Only a few foods contain vitamin D naturally. Fortunately, the body can make all the vitamin D within itself with the help of a little sunshine.

Vitamin D in foods

Most foods have negligible amounts of vitamin D. Most of the plant foods are poor sources of vitamin D. Some foods of animal origin like egg yolk, liver, fatty fish, butter and fortified milk, provide variable amounts of the vitamin. Vitamin D content of selected foods is given in below table

Vitamin D from the sun

Most of the world’s population relies on natural exposure to sunlight to maintain adequate vitamin D nutrition. The sun imposes no risk to vitamin D toxicity. For most people, exposing hands, face, and arms on a clear summer day for 10 to 15 minutes, at least 3 times a week, should be sufficient to maintain vitamin D nutrition.

Dark-skinned people require longer sunlight exposure than others, to derive the same amount of vitamin D from sunlight exposure. The ultraviolet rays of the sun, which promote vitamin D synthesis, can be blocked by heavy clouds, smoke, or fog. People who are unable to go outdoors frequently or dark-skinned people who live in cloudy or smoggy cities should consume foods fortified with vitamin D or therapeutic supplements to prevent its deficiency.

Vitamin D plays an essential role in the metabolism of calcium and phosphorus. Calcium is required for the development of bones and teeth. Blood calcium levels have to be very tightly maintained for normal functioning of the nervous system. The prohormone vitamin D, which gives rise to the hormone calcitriol, regulates blood calcium levels within a narrow range.

Vitamin D in bone growth

Vitamin D plays a special role in bone growth by making calcium and phosphorus available in the blood. When the blood calcium level becomes too low – for example, when calcium intake from diet is inadequate – the parathyroidgland secretes parathyroid hormone(PTH).

PTH promotes the activation of vitamin D to calcitriol, the active form of the vitamin. Calcitriol restores normal blood calcium in three ways – by facilitating absorption of dietary calcium from the intestine, increasing the mobilisation of calcium from the bone into the blood, and increasing the
reabsorption and retention of calcium by the kidneys. These actions increase the levels of calcium and phosphorus in the blood. Normal calcium
levels permit deposition of calcium in bones and teeth. It is also important for normal nervous function and muscle contraction.

Vitamin D in other roles

Scientists have discovered other roles for vitamin D. Vitamin D has been found in many other tissues such as the brain, nervous system, pancreas,
skin, muscles, reproductive organs, liver, kidneys, hormone secreting glands, etc. Vitamin D may have some important role in these tissues as well.

Daily allowances of vitamin D ICMR has not established a daily allowance of
vitamin D for our Indian population, because plenty of sunlight is already available in a tropical country like ours. It is not known how much of
vitamin D is exactly required by our body through diet and sunlight. The vitamin D requirementof children is placed between 200-400 International Units (100 IU = 2.5mg). The requirement is lesser for older age groups. The
allowance for older age groups is difficult to establish because of exposure to sunlight. Vitamin D requirements have been suggested for different
age groups in below table.

The requirement for vitamin D can be obtained in great measure through exposure to adequate sunlight. Only in cases where vitamin D requirement is not met through adequate exposure to sunlight or due to metabolic or genetic reasons, therapeutic supplementation of vitamin D may be necessary.

Vitamin D was first recognised by McCollum as the component of ‘good fats’ that cured rickets. Chemically, compounds with vitamin D activity are sterols. The two forms of the vitamin, which are important, are vitamin D2 (ergocalciferol) of plant origin and vitamin D3 (cholecalciferol) of animal origin. Vitamin D2 is formed when ergosterol (found in plants) is exposed to ultraviolet rays. Vitamin D3 is the chief form occurring in animal cells and develops in the skin on exposure of 7-dehydrocholesterol to ultraviolet rays from sunshine.

Dietary vitamin D is absorbed along with food fats from the intestine; bile salts are essential for effective absorption. Disease conditions in which fat absorption is affected also hinder vitamin D absorption. Vitamin D made in the skin enters the blood where it circulates attached to a specific protein.

Vitamin D itself is an inactive, storage form of the vitamin concentrated in the liver. It is rapidly hydroxylated (addition of-OH group) to 25-hydroxy vitamin D3 (calcidiol) in the liver. Calcidiol is further hydroxylated to 1, 25-dihydroxy vitamin D3 (calcitriol) in the kidney. Calcitriol is considered the active form of the vitamin, which acts as a hormone in calcium metabolism. As vitamin D is necessary for proper bone calcification, it is also called calciferol (to signify an alcohol promoting calcification). Vitamin D is soluble in fats and organic solvents but insoluble in water. It is stable to heat and is not easily oxidised.