How Carbohydrates Provide Energy for Exercise

All the energy we need for life as well as for exercise comes from the food we eat and the fluids we drink. These nutrients are commonly broken into three classes:

• Carbohydrates
• Fats
• Proteins

Each category of food is important for health and we should all consume foods from each category. The ratios in which we need to consume these foods, however, is often the topic of a debate.

Carbohydrate is arguably the most efficient source of energy for athletes. No matter what sport you play, complex carbs provide the energy that fuels muscle contractions. Once eaten, carbohydrates break down into smaller sugars (glucose, fructose, and galactose) that get absorbed and used as energy. Any glucose not needed right away gets stored in the muscles and the liver in the form of glycogen. Once these glycogen stores are filled up, any extra gets stored as fat.

Glycogen is the source of energy most often used for exercise. It is needed for any short, intense bouts of exercise from sprinting to weightlifting because it is immediately accessible. Glycogen also supplies energy during the first few minutes of any sport. During long, slow duration exercise, fat can help fuel activity, but glycogen is still needed to help break down the fat into something the muscles can use.

Adequate carbohydrate intake also helps prevent protein from being used as energy. If the body doesn’t have enough carbohydrate, protein is broken down to make glucose for energy. Because the primary role of protein is as the building blocks for muscles, bone, skin, hair, and other tissues, relying on protein for energy (by failing to take in adequate carbohydrate) can limit your ability to build and maintain tissues. Additionally, this stresses the kidneys because they have to work harder to eliminate the byproducts of this protein breakdown.

Carbohydrate has other specific functions in the body including fueling the central nervous system (CNS) and brain.

One gram of carbohydrate provides four calories of energy. Athletes often talk about carbohydrate loading and carbohydrate depletion which refers to the amount of carbohydrate energy we can store in our muscles. This is generally around 2,000 carbohydrate calories, but we can change this number through depletion and loading. During depletion (from diet, exercise, or a combination) we use up the stored carbohydrate.

If we don’t replenish these stores, we can run out of fuel for immediate exercise. Athletes often refer to this as "bonking" or "hitting the wall." In the same way, eating large amounts of carbohydrates can increase these stores. This is often referred to as carbohydrate loading or carbo-loading.

While every person is unique, and our carbohydrate storage capacity will vary, according to Dan Benardot, the author of Advanced Sports Nutrition, "Humans can store approximately 350 grams (1,400 kilocalories) in the form of muscle glycogen, an additional 90 grams (360 kilocalories) in the liver, and a small amount of circulating glucose in the blood (~5 grams, or about 20 kilocalories). The larger the muscle mass, the greater the potential glycogen storage but also the greater the potential need."

Another common figure in the research indicates that maximal glycogen storage is approximately 15 grams per kilogram of body weight (15 grams per 2.2 pounds). With this math, a 175-pound athlete could store up to 1200 grams of glycogen (4,800 calories) which could fuel high-intensity exercise for quite some time.

Carbohydrate stored as glycogen is an easily accessible source of energy for exercise. How long this energy supply lasts depends on the length and intensity of exercise and can range anywhere from 30 to 90 minutes or more.

To avoid running out of energy during exercise, start with full glycogen stores, replenish them during exercise and refill them after exercise to be ready for the next workout.

Carbohydrates are also divided into simple and complex forms. Simple sugars (carbs) are absorbed and converted to energy very quickly and provide a rapid source of energy. Fruit and sports drinks are a good source of simple carbohydrates.

Complex carbohydrates take a bit longer to be digested and absorbed into the body. They also take longer to break down and therefore provide energy at a slower rate than simple sugars. Examples of complex carbohydrates are breads, rice, and pasta.

Starch and fiber are also considered complex carbohydrates but fiber cannot be digested or used for energy. Starch is probably the most important energy source in an athlete’s diet because it is broken down and stored as glycogen. Foods high in starch include whole grain breads, cereals, pasta, and grains.