The body relies heavily on stored carbohydrates to fuel intense exercise involving a high heart rate and sustained effort. When these efforts last between one and two hours, internal fuel reserves begin to dwindle, making the intake of external carbohydrates a necessity for maintaining performance. Consuming external carbohydrates during the activity is necessary to prevent premature fatigue and a decline in power output. Understanding the specific timing and amount of intake can determine the difference between finishing strong and experiencing a significant performance drop.
Why Fueling is Essential During Exercise
During high-intensity exercise, the body’s preferred and most efficient energy source is glycogen, the stored form of glucose found in the muscles and liver. Muscle glycogen is the primary fuel for working muscles, but these limited reserves can be depleted within 90 to 120 minutes of intense effort. As muscle stores decline, the body increasingly relies on blood glucose, which the liver maintains by releasing its stored glycogen.
Introducing external carbohydrates (exogenous fuel) helps maintain stable blood glucose levels, reducing the rate at which the liver must release its limited stores. While often called “glycogen sparing,” the primary benefit is sustaining the fuel supply to the central nervous system and muscles late in the exercise bout. When blood glucose drops too low, performance suffers dramatically, leading to the well-known phenomenon of “hitting the wall.” Supplementing with carbohydrates mid-exercise allows athletes to sustain a higher output for a longer duration.
Specific Intake Guidelines for 1-2 Hours
For intense exercise lasting between one and two hours, the recommended intake is 30 to 60 grams of carbohydrates per hour. This range replenishes the glucose utilized by the muscles and maintains a consistent blood sugar level without overwhelming the digestive system. Targeting the higher end of this range is more beneficial as the exercise duration approaches the two-hour mark.
This recommendation differs from longer ultra-endurance events, which might require up to 90 grams per hour, amounts generally unnecessary for the one-to-two-hour window. The 30 to 60 grams per hour balances the metabolic need for fuel with the body’s capacity to absorb it. Consuming too little leads to fuel depletion, while consuming significantly more than 60 grams of a single type of carbohydrate risks gastrointestinal distress.
Choosing the Right Delivery Format
Consuming the required 30 to 60 grams of carbohydrates per hour involves choosing a delivery format that is easy to ingest and rapidly absorbed. Specialized sports drinks are a common option, providing necessary fluids and electrolytes alongside the carbohydrates. Gels and chews offer a concentrated, portable, semi-solid dose, typically containing 20 to 30 grams per serving.
Many modern sports nutrition products utilize a blend of multiple transportable carbohydrates, such as glucose and fructose, often in a 2:1 ratio. This combination is highly effective because glucose and fructose use different transport mechanisms in the gut, allowing the body to absorb a higher total amount of carbohydrate per hour. Easily digestible whole foods, like small pieces of banana or energy bars, can also contribute to the hourly target, provided they are trialed in training to ensure they do not cause stomach upset.
Recognizing Signs of Fuel Depletion
When an athlete fails to meet the required carbohydrate intake during the one-to-two-hour window, physical and cognitive symptoms of fuel depletion emerge. Common physical signs include a sudden drop in power output and a feeling of “heavy” or “leaden” legs. This physical decline results directly from muscles running low on their preferred glycogen fuel source.
Cognitive symptoms, often called “bonking” or “hitting the wall,” indicate the brain is being starved of energy, as it relies on a steady supply of blood glucose. These signs manifest as an inability to concentrate, cognitive fog, irritability, or malaise. Recognizing these symptoms suggests the athlete waited too long to initiate or sustain their fueling strategy, since the body requires time to absorb ingested carbohydrates and convert them into usable energy.