sucrose (as sucrose is just a disaccharide of glucose + fructose). The choice of a liquid (e.g., sports drink), semi-solid (e.g., gel) or solid (e.g.,blocks, chews, beans), is largely based on individual preference and tolerance. Sports drinks are nice because they provide carbohydrate, electrolytes and fluid– all things that the endurance athlete needs—all in one package. Most sports drinks contain about 14 grams of carbohydrate per 8 fluid ounces. Gels and chews typically contain 24-30 grams of carbohydrate per serving; but, they need to be consumed with adequate fluid (typically 8 fluid ounces per ½ serving) in order to dilute the carbohydrate concentration to an acceptable level (approximately 8% carbohydrate solution) so as to minimize the risk of GI distress. Some newer products on the market contain protein (in addition to the carbohydrates) claiming that the addition of protein enhances endurance performance. However, the limited research supporting these products was poorly constructed (i.e., the studies matched carbohydrate content but not total calorie content; thus the carbohydrate + protein groups were getting more total calories) and “performance” was measured by time to exhaustion, which does not apply to any existing endurance event.
Of course, while sports foods are easy and convenient, they are not the only choice for the endurance athlete looking to meet carbohydrate requirements during exercise. For those who would prefer to go the whole foods route there are a number of choices that will fit the bill. Fruits such as bananas, oranges, apples, grapes and melons are a great choice. Diluted fruit juice (approximately 1:1 dilution with water) can be a good substitute for a sports drink. Finally, granola bars, graham crackers and fruit bars (e.g., fig, strawberry, or apricot Newtons) are a nice change from the more typical sports bars.
The Bottom Line
For exercise of a moderate-to-high intensity, lasting more than 45 minutes, (especially if the athlete was inadequately fueled prior), carbohydrate replacement during the training/competition can be beneficial. The amount of carbohydrate needed will depend upon the duration and intensity of the training/competition. Aim for approximately 30 grams in total for an exercise duration of 45-75 minutes, 30-60 grams per hour for an exercise duration of 1-2.5 hours and upwards of 90 grams per hour if exercise lasts longer than about 2.5- 3 hours. Look for products that contain a combination of glucose and fructose or glucose and sucrose and chose those taste and work best for you. And, last but not least, use training to experiment with foods and sports products so you don’t have any unpleasant surprises during the big event(s).
Katherine Beals, PhD, RD, FACSM, CSSD. is an Associate Professor in the Division of Nutrition at the University of Utah where she specializes in Sport Nutrition. She is a Fellow of the American College of Sports Medicine and a Certified Specialist in Sports Dietetics