Knowing When to Use Solid Instead of Liquid Carbs Can Place You a Stride Ahead
BY ELLEN COLEMAN, MA, MPH, RD
© 1997. 42K(+) Press, Inc. "Carbo Science " originally appeared in the May 1997 issue of Marathon & Beyond, and is reprinted here with permission of 42K(+) Press, Inc.. For information about reprinting or excerpting this article or any other M&B article, contact Jan Seeley via email or by calling (217) 359-9345. Marathon & Beyond is the only bimonthly magazine for marathoners and ultrarunners.
ENDURO AUTOMOBILE races such as the 24 Hours of LeMans have been held since the early days of the auto industry, both as pure competition and as a method of promoting specific brands of cars. Success comes to the focused and well-paced driver who can work in concert with his car against the clock hour after grueling hour and pay attention to details, including the optimum fuel mixture for the distance, speed, and conditions.
Picking the right long-distance fuel is also critical to today's marathoners and ultramarathoners, and there's an array of solid and liquid carbohydrates to choose from. Each carbohydrate form (solid and liquid) has advantages and drawbacks. Sports drinks and other liquids are a practical source of carbohydrate because they also replace our fluid losses. High carbohydrate foods, on the other hand, can be easily carried and provide a feeling of satiety that you won't get from drinking fluids. Marathoners and ultramarathoners use both forms of carbos, so it's important to know precisely under what conditions each form of the carbohydrate will affect performance.
Muscle and liver glycogen depletion are well-recognized limitations to distance-running performance, and the longer the distance, the more profound the limitations. Eating a high-carbohydrate meal before morning runs helps replenish your liver glycogen stores and improves performance.
Consuming carbohydrates during runs provides glucose for your working muscles when they get low on glycogen. Also, eating an adequate amount of carbohydrate following a long run helps replace muscle and liver glycogen, which is essential for adequate recovery.
The massive body of research on this subject reflects the intense interest in the performance benefits and physiological effects of consuming liquid and solid carbohydrates before, during, and after exercise and provides practical recommendations about whether you should consume liquid or solid carbohydrates before, during, and after runs.
CARBOHYDRATE FORM AND BLOOD GLUCOSE LEVELS
At rest, solid food empties from the stomach more slowly than liquid food does. For this reason, solid carbohydrates may provide a slow, sustained release of blood glucose. In contrast, liquid carbohydrates may raise blood glucose levels more quickly and to a greater extent.
Based on this difference, you'd expect solid carbohydrates to be a better preexercise meal choice than liquid carbohydrates because the solid carbos provide a more continuous supply of glucose during exercise. You'd expect liquid carbohydrates to be a better choice during and after exercise because they elevate blood glucose quickly, thereby enhancing endurance and promoting glycogen replacement. On the other hand, if liquid and solid carbohydrates cause similar blood glucose responses, then you would not expect a difference in performance or glycogen restoration.
To scientifically measure such differences, we use the glycemic index (GI), which indicates how much a food increases the blood glucose level relative to glucose, which has a GI value of 100. The glycemic index is influenced by the form of the food (liquid versus solid), its fiber content, the presence of protein and fat, and the food processing and preparation methods. The glycemic index is not simply a function of whether a carbohydrate is a liquid or a solid. For example, an orange has a GI value of 66, while orange juice has a value of 67. The glycemic index is also not a function of whether the food is a starch (e.g., pasta) or simple carbohydrate (e.g., table sugar). For example, a baked potato has a GI of 98, which is close to the value of 100 for glucose.
But the glycemic index concept does have limitations. The numbers that are available are largely based on tests on simple foods. High glycemic foods often don't affect the glycemic response when they're combined with other foods in meals. For example, bread has a high GI, but when you add margarine, fruit, and milk to the mix, the GI of the meal is medium to low. Also, the glycemic index is based on equal grams of carbohydrate, not average serving sizes.
EATING ONE HOUR BEFORE RUNNING
Research published in the late 1970s by Carl Foster and colleagues at Ball State University in Muncie, Indiana, suggested that consuming 75 grams of glucose (300 calories) 30 minutes prior to exercise reduced endurance by causing hypoglycemia (low blood sugar) and early fatigue. The result of this study led to the recommendation that endurance athletes avoid consuming carbohydrates (liquid or solid) in the hour before exercise.
Insulin is secreted by the pancreas in response to an increase in blood glucose. Insulin lowers the blood glucose level by promoting the uptake of glucose into cells. When the blood glucose drops, some people begin to show the symptoms of hypoglycemia (weakness, dizziness, nausea, confusion, and partial blackout), or they become exhausted sooner. Fortunately, these insulin and glucose responses are temporary and will not harm performance unless you're sensitive to having your blood glucose lowered.
Research published in 1991, however, indicated that consuming carbohydrates an hour before exercise may help performance. Michael Sherman and colleagues at The Ohio State University in Columbus found that performance was improved by 12.5 percent when subjects consumed liquid carbohydrate (a glucose polymer drink) an hour before cycling exercise. Eating one gram of carbohydrate per kilogram of body weight helped subjects maintain blood glucose levels during exercise.
Does Carbo Form Matter?
Does the form of the preexercise carbohydrate influence performance? Diane Thomas and colleagues at the University of Sydney in Australia found that athletes consuming one gram of carbohydrate per kilogram of body weight of lentils (yes, lentils!) an hour before cycling improved exercise time by 20 minutes. Compared to an equal amount of carbohydrate from a baked potato or a glucose drink, the lentils (which have a GI of 29) provided a more gradual rise and fall in blood glucose, thereby maintaining blood glucose levels at higher levels during exercise. The Thomas study results suggest that a low-glycemic solid carbohydrate is the preferred preexercise meal. Yet, the Sherman study results indicate that performance is improved following the consumption of a high-glycemic liquid carbohydrate as well. There hasn't been a lot of research in this area (in fact, the Thomas study is the only one that has examined the GI of preexercise meal), so experiment to find your "best" preexercise meal.
There is also the obvious consideration of whether certain foods (liquid or solid) give you gastrointestinal distress. Some folks can't eat solid food prior to running without risking a bathroom break and/or gut distress—nausea, cramps, or diarrhea. My advice is that you try different liquid and solid carbohydrates with high and low glycemic indexes to find out what works well for you. The research by Sherman and Thomas suggests that you should consume one gram of carbohydrate per kilogram of body weight an hour before exercise (to get your weight in kilograms, divide your body weight by 2.2).
If you think you are sensitive to having your blood glucose lowered, you have several options. You can consume a low-glycemic carbohydrate, take in a liquid or solid high-glycemic carbohydrate a few minutes before exercise, or wait until you're exercising to eat a carbohydrate. The rise in the exercise hormones (epinephrine and growth hormone) blocks the release of insulin, thereby countering the insulin's effect in lowering blood glucose.
Have you got a human interest story about a runner you know or a race report to share ? ontherunevents.com is happy to share stories with our readers. Send us your stories, your experiences!
Copyright © 1999 ontherunevents.com, All rights reserved.