Vitamin D is a fat-soluble vitamin that is naturally present in very few foods, fortified in others, and available as a dietary supplement. It is also synthesized by the body when ultraviolet (UV) rays from sunlight interact with the skin, triggering vitamin D production. However, vitamin D obtained from sun exposure, food, and supplements is biologically inactive and must undergo metabolic processes in the liver and kidneys to become its active form, which the body can utilize.

Vitamin D plays a crucial role in calcium absorption in the gut and helps maintain optimal serum calcium and phosphate levels, ensuring proper bone mineralization. It is essential for bone growth and remodeling, supporting overall skeletal health. A deficiency in vitamin D can lead to thin, brittle, or misshapen bones. When combined with calcium, vitamin D also helps reduce the risk of osteoporosis, particularly in older adults.

Vitamin D plays multiple roles in the body beyond bone health. It helps regulate cell growth, supports neuromuscular and immune function, and contributes to reducing inflammation. Additionally, vitamin D influences the expression of many genes involved in cell proliferation, differentiation, and apoptosis (cell death), highlighting its importance in overall cellular health.

Vitamin D exists in two forms:

1. Vitamin D2, derived from the UV irradiation of the yeast sterol ergosterol and found naturally in sun-exposed mushrooms.

2. Vitamin D3, synthesized in the skin when exposed to sunlight and naturally present in oil-rich fish such as salmon, mackerel, and herring.

The best indicator of vitamin D status is the serum concentration of 25-hydroxyvitamin D [25(OH)D], as it reflects both cutaneous production and intake from food and supplements. Due to its long circulating half-life of approximately 15 days, cholecalciferol (Vitamin D3) supplements can be administered weekly.

The American Society of Endocrinology recommends that adults who are vitamin D deficient be treated with 50,000 IU of vitamin D2 or vitamin D3 once a week for 8 weeks, or the equivalent of 6,000 IU/day of vitamin D2 or vitamin D3. This regimen aims to achieve a blood level of 25(OH)D above 30 ng/ml. Following the initial treatment phase, maintenance therapy of 1,500–2,000 IU/day is recommended.

References:

Institute of Medicine, Food and Nutrition Board. Dietary Reference Intakes for Calcium and Vitamin D. Washington, DC: National Academy Press, 2010.

Michael F. Holick, et al Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline, The Journal of Clinical Endocrinology & Metabolism, Volume 96, Issue 7, 1 July 2011, Pages