What type of nutrient utilization improves with high-volume endurance training?

High-volume endurance training primarily enhances the body's ability to utilize lipids, or fats, as a source of energy during prolonged exercise. As endurance training increases, the body's capacity to oxidize lipids improves, leading to several physiological adaptations.

When athletes undergo extensive endurance training, their muscle cells become more efficient at transporting and using fatty acids for fuel. This adaptation occurs due to an increase in the number and size of mitochondria—the powerhouses of the cell responsible for energy production—along with upregulation of enzymes involved in fat metabolism. Endurance-trained individuals also have a greater ability to mobilize and transport lipids from adipose tissue to muscle cells.

On the other hand, while carbohydrate and protein utilization can also be affected by training, they do not increase to the same extent through high-volume endurance training as lipid utilization does. Carbohydrates serve as a primary energy source during high-intensity exercise, while protein is mainly used for tissue repair and growth rather than as a direct fuel source during endurance activities.

Water is essential for various bodily functions and hydration during exercise but does not serve as a nutrient that the body utilizes for energy production. Therefore, the correct answer focuses specifically on the body’s enhanced ability to utilize lipids more effectively due to the