Final answer:
Vitamin D acts as a hormone in the human body. It is synthesized in the skin and activated in the kidneys, where it aids in calcium absorption and bone health. Vitamin D deficiency can cause rickets or osteomalacia due to its crucial role in maintaining calcium levels.
Step-by-step explanation:
Among the options provided, Vitamin D acts as a hormone in the human body. The skin synthesizes Vitamin D when exposed to UV radiation. This synthesis starts with the conversion of cholesterol in the skin to cholecalciferol, also known as Vitamin D3. It is then transformed in the liver to an intermediate form called calcidiol, and finally, in the kidneys, it is converted to its active form, calcitriol, which behaves as a hormone.
Calcitriol is involved in calcium absorption in the digestive tract, its reabsorption in the kidney, and maintaining normal serum concentrations of calcium and phosphate, which are crucial for bone health and other physiological processes. Because it has receptors in most cells and impacts various body functions, Vitamin D is suggested to be classified as a hormone rather than a vitamin.
Deficiency in Vitamin D can lead to diseases such as rickets in children and osteomalacia in adults, highlighting its importance for bone health. Supplements and certain foods can help maintain sufficient levels of Vitamin D, especially when sunlight exposure is limited.