Protein is fundamental for tissue construction and repair, especially muscle, composed of amino acids. It also serves as an energy source, particularly during periods of increased physical activity.
Protein’s Contribution to Exercise Fuel
The body primarily relies on carbohydrates and fats for energy during exercise. Carbohydrates, stored as glycogen in muscles and the liver, are the most readily available fuel source, while fats provide a more concentrated, long-lasting energy supply. Protein typically contributes a smaller percentage to overall energy expenditure, often ranging from a few to about 10% of total energy for sustained exercise.
However, under certain circumstances, protein’s contribution to fuel can increase. When carbohydrate stores are low, such as during prolonged exercise or periods of fasting, the body can convert amino acids from protein into glucose. This process occurs mainly in the liver. This helps maintain blood sugar when carbohydrate glucose is scarce.
Amino acids, particularly branched-chain amino acids, can also be used directly by muscle cells to produce energy, similar to how fats and carbohydrates are used. This adaptability allows the body to sustain activity when preferred energy sources are depleted. The amount of protein used for energy can increase significantly under intense exercise conditions, potentially supplying up to 15% of energy needs.
Exercise Characteristics Impacting Protein Use
Exercise characteristics directly influence protein use for energy. Higher exercise intensity can lead to increased protein breakdown, especially when carbohydrate reserves are running low. This is because high-intensity efforts demand rapid energy production, and if carbohydrates are insufficient, the body turns to other sources, including amino acids.
As exercise duration extends, the reliance on protein as a fuel source generally increases. During prolonged aerobic activities, muscle protein is broken down, releasing amino acids that can be converted into glucose to maintain blood glucose. For example, protein contribution can increase from around 2% during shorter activities to between 5% and 10% of total energy during long-distance running or cycling.
Different types of exercise also affect protein utilization uniquely. In endurance activities, protein’s energy contribution becomes more noticeable over long durations, helping to sustain performance when glycogen stores diminish. Conversely, during resistance training, protein is primarily used for muscle repair and synthesis after the workout, though some protein breakdown does occur during the exercise itself. Intense resistance exercise causes micro-tears in muscle proteins, which then require amino acids for rebuilding and strengthening.
Nutritional and Physiological Determinants of Protein Use
The body’s internal state and dietary intake play a significant role in determining protein’s use as fuel. Carbohydrate availability is a major factor; when muscle and liver glycogen stores are low, the body’s reliance on protein for energy increases. This inverse relationship means that a sufficient intake of carbohydrates can spare protein from being used for fuel, allowing it to fulfill its primary roles in tissue repair and building.
An individual’s training status and fitness level also influence protein utilization. Trained individuals often become more efficient at burning fat for fuel, which helps spare protein. This improved fat oxidation capacity means they can rely less on amino acids for energy during prolonged exercise compared to less trained individuals.
Dietary protein intake itself affects protein metabolism during activity. While adequate protein is important for muscle maintenance and repair, consuming extremely low amounts can lead to more protein being broken down for energy during exercise. For athletes, recommended daily protein intake typically ranges from 1.2 to 2.0 grams per kilogram of body weight to support muscle recovery and adaptation.
Hormonal influences further regulate protein metabolism during exercise. Cortisol promotes the breakdown of body proteins, particularly muscle proteins, into amino acids during activity. In contrast, hormones like insulin and growth hormone promote anabolic processes, encouraging protein synthesis and reducing its breakdown, supporting muscle growth and recovery.