Vitamin D

Vitamin D is a essential fat-soluble vitamin. It is primarily synthesized by the body through UV exposure, which converts 7-dehydrocholesterol in the skin into cholecalciferol (D3). This precursor travels to the liver to become calcifediol, and then to the kidneys to be converted into the active form, calcitriol, by the enzyme 1-alpha-hydroxylase.

Calcitriol maintains healthy levels of calcium and phosphate by increasing their absorption in the gastrointestinal tract, promoting renal reabsorption in the kidneys, and facilitating bone remodeling. Low calcium levels trigger the parathyroid gland to release PTH, which in turn stimulates 1-alpha-hydroxylase activity to boost Vitamin D activation.

Vitamin D deficiency is common due to limited sunlight exposure and skin pigmentation factors. In children, it causes rickets, characterized by bendy bones due to inadequate mineralization, while in adults, it leads to osteomalacia, resulting in brittle bones as minerals are leached from skeletal structures.

While toxicity is rare from sunlight, it can occur through excessive supplementation. Symptoms of Vitamin D overdose are summarized as 'stones' (kidney stones), 'bones' (skeletal issues), 'abdominal moans' (gastrointestinal distress from muscle contraction), and 'psychic groans' (neurological depression caused by excess calcium interfering with nerve firing).