Carbohydrates are the body's preferred fuel source during moderate-to-high intensity exercise. They are stored as glycogen in muscles (approximately 300–400 g) and the liver (about 75–100 g), providing readily available energy for athletic performance. Glycogen is the primary fuel for exercise above roughly 65% VO2max, and its depletion is a major cause of fatigue and reduced performance. The crossover concept in exercise metabolism describes how the body shifts from fat oxidation to carbohydrate oxidation as the primary fuel source as exercise intensity increases, with the crossover point typically occurring around 65% VO2max, varying with training status and diet. The Randle cycle (glucose-fatty acid cycle) similarly describes the competition between glucose and fatty acids for oxidation: when fat availability is high, fat oxidation increases and glucose oxidation decreases, and vice versa.
Carbohydrates are classified as simple or complex based on their chemical structure and digestion rate. Simple carbohydrates (monosaccharides and disaccharides) are quickly digested and raise blood sugar rapidly, as found in fruit, table sugar, and honey. Complex carbohydrates (polysaccharides) digest more slowly and provide sustained energy, with oats, rice, and potatoes being common examples. The glycemic index (GI) ranks carbohydrates on a scale of 0–100 based on how quickly they raise blood glucose: high GI foods (above 70) cause rapid spikes, while low GI foods (below 55) produce slower, sustained rises. Athletes should consume high GI foods during and immediately after exercise, when rapid glycogen replenishment and quick energy are most beneficial. Fiber, while important for gut health, blood sugar regulation, and satiety at 25–35 g per day, should be avoided 2–3 hours before exercise to prevent gastrointestinal discomfort.
Endurance athletes typically need 6–10 g of carbohydrate per kg of body weight per day, depending on training volume and intensity, with needs potentially exceeding 10 g/kg/day during ultra-endurance events. Carb loading is a strategy to maximize glycogen stores before endurance events lasting longer than 90 minutes, using 8–12 g of carbs per kg per day for 1–3 days before competition while tapering training, which can lead to glycogen supercompensation — loading muscles with 150–200% of normal glycogen levels. During exercise lasting longer than 60 minutes, athletes should consume 30–60 g of carbohydrates per hour from easily digestible sources such as sports drinks, gels, and bananas. For events longer than 2.5 hours, intake can increase to up to 90 g per hour by using a 2:1 glucose-to-fructose ratio, which utilizes two different intestinal transport pathways (SGLT1 and GLUT5) for greater absorption. Gut training — practicing race-day nutrition during training — can improve gastrointestinal tolerance and nutrient absorption over time.