Vitamin D – Functions, Sources & Deficiency

Vitamin D – Functions, Sources & Deficiency

Vitamin D is a unique vitamin, differing from other vitamins in it’s very definition. A vitamin is defined as “a substance that must be obtained from food because of a lack of capacity in the human body to synthesise it”. Vitamin D can not only be obtained from the diet, but it can be synthesised in our own body.

Vitamin D exists in the nature in several forms. For humans, the two nutritionally important forms of Vitamin D are “Calciferol” and “Cholecalciferol”.

While calciferol is derived from plant sources, cholecalciferol is found in animal fats and fish liver oils. An important fact of cholecalciferol is that it can be produced from the cholesterol in our skin by exposure to UV rays of the sunlight.

Vitamin D is a metabolically inactive compound. It has to undergo transformations in our body to active forms. This transformation occurs in two stages. The first one happens in the liver and the second one in the kidneys,

Cholecalciferol is thus not a dietary requirement. It can be produced in human body and the only ingredient you need to make enough Vitamin D is 5 minutes of exposure to sunlight.


The main functions of Vitamin D can  be divided based on it’s site of action.

  1. Intestines : Promotes intestinal absorption of calcium and phosphorous.
  2. Bone : Increases bone mineralization, helps in the maturation of collagen.
  3. Kidneys : Increases absorption of minerals like phosphate, calcium.

Vitamin D also plays a major role in normal growth.


Sunlight : As explained earlier, the main source of Vitamin D for humans is their skin itself. The UV-B rays of sun converts the cholesterol in the skin to cholecalciferol. Dark skinned persons have a disadvantage because the UV rays get filtered by the melanin in their skin. Air pollution and other environmental factors also have a role to play in the amount of UV rays in the sunlight. Thus, the time required for adequate Vitamin D synthesis varies from person to person. Ideally, the exposure to sunlight should not produce redness or burn the skin. Over-exposure to sunlight can lead to skin cancers.

Diet : Liver, butter, yolk of egg, cheese, fish liver oil are some of the rich sources of Vitamin D.

Other sources include : whole milk, fish fat, cod liver oil, shark liver oil, halibut liver oil

Daily requirement

It is recommended to have a daily intake of just 400 IU (10 micrograms) for persons who are at risk of Vitamin D deficiency, including pregnant and breast-feeding women.

NOTE : Kindly avoid Vitamin A containing supplements in pregnancy (like cod liver oil)

 Vitamin D deficiency

People who are at higher risk of Vitamin D deficiency are young children, pregnant and breast-feeding women and older people. Persons who are mostly confined to indoor activities and those who cover their skin due to cultural practices are also at risk. The 2 main deficiency conditions of Vitamin D are Rickets and Osteomalacia.


Usually affects children belonging to the age group of 6 months to 2 years. Rickets primarily affects the growing bones by decreasing it’s calcification. It leads to growth failure, bone deformity, seizures, flabbiness of muscles. Children suffering from Rickets will also have delayed teething and walking.

Rickets, vitamin d
A child with bowed legs

Bony deformities of Rickets are :

  • Curved legs,
  • Deformed pelvis,
  • Pigeon chest,
  • Harrisson’s sulcus,
  • Rickety rosary,
  • Spinal deformities like kyphoscoliosis


Vitamin D deficiency in adults manifests as Osteomalacia. It usually affects women especially during pregnancy and lactation. Elderly persons are also at the risk of Osteomalacia. Deformities are similar to those of Rickets.



Vitamin D  deficiency can be prevented by 2 effective ways :

  1. Exposure to sunlight.
  2. Periodic supplementation of Vitamin D in the form of fortified foods like milk.

Since the margin between “optimal” and “excess” of Vitamin D in our body is low, it is always recommended to consult your doctor before starting any kind of supplements.

This article is a part of the Vitamin series. Articles on the functions, sources and deficiency states of Vitamin A and Vitamin C have already been published. Hope you liked this one too.



Image Credit – Library of Congress & athriftymrs.com

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