What do proteins primarily serve as in the body apart from energy?

Proteins play a crucial role in the body primarily as the building and repairing materials for tissues. They are essential components of every cell and are involved in the formation of muscles, skin, organs, and other tissues throughout the body. When the body needs to grow or heal, proteins facilitate these processes by providing the necessary amino acids that are the building blocks of new tissues. This function is fundamental to maintaining overall health, supporting recovery from injury, and contributing to growth and development.

The other options, while they relate to some functions of proteins, do not encompass the primary role as effectively. For instance, proteins do not serve as a source of carbohydrates; rather, carbohydrates are their distinct macronutrient category. While proteins contribute to the formation of hemoglobin, the specialized protein responsible for oxygen transport in blood, this is more of a specific function rather than the broad primary role of proteins. Lastly, while proteins do play a role in bone health, they are not the primary building blocks for bones; this function is mainly attributed to minerals like calcium and phosphorus. Therefore, the answer focusing on building and repairing tissues represents the most comprehensive and significant role of proteins in the body.