Exposure to sunlight cheapest and easily available source of Vitamin D

Dr Harshal Aswar

Vitamin D (1, 25 dihydroxy vitamin D) is synthesized in the skin on exposure to sunlight (UVB spectrum wavelength 290-320 nm). Infact it is a hormone controlling mineral ion metabolism and other pleomorphic function. The community-based Indian studies of the past decade done on apparently healthy controls reported a prevalence ranging from 50% to 94%.

On exposure to sunlight 7- Dehydrocholesterol undergoes photolysis and Previtamin d2 ( Cholecalciferol) is formed. Previtamin D2 undergoes 25- hydroxylation and 1 alpha hydroxylation in liver and kidneys forming vitamin D2 and vitamin D3 (Calcitriol) respectively. Vitamin D3 has highest biological activity compared to its metabolites. Vitamin D controls its inactivation by 24- hydroxylation limiting its biological effects.

Vitamin D Metabolism

Metabolic activation of vitamin D to Calcitriol and its effects on calcium and phosphate homeostasis. The result is an increase in the serum calcium and phosphate concentrations.

Sources:  Except for fish other food products contain only limited amount of Vitamin D unless fortified. Major source remains photolysis reaction upon exposure to sunlight (UVB). When an adult wearing a bathing suit is exposed to one Minimal Erythemal Dose (MED) of UV radiation (a slight pinkness to the skin 24 h after exposure), the amount of vitamin d produced is equivalent to ingesting between 10,000 and 25,000 IU. A variety of factors reduce the skin’s production of Vitamin D, including increased skin pigmentation, aging, and the topical application of a sunscreen. UVB spectrum with wavelength of 290- 320 is maximum  in sunlight during the brightest hours i.e 1 pm to 3 pm. Minimal Erythema Dose is generally understood as the amount of UV radiation that will produce minimal erythema (sunburn or redness caused by engorgement of capillaries) of an individual’s skin within a few hours following exposure to UVB light. This can be done over 10- 20 min of sun exposure during brightest hours. Erythema goes off on itself within 24 hrs.

Recommended Dietary Allowances (RDA): National Academy Of Medicine 2010 report recommended daily intake of Vitamin D as 600 IU from 1- 70 years of age and 800 IU for those over 70 years. Safety range for Vitamin D is broad. Toxicity is seen those taking doses in the range of 40,000 units daily; however upper limit of Vitamin D daily intake is set at 4000 IU.

Functions: Vitamin D in its active form i.e. Vitamin D3 is required for absorption of calcium and phosphate from intestines (duodenum), distal tubules in the kidneys and maintainace of calcium and phosphate homeostasis. For this function it may cause resorption of bones via RANK ligand mediated osteoclast activity. Apart from bone mineral ion metabolism it is required for maintaining functions of many non skeletal tissues like muscles including heart muscles, immune functions, cell proliferation and differentiation.

Deficiency state: The National Academy of Medicine has defined vitamin D sufficiency as Vitamin D levels > 50 nmol/l (> 20 ng/ml).

 Vitamin D levels in ng/mL

Deficiency of Vitamin D in children before epiphyseal fusion, results in Growth Retardation and expansion of growing ends of bones. This is known as Rickets. Osteomalacia is impaired mineralization of bone matrix causing bowing and frequent fractures of long weight bearing bones. Proximal myopathy, weakness of shoulder and hip muscles is a striking features of vitamin D deficiency. Vitamin D is also required for proper functioning of Immune, Muscular, and Cardiovascular systems, although more research is needed in these areas. In long standing Vitamin D deficiency , secondary hyperparathyroidism ensues causing osteoporosis .

Excess/toxicity: Main consequences are due to hypercalcemia. Weakness, nausea, vomitting , constipation, nephrolithiasis and renal failure are few symptoms caused by Vitamin D excess.

Risk factors for deficiency of Vitamin D:

  1. Lack of vitamin D in the diet, often in conjunction with inadequate sun exposure

Infants, children, and older adults are at risk for low vitamin d levels because of inadequate vitamin D intake. Human breast milk contains low levels of vitamin D, and most infant formulas do not contain adequate vitamin D. Older adults often do not consume enough vitamin D-rich foods, and even when they do, absorption may be limited. People who do not or cannot consume dairy products are also at risk for vitamin D deficiency.

  •  Inability to absorb vitamin D from the intestines

Diseases affecting the body’s ability to absorb adequate amounts of vitamin D through the intestinal tract are Celiac disease, Crohn’s disease, and Cystic fibrosis.

  •  Inability to process vitamin D due to kidney or liver disease

The liver and kidney have important enzymes that change vitamin D from sun-exposed skin or food to the biologically active form of vitamin D. People with chronic kidney and liver disease are at increased risk of low active vitamin D levels because they have decreased levels of these enzymes.


Vitamin D is cheapest and widely available. Adequate exposure to sunlight i.e  10- 20 min in brightest hours is  sufficient. Children, old adults often due to low intake suffer deficiency.

(The author is a Senior Resident, Department of Medicine in the Faculty of Medicine and Health Sciences, SGT University, Gurugram)

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