What nutrient is primarily responsible for building and repairing tissues in the body?

The nutrient primarily responsible for building and repairing tissues in the body is proteins. Proteins are made up of amino acids, which are essential for various physiological functions, including the growth and maintenance of tissues. They play a crucial role in the regeneration of muscle, skin, organs, and other tissues. Enzymes and hormones, which regulate metabolic processes, are also proteins.

While carbohydrates serve as a primary energy source and fats provide energy and support cell structure, they do not have the direct role in tissue building and repair that proteins do. Vitamins, on the other hand, are involved in various biochemical reactions and support processes such as immune function and skin health, but they do not contribute to the structural formation of tissues. Therefore, proteins are uniquely vital for tissue health and recovery, making them the correct answer.