What is considered a macronutrient that acts as a secondary energy source during exercise?

Protein is indeed considered a macronutrient that can act as a secondary energy source during exercise, particularly in circumstances where carbohydrates and fats are not available or when the body is under prolonged physical stress. While the primary role of protein in the body is to support growth, repair, and maintenance of tissues, it can be utilized for energy when other sources are depleted. During intense or extended exercise, the body may begin to break down protein from muscle tissue to convert into glucose through a process called gluconeogenesis, especially when glycogen stores are low.

In comparison, carbohydrates and fats serve as the primary sources of energy during exercise; carbohydrates are typically the preferred fuel, especially for high-intensity activities, while fats are used predominantly during lower-intensity, longer-duration efforts. Vitamins do not provide energy and are important for various metabolic functions and the maintenance of one's health. Thus, in the context of exercise and energy utilization, protein's role as a secondary energy source becomes significant in certain scenarios.