Which of the following supplements may act as a precursor during exercise?

HMB, or beta-hydroxy beta-methylbutyrate, is recognized for its role as a metabolic precursor, particularly during exercise. It is a metabolite of the amino acid leucine, which is significant in muscle protein synthesis and recovery. When the body undergoes stress during exercise, such as resistance training, HMB serves as an important component that can help to enhance muscle repair and reduce muscle breakdown.

The usage of HMB can provide benefits in maintaining muscle mass and may also contribute to improved strength and performance in athletes. It's focused on reducing exercise-induced muscle damage, which can be particularly beneficial during intense workouts or competitive events.

In contrast, while casein, glutamine, and whey protein have their own distinct roles in nutrition, muscle recovery, and overall health, they do not specifically act as precursors in the same manner as HMB does during the exercise phase. Casein is a slow-digesting protein beneficial for muscle recovery over extended periods, whey is a fast-digesting protein that supports muscle growth and recovery post-exercise, and glutamine plays a role in immune function and gut health, but none serve directly as a precursor to enhance muscle preservation during exercise like HMB.