Diet Site

Carbohydrates and Exercise

by Traci Kaufman, R.D.


Improve Athletic Performance with Diet

Athletic Performance Now that the weather is warming up, people are exercising more outdoors. The purpose of this article is to cover the two most important nutrients when exercising: carbohydrates and fluid. These nutrients could make a difference in your endurance and performance.



Carbohydrate is one of the most important nutrients to athletic performance. Carbohydrate plays the major role in supplying your brain and body with power. The body cannot supply enough carbohydrate on its own and therefore it needs to come from foods. Exercising with low levels of carbohydrate leads to fatigue. Carbohydrates are the ideal fuel for muscular work.

Carbohydrate is stored as glycogen, which is stored in the muscles and liver. Liver glycogen is used to maintain blood sugar, which, in turn, fuels the brain, nervous system and other cells. Optimal blood sugar levels are important for clear, brain function and therefore critical to sharp, high speed mental performance. Low blood sugar results in weakness and fatigue. Muscle glycogen fuels muscle cells during exercise. Muscle glycogen and fat supply energy during endurance activities. Maximizing glycogen stores is one of the primary goals of sports nutrition.

When exercising hard there is a continual loss of glycogen from the active skeletal muscle during the prolonged exercise. When the glycogen stores become depleted the athlete will not be able to exercise intensely and will experience fatigue. A gradual decline in muscle glycogen is related to the chronic fatigue often experienced by athletes during repetitive strenuous training conditions. Chronic fatigue often limits an athlete's ability to comply with a progressive training program and subsequently to compete at maximal potential. If you do not eat enough carbohydrates to refill the stores that are depleted in each workout, you may not have enough carbohydrates available during ensuing workouts. Therefore, consuming carbohydrates during endurance exercise can postpone fatigue and prolong peak performance.

Diet and endurance training influence the amount of glycogen stored in muscle and the time it takes to exhaustion. A high carbohydrate diet can raise the initial muscle glycogen concentration and thus there will be a greater time to exhaustion. Diet provides the body with the needed fuels, while training promotes muscles to store more carbohydrate and help improve the body's utilization of fuel. More muscle glycogen will help increase endurance. An individual that is more fit uses less glycogen, is better able to conserve the limited glycogen stores in the body, and utilizes more fat as a fuel source during endurance events.

Athletes who follow a high-carbohydrate diet can maintain high-intensity exercise for a longer period than those on a lower-carbohydrate diet. There is substantial evidence for a benefit of carbohydrate intake for the performance of brief, high power events if the competitor has been consuming a reduced energy diet.

Total carbohydrates are made up of simple sugars, complex carbohydrates, and fiber.

Simple carbohydrates are commonly known as sugars. Sources of simple carbohydrates include table sugar, candies and other sweets, sodas and bakery goods. These foods provide empty calories, i.e., calories that supply no vitamins and minerals and should therefore be minimized.

Complex carbohydrates include all the complex starches and fiber, such as those found in grains, cereals, breads and starchy vegetables like potatoes, corn, peas and beans. Milk, fruit and vegetables also contain carbohydrate.

Complex carbohydrates contain many essential nutrients and are the body's most effective source of energy to the athlete. Complex carbohydrates increase glycogen stores more efficiently than sugars, or simple carbohydrates.

Complex carbohydrates are ideal because they are quickly digested and absorbed into the bloodstream, leaving the stomach quickly so there is less chance of indigestion and nausea during the event.

Carbohydrates should make up the largest portion of the athlete's diet. Research suggests that to maintain adequate carbohydrate stores during heavy training, carbohydrate intake should range from 7-10 grams/kilogram of body weight/day or 55-70% carbohydrate.

Athletes who train exhaustively on successive days, or who compete in more prolonged endurance events, would benefit from a diet that contains 65% to 70% of total calories from carbohydrates.



Of all the physiological factors that can cause early fatigue during exercise, dehydration is arguably the most important. Dehydration is a common occurrence, even during exercise in the cold. Dehydration decreases performance, impairs cardiovascular function that can impair physical performance, and pose serious health problems.

Athletes can lose a large amount of fluid when exercising. An athlete can become dehydrated in as little as 30 minutes.

Dehydration occurs when fluid (sweat) loss exceeds 1% of body weight (800 ml in the case of an 80-kilogram male). Work capacity and temperature control can be impaired with a loss of as little as 2% of body weight and can cause a 5-10% drop in performance. A 1/10th-temperature change can decrease performance.

During any form of exercise, working muscles produce heat, and body temperature rises. Getting rid of this heat requires fluid evaporation from the skin (sweating) which cools the body. When fluid evaporates from the skin, the body therefore loses valuable water (as high as 1-2 quarts/hour) during heavy exercise. When the athlete fails to ingest enough fluid and dehydration becomes sufficiently severe, sweating decreases in an attempt to conserve body water. As a result, blood thickens, heart rate increases and body temperature rises and you get fatigue, headache, nausea, chills, stomach discomfort and increasing the chance of heat cramps, exhaustion, or stroke.

Recent research illustrates that maintaining normal or near hydration during exercise maintains cardiovascular and thermoregulatory responses and improves exercise performance. It is always the athlete's best interest to prevent unnecessary increases in core temp that will ultimately lead to premature fatigue.

As you exercise, be alert for these conditions. They'll increase your loss of fluid through sweat, which could make your body become dehydrated faster.

Temperature: The higher the temperature, the greater your sweat losses.

Intensity: The harder you work out, the greater your sweat losses.

Body size: The larger the athlete, the greater the sweat losses. Males generally sweat more than females.

Duration: The longer the workout, the greater your fluid losses.

Fitness: Well-trained athletes sweat more. And they start sweating at a lower body temperature. Why? The function of sweating is to cool the body. The well-trained athlete cools his or her body more efficiently than an untrained person.

Most active people have experienced dehydration and probably haven't been aware of it. Exercise blunts the thirst mechanism and cannot rely on thirst as an indicator of dehydration. Providing adequate fluid intake before, during and after exercise can prevent dehydration. Fluid replacement practices should be designed to match fluid intake and sweat loss. This approach will require sports competitors to ingest more fluid, often more that they are accustomed to drinking.

The rate of fluid replacement should match rate of sweating (at least 80%)

Each pound of weight loss corresponds to 450 ml (15 ounces) of dehydration (8 ounces = 1 cup)

Plain, cool water (5 degrees to 10 degrees C; 40 degrees to 50 degrees F) is an effective fluid replacement, and it is the most readily available and least costly alternative. Drink plain water before or during activities lasting 60 minutes or less.

Electrolytes (sodium, potassium, chloride, and magnesium) are lost in sweat, but the loss of water is considerably greater. However, in sport or training that requires more than 1 hour of continuous effort, a carbohydrate-electrolyte solution should be provided to improve performance.


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Traci Kaufman, Registered Dietitian, received her bachelor's degree in dietetics and nutrition from the University of Wisconsin-Madison. She has worked as a clinical nutritionist at UCI Medical Center-Irvine in Orange California, and served as team nutritionist for the Los Angeles Rams. Traci is an active member of the American Dietetic Association and two Dietetic Practice Groups: Sports, Cardiovascular, and Wellness Nutritionist (SCAN), and Dietitians in General Clinical Practice. Traci resides in Southern California.


This page created 1999

This page modified October 2006

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