Vitamin D in health and disease

Yogita Dhas, (SIU-JRF)

In the last few years, numerous studies have shown that modest exposure to sunlight may actually be good for you, helping the body to produce vitamin D. It needs to keep bones healthy and protect against many diseases.

Basically, vitamin D is unique among hormones because it can be made in the skin from exposure to sunlight. Vitamin D was first identified and characterized in 1923 by Goldblatt and Soames. Vitamin D exists in two basic forms; vitamin D3 (cholecalciferol) and vitamin D2 (ergocalciferol). Vitamin D is produced endogenously in the skin from the cholesterol precursor 7-dehydrocholesterol, naturally, present in the skin or obtained from lanolin. Vitamin D3 production beneath the skin depends on skin pigmentation, sunscreen use, time of day, season, latitude, altitude, and air pollution. The foods sources of vitamin D include cod liver oil and fatty fish (e.g., salmon, mackerel, and tuna); foods fortified with vitamin D; and supplements. Both, forms of vitamin D (D3 & D2) require the same activation process, involving first 25-hydroxylation in the liver, followed by 1α-hydroxylation in the kidney, to make the biologically active form. Following the final converting step in the kidney, calcitriol (the physiologically active form of vitamin D) is released into the circulation. By binding to vitamin D-binding protein (VDBP), a carrier protein in the plasma, calcitriol is transported to various target organs.

Vitamin D deficiency is common in metropolitan cities of India and worldwide due to an indoor lifestyle, less consumption of vitamin D rich diet and use of sunscreen. The estimated prevalence of vitamin D deficiency in India is 50% to 70% in adult population, despite the fact that it is located between 8.4° and 37.6° north latitude and majority of the Indian population exposed to ample sunlight throughout the year. For Indian skin tone, minimum "direct sun exposure" required daily is more than 45 min to bare face, arms, and legs to sun's UV-B rays (wavelength 290-310 nm) because darker skin has high melanin content. Melanin acts as a natural sunscreen and produces a significantly lesser amount of vitamin D when compared with the individuals with fairer skin.

Vitamin D deficiency causes a decrease in the intestinal absorption of calcium and phosphorus, resulting in an increase in PTH levels, resulting in osteopenia and osteoporosis. Vitamin D deficiency is responsible for muscle weakness, an increase in body sway, and an increased risk of falling. Also, vitamin D deficiency has been associated with an increased risk of diabetes mellitus, cardiovascular diseases, certain cancers, cognitive decline, depression, pregnancy complications, autoimmunity, allergy, and even frailty. Therefore, there is need to implement measures to educate the Indian population about the status of vitamin D and also the modes to attain vitamin D sufficiency.