Question corner: Sunshine vitamin

Football supporters wearing Mexican style hats arrive in the sunshine to see Manchester United's English Premier League soccer match against Norwich City at Old Trafford Stadium, Manchester, England, Saturday Oct. 1, 2011. Britain has experienced the hottest October day on record. National weather service the Met Office says Saturday's temperature reached 29.6 degrees Celsius (85.3 Fahrenheit) at Gravesend in southeast England. (AP Photo/Jon Super)

How does exposure to sun help to produce Vitamin-D in humans?

S.M. Sivadarshini

Tiruchirappalli, Tamil Nadu

Vitamin D, a fat soluble vitamin resembles sterols in structure and functions like a hormone. Ergocalciferol and cholecalciferol, both sterols are the precursor substances for the synthesis of vitamin D and are referred to as provitamins. Ergocalciferol is designated as vitamin D ₂ and cholecalciferol also as vitamin D ₃ .

Ergocalciferol is the provitamin found in plants and cholecalciferol is the provitamin present in animals. In man the provitamin cholecalciferol (also called calciol) is synthesized from 7-dehyrocholesterol an intermediate product of cholesterol biosynthesis. The conversion of 7- dehyrocholesterol into provitamin cholecalciferol takes place in the skin (dermis and epidermis) on exposure to sunlight.

The provitamins D ₂ and D ₃ as such are not biologically active. They are metabolized identically in the human body and converted into active forms of vitamin D. The active vitamin D is calcitriol.

Conversion of provitamins into active calcitriol takes place in two steps. The first step occurs in the liver where the provitamin is converted into hydroxycholecalciferol and in the second step, which occurs in the adrenal cortex of our kidney, the hydroxycholecalciferol is converted into active vitamin called calcitriol.

Anyhow, for the synthesis of provitamins from which active vitamin D is formed, sunlight is very essential. The incident UV-rays in the sunlight helps for this. Humans make 90 per cent of their vitamin D naturally from sunlight exposure to their skin – specifically, from ultraviolet B exposure to the skin, which naturally initiates the conversion of cholesterol in the skin to vitamin D ₃ .

Few foods naturally contain or are fortified with supplemental vitamin D. It is just 10 per cent of what the most we need daily. In contrast, sun exposure to the skin makes thousands of units of vitamin D naturally in a relatively short period of time. Therefore Vitamin D is regarded as Sun Shine Vitamin.

S. Palaniappan

Pudukottai, Tamil Nadu