Sports Nutrition How Do Carbs Fuel Exercise? By Elizabeth Quinn, MS Elizabeth Quinn, MS Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics. Learn about our editorial process Updated on October 12, 2021 Medically reviewed Verywell Fit articles are reviewed by board-certified physicians and nutrition and exercise healthcare professionals. Medical Reviewers confirm the content is thorough and accurate, reflecting the latest evidence-based research. Content is reviewed before publication and upon substantial updates. Learn more. by Melissa Rifkin, MS, RD, CDN Medically reviewed by Melissa Rifkin, MS, RD, CDN Melissa Rifkin is a Connecticut-based registered dietitian with over 15 years of experience working in the clinical setting. Learn about our Medical Review Board Print Cultura RM Exclusive / Danielle Wood / Getty Images Table of Contents View All Table of Contents What Do Carbs Do? How Carbs Fuel Exercise Calculating Your Carb Needs Dietary Sources of Carbs Carbs in a Balanced Diet Frequently Asked Questions 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 systemAiding digestionManaging blood cholesterol levelsHelping 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 muscles90 grams (360 calories) of carbs are stored in the liver5 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. Your Complete Guide to Carbohydrates 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. Holesh JE, Aslam S, Martin A. Physiology, carbohydrates. In: StatPearls [Internet]. StatPearls Publishing. Goyal MS, Raichle ME. Glucose requirements of the developing human brain. J Pediatr Gastroenterol Nutr. 2018;66(Suppl 3):S46-S49. doi: 10.1097/MPG.0000000000001875 Murray B, Rosenbloom C. Fundamentals of glycogen metabolism for coaches and athletes. Nutr Rev. 2018;76(4):243-259. doi:10.1093/nutrit/nuy001 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 Jensen J, Rustad P, Kolnes A, Lai Y. The role of skeletal muscle glycogen breakdown for regulation of insulin sensitivity by exercise. Front Physiol. 2011;2. doi:10.3389/fphys.2011.00112 Kanter M. High-quality carbohydrates and physical performance. Nutr Today. 2018;53(1):35-9. doi:10.1097/NT.0000000000000238 By Elizabeth Quinn, MS Elizabeth Quinn is an exercise physiologist, sports medicine writer, and fitness consultant for corporate wellness and rehabilitation clinics. See Our Editorial Process Meet Our Review Board Share Feedback Was this page helpful? Thanks for your feedback! What is your feedback? Other Helpful Report an Error Submit