Why Carbohydrates Are Important for Exercise

How Much You Need to Achieve Maximal Output

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All the energy we need for life comes from the foods we eat and the fluids we drink. These nutrients are broadly broken into fats, proteins, and carbohydrates. Carbohydrates play an especially important role as they provide the quick energy needed for exercise.

Carbohydrates found in foods like grains, fruits, vegetables, beans, and dairy products are your body's favorite source of energy, but this is not the only role that carbs play. They also ensure mental sharpness and aid in the metabolism of fat for energy.

How Carbohydrates Fuel Exercise

Complex carbohydrates are an efficient source of energy that fuel muscle contractions. Once eaten, carbs are broken down into smaller sugars (glucose, fructose, and galactose) to be used as energy for immediate tasks. Any unused glucose will be converted into glycogen and stored in the muscles and liver for future use.

Glycogen is the energy source most often used for short, intense bouts of exercise such as sprinting or weightlifting. Because glycogen is stored in muscles, it is immediately accessible. During bursts of activity, the stored glycogen will be converted back to glucose and burned for fuel. This is the typical energy source for the first few minutes of any sport.

During endurance exercise, glycogen can also break down fat into something the muscles can use for fuel. Protein can also be broken down and used as a last resort, but this stresses the kidneys and limits the body's ability to build and maintain muscle tissue.

Beyond muscle contraction, carbs supply energy to the brain. If you have ever felt low energy or experienced a brain fog during exercise, it is likely because you are not getting enough carbs.

The ample intake of carbohydrates ensures you have access to the energy needed for exercise. It also helps maintain mental sharpness for endurance sports.

Calculating Intake Needs

One gram of carbohydrates provides four calories of energy. Maximal glycogen storage is approximately 15 grams per kilogram of body weight (15 grams per 2.2 pounds). This would mean that a 175-pound athlete could store up to 1,200 grams of glycogen (4,800 calories), fueling high-intensity exercise for quite some time.

Larger muscle mass provides greater glycogen storage but also increases the demands for energy. While every person is unique, the average carbohydrate storage capacity in the body roughly breaks down as follows:

  • 350 grams (1,400 calories) of carbs are converted to glycogen in muscles
  • 90 grams (360 calories) of carbs are stored in the liver
  • 5 grams (calories) of carbs are broken down and circulate in the blood as glucose

Exercise and diet changes can deplete these energy stores. If you don’t replenish the stores, you will run out of fuel for immediate exercise. Athletes often refer to this as "hitting the wall." By contrast, eating large amounts of carbohydrates can increase these stores. This is typically referred to as "carb-loading."

Dietary Sources

Carbohydrates can be broken down into two types.

Simple Carbohydrates

Simple carbohydrates are absorbed and converted very quickly, providing a rapid source of energy. While some of these are found naturally in milk and fruit, most of the simple carbs in American diets are added to foods, typically sugar, corn syrup, or fruit juice concentrations. Sports drinks and sweetened fruit juices are quick sources of simple carbs.

Complex Carbohydrates

Complex carbohydrates take longer to be digested, absorbed, and metabolized. Thus, they provide energy at a slower rate and are often stored as glycogen. Ideal sources include foods high in starch, such as whole-grain bread, cereals, pasta, and grains.

Of the two, complex carbs pack more nutrients than simple carbs. They are higher in fiber and are more slowly digested, meaning that they are less likely to cause spikes in blood sugar.

While simple carbs can provide you with the fuel you need for explosive bursts of energy, they are quickly spent and may be less appropriate for people with type 2 diabetes.

A Word From Verywell

To maintain energy, eat carbohydrates before and after intense exercise. It is equally important to eat a balanced diet with the appropriate proportion of carbs, proteins, and healthy fats.

Generally speaking, that would mean at least 50% of your daily energy intake would come from carbohydrates, 35% or less would come from fats, and the remainder would come from proteins.

For athletes, the proportion may need to be adjusted to accommodate the increased energy needs, increasing carbohydrates to 60% while decreasing fats to 30% or less.

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Article Sources
Verywell Fit uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-259. doi:10.1093/nutrit/nuy001

  2. Ferretti F, Mariani M. Simple vs. Complex Carbohydrate Dietary Patterns and the Global Overweight and Obesity Pandemic. Int J Environ Res Public Health. 2017;14(10) doi:10.3390/ijerph14101174

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