Carbohydrates are the body’s most readily available source of energy. For runners, who place high and sustained demands on their energy systems, carbohydrate availability is crucial for performance, recovery, and adaptation to training. The key debate is not whether a runner needs carbs, but rather how many and when to consume them for optimal results. Understanding the science behind carbohydrate storage and utilization is essential for creating an effective fueling strategy.
The Role of Glycogen in Running Performance
The body converts consumed carbohydrates into glucose, which is then stored primarily in the muscles and liver as glycogen. Muscle glycogen is the most direct and quickly accessible fuel source for working muscles during a run. The body has a limited storage capacity for this high-octane fuel, which is why it becomes a limiting factor in endurance events.
While the body possesses vast stores of fat, which can also be metabolized for energy, fat metabolism is a slower and more oxygen-intensive process. This makes fat an insufficient fuel source for high-intensity efforts or sustained moderate-to-fast running paces. At exercise intensities above approximately 60% of an individual’s maximum effort, the reliance on carbohydrate-derived glycogen increases significantly.
When muscle and liver glycogen stores become severely depleted, the runner experiences a rapid loss of energy and performance, a phenomenon commonly known as “hitting the wall” or “bonking.” This sudden fatigue occurs because the body can no longer generate adenosine triphosphate (ATP) quickly enough to sustain the muscular demands of running. Although fat continues to burn, its rate of energy production cannot match the demands of the pace, forcing a drastic slowdown. Fat metabolism also requires the presence of glucose to proceed efficiently, making glucose availability a limiting factor even when relying on fat stores.
Fueling Strategies Before, During, and After a Run
Strategic timing of carbohydrate intake around a run is just as important as the total amount consumed. Before a run, the goal is to top off muscle glycogen stores and maintain stable blood sugar without causing gastrointestinal distress. Complex carbohydrates, such as oatmeal or whole-grain toast, should be consumed two to four hours prior to provide a steady energy release for sustained activity.
Closer to the start, specifically 30 to 60 minutes before, a small, easily digestible snack of simple carbohydrates, like a banana or a few pretzels, can provide a quick energy boost. These immediate pre-run snacks should be low in fat and fiber to minimize the chance of stomach issues during the run.
For runs lasting longer than 60 to 90 minutes, consuming carbohydrates during the exercise becomes necessary to spare existing glycogen stores and maintain blood glucose levels. Runners should aim to ingest 30 to 60 grams of carbohydrates per hour after the first 45 minutes of running. This mid-run fuel often comes from simple sugars found in energy gels, chews, or sports drinks, which are quickly absorbed into the bloodstream.
The post-run period focuses on recovery, specifically the rapid replenishment of depleted glycogen stores. Consuming carbohydrates within 30 to 60 minutes after a long or hard run is a practice that optimizes this process. Combining carbohydrates with protein in a ratio of approximately 3:1 is recommended to accelerate glycogen resynthesis and support muscle repair. For instance, a recovery drink like chocolate milk provides both the necessary carbohydrates and protein to jumpstart recovery. Following this immediate window, a balanced meal with complex carbohydrates, lean protein, and healthy fats helps to complete the refueling process within a couple of hours.
Adjusting Carbohydrate Intake for Training Volume
A runner’s overall carbohydrate needs must be periodized, meaning the quantity should fluctuate based on the daily training load, a concept known as “fueling for the work required.” On rest days or days with short, low-intensity runs, the body’s needs are relatively low, and a focus on balanced, nutrient-dense meals is appropriate. During these low-volume periods, a runner might aim for 3 to 5 grams of carbohydrate per kilogram of body weight.
In contrast, an endurance runner preparing for a marathon or undertaking high-volume training requires significantly more daily carbohydrate intake to ensure full glycogen stores. For heavy training days or the 24 hours leading up to a long run, intake should be increased to 7 to 12 grams per kilogram of body weight.
Carbohydrate loading is a specific, temporary strategy used in the days immediately before a competition to maximally stock the body’s glycogen reserves. This involves increasing the percentage of calories from carbohydrates to a very high level while simultaneously reducing training volume, or tapering. This adjustment ensures runners have the necessary fuel for peak performance.