Carbohydrate Replacement During Exercise

Definition

Carbohydrate replacement involves the ingestion of some form of carbohydrate during an exercise or sporting event.

Purpose

The purpose of carbohydrate replacement is to provide a person with additional glucose to use as a “fuel” for the exercising skeletal muscles during an exercise or sporting event, especially during the latter stages of such an event when natural, existing supplies of carbohydrates begin to run low and fatigue begins to set in.

Description

The study of carbohydrate supplementation has been an active field of research since the mid-1980s. Hundreds of research studies have focused on one aspect or another of the topic. Many of these studies resulted from the development of sports drinks, artificial beverages designed and developed to meet the needs of athletes. The first, and probably most famous, of these sports drinks is Gatorade, a beverage developing in 1965 by researchers at the University of Florida for use by the school's football team.

Carbohydrate replacement is an issue of importance to any athlete who exercises for some extended period of time (usually more than 90 minutes) beyond the initial time during which natural carbohydrate supplies are adequate to meet the body's need for a fuel from which to produce energy.

All forms of exercise, from a slow-paced walk to a 100-meter sprint or a marathon run, requires an increase in the output of energy by the body's cells. That energy comes primarily from the conversion of glucose (blood sugar) into carbon dioxide and water, a chemical reaction in which energy is released. Cells obtain glucose in one of two ways. First, carbohydrates in food, such as starch and sugars, are broken down in the digestive system to produce simpler substances, primarily glucose. The term “carbohydrate” refers to three types of biochemical compounds: monosaccharides, such as glucose and fructose; disaccharides, such as sucrose (table sugar) and lactose (milk sugar); and polysaccharides, such as starch. The sequence of events that occurs in the digestive system is: polysaccharides, disaccharides, monosaccharides. The glucose formed in this series of reactions passes through the lining of the digestive system into the bloodstream, where it is transported to cells.

Glucose that is not immediately used by cells is converted to another form, the chemical called glycogen. Glycogen is stored primarily in the liver and muscle tissue until it is needed by cells. During exercise, cells make use not only of the glucose delivered through the bloodstream (for example, from breakfast food), but also from glycogen that has been kept in reserve in the liver and muscle tissues.

The ability of an athlete to complete an 800-meter run, to play 25 shifts in an ice hockey game, or to stay with a cycling peloton for 2 hours depends to a large extent on the amount of carbohydrate available for meeting the body's energy requirements. In situations where the body's carbohydrate resources have been depleted, other sources of energy are available (fat and protein metabolism), but these sources are inefficient and do not produce as much energy. The athlete who understands the role of carbohydrates in energy production attempts to retain as much of that resource as possible during the early stages of an exercise or an event in order to have adequate supplies of carbohydrate during the late stages of the event.

Recommended dosage

For relatively moderate exercises that last less than one hour, there is little evidence to suggest that carbohydrate supplementation improves an athlete's performance to a significant extent. One study of intense short-term exercise found an improvement of 2.3% among athletes who ingested carbohydrate during exercise. Some researchers have raised the possibility that glucose may stimulate the nervous system in such instances, producing a psychological sense of endurance that does not reflect any actual physiological changes in the body. This issue is still a matter of dispute among researchers.

For exercises and sporting event lasting more than two hours, the evidence regarding carbohydrate supplementation is much more clear: The ingestion of carbohydrate during an exercise or sporting event increases an athlete's endurance significantly over the ingestion of pure water alone or the ingestion of no fluids. Many studies have been conducted to determine the optimal consumption of carbohydrates, and the result of those studies indicates that an athlete should attempt to consume between 60–70 g/h (grams of carbohydrate per hour) for maximum effect. Below that amount, the overall effect of carbohydrate consumption corresponds to the amount of carbohydrate ingested. That is, the more carbohydrate consumed, the greater the improvement in an athlete's performance. Beyond the recommended level of 60–70 g/h, there is little or no improvement. The reason for this observation is that the body can metabolize carbohydrate only at some given rate, which appears to be about 60–70 g/h. This amount of carbohydrate is optimally taken in by drinking 625–1,250 mL per hour of a beverage containing 4%–8% carbohydrate.

KEY TERMS
Carbohydrate—
A chemical compound consisting of carbon, hydrogen, and oxygen, the latter two in approximately a two-to-one ratio.
Glucose—
A monosaccharide with the chemical formula C6H12O6, the primary source for the production of energy in cells; also known as blood sugar.
Glycogen—
A chemical compound in which glucose is stored for future use in the liver and muscle tissue.
Hypertonic—
Having a greater concentration than some other reference solution, such as body fluids.
Hypotonic—
Having a lesser concentration than some other reference solution, such as body fluids.
Isotonic—
Having the same concentration as some other reference solution, such as body fluids.

An interesting exception to this generalization involves the use of two different types of carbohydrates. In some studies, athletes have been given carbohydrate supplements consisting of two different kinds of carbohydrate, such as glucose and fructose, or glucose and sucrose. In such cases, the maximum benefit obtained from supplementation increases dramatically, usually to twice the level seen with a single carbohydrate. Thus, if the consumption of 60–70 g/h of glucose improves performance by 8%, the consumption of a solution containing 60–70 g of glucose and 60–70 g of fructose is likely to result in an improvement of up to 16%. The apparent explanation for this phenomenon is that the body uses specific protein transporters to carry each different kind of carbohydrate to cells. The addition of 60–70 g of glucose apparently saturates all transporters for that carbohydrate in the body, but has no effect on the transporters for fructose or sucrose. Either of those compounds can be added to the carbohydrate supplement with a corresponding increase in the benefit obtained from supplementation.

Some recent research has shown an unexpected additional benefit of carbohydrate supplementation during exercise. The evidence suggests that such drinks enhance immune function, which is helpful in light of research that indicates an individual's immune system functions somewhat less efficiently after exercise.

Types

A number of sports drinks are available with varying numbers, kinds, and amounts of nutrients, such as carbohydrates, vitamins, minerals, and electrolytes. Those drinks are generally classified as isotonic (having the same concentration of salts and minerals as body fluids), hypertonic (having greater concentration of salts and minerals than body fluids), and hypotonic (having lower concentrations of salts and minerals than body fluids).

Sport drinks that provide carbohydrate replacement during an exercise come in liquid, gel, and powdered form. Powdered sport drinks can be made in any concentration desired. A trainer or athlete needs to determine the proper concentration required that will provide both adequate hydration and adequate carbohydrate supplementation. Trainers and athletes also need to be aware of the role that sport drinks or other carbohydrate supplements play in an exercise or contest and prepare adequate supplies for the athlete. This means having adequate amounts of a sport drink or supplement available for an athlete and having it available in a location that is easily accessible during the event.

Precautions

QUESTIONS TO ASK YOUR DOCTOR

Interactions

Although uncommon, it is important to check with a medical professional to verify that there are no negative interactions between medications an individual takes and the supplements they intend to use. For persons with food allergies, it is necessary to review the ingredients as many supplements may contain milk product, gluten, or other potentially harmful allergens.

Complications

One potential risk associated with carbohydrate supplementation is the possibility of an athlete consuming too much of a product, resulting in weight gain during an exercise or competition. Since relatively large quantities of a sport drink or its equivalent are usually recommended, an athlete needs to be careful not to ingest more than the required amount. A second risk associated with the use of carbohydrate supplements is the possibility of gastric upset. A number of studies have shown that the ingestion of carbohydrate supplements during exercise or a contest is highly correlated with the development of nausea, vomiting, belching, heartburn, chest pain, bloating, abdominal cramps, side ache, and diarrhea. Trainers and athletes need to determine—on a trial and error approach, if necessary—the correct concentration of sport drink or equivalent to provide the carbohydrate supplement needed by the athlete without causing gastrointestinal disturbances.

Results

Determination of the correct amount of carbohydrate supplementation for a given athlete has a high potential for significantly improving an athlete's endurance without producing harmful side effects.

See also Calories ; Carbohydrates ; Fat ; Immune system ; Metabolism and energy ; Protein .

Resources

BOOKS

Dunford, Marie. Fundamentals of Sport and Exercise Nutrition. Champaign: Human Kinetics, 2010.

Jeulendrup, Asker, and Michael Gleeson. Sport Nutrition: An Introduction to Energy Production and Performance, 2nd ed. Champaign: Human Kinetics, 2010.

PERIODICALS

Hawley, J. A., K. D. Tipton, and M. L. Millard-Stafford. “Promoting Training Adaptations Through Nutritional Interventions.” Journal of Sports Science 24, no. 7 (2006): 709–721.

Wallis, G. A., S. E. Yeo, A. K. Blannin, and A. E. Jeukendrup. “Dose-response Effects of Ingested Carbohydrate on Exercise Metabolism in Women.” Medical Science and Sports Exercise 39, no. 1 (January 2007): 131–138.

ORGANIZATIONS

Gatorade Sports Science Institute, 617 W Main St., Barrington, IL, 60010, (800) 616-4774, http://www.gssiweb.com .

National Association of Sports Nutrition, 8898 Clairemont Mesa Blvd., Ste. J, San Diego, CA, 92123, (858) 694-0317, http://nasnutrition.com .

David E. Newton, AB, MA EdD

  This information is not a tool for self-diagnosis or a substitute for professional care.