Protein catabolism yields ___________ cal/g.

Protein catabolism yields approximately 4 calories per gram. This value reflects the energy released during the breakdown of proteins into their constituent amino acids, which are then utilized by the body for various metabolic processes, including energy production, synthesis of new proteins, and other critical functions.

The energy content from proteins is lower than that from fats, which yield about 9 calories per gram, and higher than that from carbohydrates, which also yield around 4 calories per gram. The 4 cal/g figure takes into account the complexity of protein molecules and their metabolism in the body, where some energy is lost as heat during the conversion processes. This understanding is critical for nutritional and metabolic studies, as it helps in calculating the energy balance related to diet and metabolism.