How Do Carbs Fuel Exercise?

granola with fruit and yogurt

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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.

What Do Carbs Do?

Carbohydrates serve a variety of important functions, including:

  • Providing energy to fuel the body including the brain, heart, and central nervous system
  • Aiding digestion
  • Managing blood cholesterol levels
  • Helping control blood glucose and insulin metabolism

Not getting enough carbohydrates can have consequences such as weakness, fatigue, constipation, vitamin deficiencies, and difficulty concentrating.

The human brain utilizes 20% to 25% of the body's glucose.

How Carbohydrates Fuel Exercise

Complex carbohydrates are an efficient source of energy that fuels 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 is 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 is 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.

Consuming enough carbohydrates ensures you have access to the energy you need for exercise. It also helps maintain mental sharpness for endurance sports.

Calculating Your Carbohydrate Needs

One gram of carbohydrates provides four calories of energy. The body can store a maximum of 15 grams of glycogen 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 of Carbohydrates

There are two different types of carbohydrates found in food: simple and complex. 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.

Simple Carbohydrates

Simple carbohydrates are absorbed and converted very quickly, providing a rapid source of energy. Some naturally occur in milk and fruit, but most of the simple carbs in American diets are sweeteners that are added to foods, such as sugar, corn syrup, or fruit juice concentrations. Sports drinks and sweetened fruit juices are quick sources of simple carbs.

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.

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.

Carbohydrates in a Balanced Diet

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 means at least 50% of your daily energy intake should come from carbohydrates, 35% or less from fats, and the remainder from protein.

For athletes, the proportion may need to be adjusted to accommodate increased energy needs. So, for example, an athlete might need to get 60% of their calories from carbs and limit fats to 30% or less.

Frequently Asked Questions

What do carbohydrates do?

Carbohydrates provide energy for your body, brain, heart, and nervous system, as well as assist with digestion and help control blood cholesterol, blood glucose, and insulin metabolism.

What foods don’t contain carbohydrates?

Meat, fish, some cheeses, eggs, oils, and plain coffee or tea don't contain carbohydrates. Foods that are low in carbohydrates include non-starchy vegetables, high-fat fruits (think avocado and coconut), nuts, and seeds.

What does the body do with excess carbohydrates?

Glucose is stored as glycogen, a readily available form of glucose, in the liver and muscles for quick energy when needed.

Do carbs turn into sugar?

Carbohydrates are converted into blood sugars (such as glucose, fructose, and galactose) in the body for immediate energy needs. Glucose is then converted into glycogen and stored for use in the future,

A Word From Verywell

Carbohydrates are an important source of energy. How many carbs the body requires differ from person to person, so talk to your doctor or a registered dietitian to determine what your unique dietary carbohydrate needs are.

4 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.
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  2. Goyal MS, Raichle ME. Glucose requirements of the developing human brainJ Pediatr Gastroenterol Nutr. 2018;66(Suppl 3):S46-S49. doi: 10.1097/MPG.0000000000001875

  3. Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-259. doi:10.1093/nutrit/nuy001

  4. 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

Additional Reading

By Elizabeth Quinn, MS
Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics.