VITAMIN D- the Vitamin you know, the FACTS you DON’T !!

Vitamin D is one of the hormones that are synthesized in the body itself. Interestingly, Vitamin D functions as steroidal hormone inside our body. Vitamin D plays an important role in regulating the calcium levels in the body. Any defect in the levels of vitamin D in body can lead to imbalance in the calcium levels of the body.

Fat soluble VitaminWater soluble Vitamin
Vitamin AVitamin B complex
Vitamin DVitamin C
Vitamin E 
Vitamin K 
Vitamins classification on basis of solubility

Forms of Vitamin D

Vitamin D is available in 2 forms. Vitamin D2 and Vitamin D3.

Vitamin D2Ergocalciferol

Vitamin D2 is found in commercial products and is less effective. This is actually derived from the fungus- Ergot. The ergosterol present in the fungus is treated with UV light. And this leads to formation of Vitamin D2 (Ergocalciferol).

Vitamin D3Cholecalciferol

Vitamin D3 form is what is produced by the skin and is also present in food.

Most of the Vitamin D is produced in the body itself. But in certain cases such as deficiencies, Vitamin D may be required to be taken as food/medication.

Vitamin D is formed in SKIN!

It is a well known fact that, Vitamin D is produced by the skin after exposure to sun.

But this is a partial story!!

Sun and Skin do form the first and crucial step in the metabolic chain of formation of Vitamin D. This process occurs in the Stratum Corneum layer of the skin. But Liver and Kidney also play an important role in formation of active form of Vitamin D. The active form of Vitamin D [1,25-hydroxy cholecalciferol {1,25-OH)2D3} or Calcitriol ] is formed in KIDNEY.

Best time of Sun light for effective formation of Vitamin D in skin?

The light from Sun contains a variety of radiations ranging from infrared to ultraviolet rays. The UV rays present here play a crucial role in converting the inactive from of Vitamin D to its active form.

Sunlight before 10am and after 3pm (Safe Sun) for atleast 15 min without sunscreen is considered good and adequate for formation of enough Vitamin D. During this time, sun emits maximum amount of beneficial UV rays – 290 to 315 mm (UV-B radiation) and the least amount of harmful Infrared rays.

During November to March, the wavelength of UV rays shifts to longer wavelengths which will not produce the vitamin. Hence Vitamin D deficiency is more seen in winter season.

It for this reason Vitamin D is also called as the “Sun-Shine Vitamin”.

Can you have Vitamin D toxicity because of high exposure to SUN?

We have discussed how Vitamin D is produced in the skin after exposure to Sun. But luckily our body already has a plan where excess Vitamin D is not produced by the skin.

The excess Vitamin D3 produced by the skin after over exposure to sun, is destroyed by sun itself!! Hence,you can never have Vitamin D toxicity due to higher exposure to Sun!!

If otherwise would have been true, then those workers who work in sun throughout the day would be suffering from Vitamin D toxicity !

Can Sun-Screen Creams interfere with Vitamin D production?

We have discussed how Vitamin D is produced in the skin after exposure to Sun and this is proportional to the area of skin that is exposed. Any form of layer like a cloth or even a layer of sunscreen can interfere with this production.

Sunscreens provide a transparent layer over skin so that the sun rays are repelled back. As a result, the necessary amount of UV rays are not absorbed into the skin.

Foods containing Vitamin D

Vitamin D is NOT present in any of the vegetarian sources of food.

It is present only in animal sources such as

  • animal liver and liver oils
  • egg yolk.

Fish liver oil and Halibut liver oil are considered the richest dietary sources of vitamin D.

Hence, those who follow a strict vegetarian diet should make sure to have adequate sun exposure in order to not face Vitamin D deficiency.

Vitamin D Metabolism

Vitamin D is an endogenously synthesized Vitamin. This means, unlike Vitamin C, which needs to be consumed via food; Vitamin D is produced inside our body and is not required to taken via food (unless there is deficiency).

The greatest source of Vitamin D is Skin. The production of Vitamin D in the skin is directly proportional to the exposure to sunlight and inversely proportional to the pigmentation of the skin.

When the sunlight falls on our skin, 7 dehydro-cholesterol is converted to Vitamin D3 in Stratum Corneum layer of skin under the effect of the UV radiation.

Vitamin D3 formed in the skin with the help of UV rays, is now transported to LIVER. Here, Vitamin D3 is converted to 25-hydroxy cholecalciferol (25-OH-D) under the effect of 25-hydroxylase by hydroxylation at the 25th position. This is the form of Vitamin D in which it circulates in the body.

But this is not the active form of Vitamin D in the body. 25-OH-D is circulated in the body in this form attached to alpha 2 globulin protein present in blood plasma.

This 25-OH-D is converted to active form in KIDNEYS. 25-OH-D undergoes hydroxylation at 1st position and is converted to 1,25-hydroxy cholecalciferol [1,25-OH)2D3] or Calcitriol in the kidneys by help of enzymes alpha 1 hydroxylase. CALCITRIOL is the metabolically active from of Vitamin D.

This conversion that takes place in kidneys, is a key step in the metabolism of Vitamin D.

Looking at the above discussed metabolism of Vitamin D, it is quite evident that, not just Sun, but good functioning Liver and Kidney are equally important for Vitamin D formation.

Hence any liver or kidney related problem can lead to symptoms which are seen in Vitamin D deficiency.

Why is 25-Hydroxy Cholecalciferol(25-O-H) used as marker of Vitamin D?

Uv Radiations taht fall on skin convert & dehydro-cholesterol to Vitamin D3. This, is then transported to LIVER. Here, Vitamin D is converted to 25-hydroxy cholecalciferol (25-OH-D) under the effect of 25-hydroxylase by hydroxylation at the 25th position. This is the form of Vitamin D which circulates in blood in the body attached to alpha 2 globulin protein present in blood plasma. Hence, in any condition related to increase or decrease of Vitamin D, 25-OH-D is used as a marker of level of Vitamin D in the body.

But this is not the active form of Vitamin D in the body.

Function of Vitamin D

Vitamin D, Parathyroid hormone and Calcitonin are the three vital hormones that play key role in maintaining the necessary calcium levels in the body. These are required for the proper calcium homeostasis of the body. Out of these three, Vitamin D and Parathyroid hormone also work in maintaining the level of phosphate in the body.

The overall effect of vitamin D is to increase the level of calcium and phosphate in the body.

Vitamin D acts on following organs-


Vitamin D is mostly absorbed via the small intestines. Vitamin D acts on the brush borders of the intestines and increases the absorption of calcium by increasing the transcription of TRPV6 on duodenum (first part of small intestine).


Vitamin D increases the reabsorption of calcium and phosphorus from the distal tubules of the nephrons.


Vitamin D plays role in mineralization of osteoid matrix and epiphyseal cartilage in bones. Active form of Vitamin D (Calcitriol) co-ordinates remodeling of bone and increases the mineral density of bones. It does this by acting on all three types of cells – osteoblasts, osteoclasts and osteocytes. Vitamin D is responsible for increased production of bone matrix proteins such as collagen, osteocalcin and osteopontine.

Vitamin D acts on the RANK ligand receptors present on the osteoclasts. This helps in calcium reabsorption from the bone. This helps in mobilzing calcium from old bone cells to formation of new bone cells.

Other actions of Vitamin D

Vitamin D has been found to be a modulator of immune response. This may be a reason why increased Vitamin D in the body is considered to be useful in fighting against tuberculosis. Hence a tubercular patient is advised to sit in sun.

In some other studies, Vitamin D supplementation in steroid resistant patients has been found to be beneficial.

Some recent studies have shown the beneficial effect of Vitamin D in treatment of COVID-19 patients. Also some studies also show that those with less vitamin D in the body may have higher chances to getting COVID-19 and also have more severe symptoms than those having no Vitamin D deficiency.

Daily Requirement of vitamin D

  • Child – 10 microgram (400 IU) per day
  • Adults – 5 to 10 microgram (200 IU) per day
  • Pregnant Females, Lactating females – 10 micro gram per day
  • People above age of 60 – 600 IU per day

Who is more prone to suffer from Vitamin D deficiency?

  • Obese people
  • Dark skinned people
  • Old aged people
  • People suffering from kidney and liver diseases
  • People who keep their body covered in clothes
  • Excess use of sunscreen
  • Gastro intestinal disorders causing defect in absorption

Obese people more likely to suffer from Vitamin D deficiency?

Vitamin D is a fat soluble vitamin. This means, in obese and overweight patients, Vitamin D is required in higher quantity than the normal individual. Such individuals, may have adequate total amount of Vitamin D in their bodies, but most of the Vitamin D remains soluble in the fat and hence becomes useless.

Hence, Obese and overweight people many suffer from Vitamin D deficiency symptoms inspite of having normal total Vitamin D level.

Vitamin D Deficiency

Vitamin D helps increasing the calcium levels in the body. Vitamin D deficiency leads to formation of Rickets and Osteomalacia.

The level of Vitamin D in blood below 30 nanogram per ml is considered deficiency and that below 10 nanogram per ml is considered severe deficiency.

This leads to symptoms such as-

  • Rickets-
    • Craniotabes
    • Frontal bossing
    • Rachitic rosary
    • Pigeon chest deformity
    • Lumbar lordosis
    • Bowing of legs
    • Seizures/ convulsions
    • Delayed motor development in child
  • Osteomalacia
    • Bone pain
    • Muscle weakness
    • Bone fracture after minimal trauma
    • Kyphosis
    • Coxa vara
    • Pigeon chest

Hypervitaminosis D – Vitamin D toxicity

Symptoms of Vitamin D toxicity include-

  • Weakness
  • Polyuria
  • Intense thirst
  • Difficulty in speaking
  • Confusion
  • Hypertension
  • Weight loss
  • Muscle weakness

Increased Vitamin D leads to increased calcium in the body. Increased calcium over long time can lead to accumulation of calcium in various body parts such as blood vessels and muscles. This is accumulation of calcium at abnormal locations is called as metastatic calcification / calcinosis.

Laboratory examination in such conditions show increased calcium in blood, increased calcium in urine, increased level of 25-D and low PTH.

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