What is the primary vitamin that also functions as a hormone and regulates calcium metabolism?

Vitamin D serves as a fundamental component in the regulation of calcium metabolism in the body. Unlike most vitamins, which primarily act as coenzymes or cofactors in metabolic reactions, vitamin D has a unique role as a hormone. It is synthesized in the skin when exposed to sunlight and can also be obtained from dietary sources.

Once activated in the body, vitamin D enhances the absorption of calcium and phosphorus in the intestines, plays a crucial role in bone mineralization, and is essential for maintaining proper levels of calcium in the blood. This regulation is vital for various physiological processes, including muscle function and nerve signaling, as well as overall bone health. Consequently, when calcium levels decrease, vitamin D helps to increase absorption from food and also promotes the mobilization of calcium from bones when necessary.

The other vitamins listed do not have this significant role in calcium regulation. Vitamin A is primarily involved in vision and immune function, vitamin C is essential for collagen synthesis and acts as an antioxidant, while vitamin E also serves primarily as an antioxidant. None of them have the same hormone-like action regarding calcium metabolism as vitamin D does.