Vitamin D: Functions & Diseases

In Australian Dental Council’s examination, the indirect question related with Vitamin D are asked but in NEET MDS there may be direct question.


Calciferol: Vitamin DCalcidiol : 25-hydroxyvitamin D [25(OH)D]Calcitriol: 1,25-dihydroxyvitamin D [1,25(OH)2D]Vitamin D2: ErgocalciferolVitamin D3: Cholecalciferol

Vitamin D is a fat-soluble vitamin that is naturally produced in the body under skin when a person is exposed to ultraviolet light. It is found in food, and also available as food supplements. Naturally occurring vitamin D is biologically inactive and needs to undergo two hydroxylation processes to become active. First, naturally occurring vitamin D “calciferol” is hydroxylated in the liver and converted to “calcidiol”, which further undergoes hydroxylation in the kidneys and becomes “calcitriol”.

In foods and dietary supplements, vitamin D has two main forms, D2 (ergocalciferol) and D3 e(cholecalciferol). Both forms are well absorbed in the small intestine. Absorption occurs by simple passive diffusion and by a mechanism that involves intestinal membrane carrier proteins. The presence of fat along with vitamin D in the gut enhances vitamin D absorption. But some vitamin D is absorbed in the gut even without the presence of dietary fat. Neither aging nor obesity alters vitamin D absorption from the gut.

Its active form plays a role in calcium and phosphorus metabolism, parathormone activity, cell differentiation and proliferation. Besides, it has very important role in the maintenance of the health of the brain, heart, pancreas blood sugar level. Its deficiency may lead to schizophrenia, diabetes, multiple sclerosis and rarely cancer.

Vitamin D facilitates the absorption of calcium and phosphorus through the gut membrane. It also maintains the serum calcium and phosphorus level which helps maintain the bone mineralisation and prevents hypocalcemic tetany. It helps osteoblast and osteoclast to make and reshape the bone. Insufficient Vitamin D in blood serum may lead to thin, brittle and misshapen bone. Its deficiency may lead to “rickets” in children and osteomalacia in adults. Vitamin D, along with calcium prevents osteoporosis in older adults.

Vitamin D performs other important roles too in the body. It reduces inflammation, and modulates cell growth, neuromuscular and immune function and glucose metabolism. In part, along with other chemical compounds, it modulates the gene encoding proteins that regulate the cell proliferation, differentiation, and apoptosis. 

Till date, no research has definitely identified the serum concentration of 25(OH)D that has correlation with deficiency (rickets), adequacy for bone health, and overall health. After reviewing data on vitamin D needs, an expert committee of the Food and Nutrition Board (FNB) at the National Academies of Sciences, Engineering, and Medicine (NASEM) concluded that people are at risk of vitamin D deficiency at serum 25(OH)D concentrations less than 30 nmol/L (12 ng/mL). Levels of 50 nmol/L (20 ng/mL) or more are sufficient for most people. In contrast, the Endocrine Society stated that, for clinical practice, a serum 25(OH)D concentration of more than 75 nmol/L (30 ng/mL) is necessary to maximize the effect of vitamin D on calcium, bone, and muscle metabolism. The FNB committee also noted that serum concentrations greater than 125 nmol/L (50 ng/mL) can be associated with adverse effects.

Table 1: Serum 25-Hydroxyvitamin D [25(OH)D] Concentrations and Health Status#

nMole/Lng/mLHealth Status
<3012Associated with vitamin D deficiency, which can lead to rickets in infants and children and osteomalacia in adults.
>30-<5012-20Generally considered inadequate for bone and overall health in healthy individuals
≥50>20Generally considered adequate for bone and overall health in healthy individuals
>125>50Linked to potential adverse effects, particularly at >150 nmol/L (>60 ng/mL)

*Serum concentrations of 25(OH)D are reported in both nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL). One nmol/L = 0.4 ng/mL, and 1 ng/mL = 2.5 nmol/L.

Table 2: Recommended Dietary Allowances (RDAs) for Vitamin D

0-12 months*10 mcg 400 IU)10 mcg (400 IU)
1–13 years15 mcg (600 IU)15 mcg (600 IU)
14–18 years15 mcg (600 IU)15 mcg (600 IU)15 mcg (600 IU)
19–50 years15 mcg (600 IU)15 mcg (600 IU)15 mcg (600 IU)15 mcg 600 IU)
51–70 years15 mcg (600 IU)15 mcg (600 IU)
>70 years20 mcg (800 IU) 20 mcg (800 IU)

*Adequate Intake (AI)

Sources of Vitamin D


Fatty fish-trout, salmon, tuna and mackerel; and fish liver oil are the best source of vitamin D. Beef, liver, cheese and egg yolk also have small amount of vitamin D. Mushrooms have variable amounts of vitamin D but ultraviolet treated mushrooms available in the market have increased levels of vitamin D. Besides, one can also use the powder form of UV treated mushrooms as an additional source.

The fortification of packaged food is another measure of providing the vitamin D. Milk, orange or other juices, ready to eat cereals are also fortified with vitamin D. Plant milk alternatives fortified with vitamin D, e.g., soy, almond or oats are also available in market for use.

Sun Exposure 

The type B ultraviolet (UVB) light with wavelength approximately from 290 to 320 nanometers penetrates the bare uncovered skin of humans and converts cutaneous 7-dehydrocholesterol to previtamin D3 which in turn becomes vitamin D3. Therefore, some of the vitamin D requirements of a human can be fulfilled by exposing oneself to sunlight in morning and evening at the time of dawn and dusk in India and between 10 am to 4 pm in areas where sunlight is less and days are shorter. Since Australia is near the south pole, this method of getting vitamin D is not advisable as the other radiation may cause cancer. Sun exposer is not a reliable source of vitamin D as the time of the day, cloud cover, season, length of the day, smog, melanin content of the skin and use of sunscreen can affect the UVB exposure. Besides older people and people with dark skin are not able to produce as much as other people. One factor should be noted, the UVB radiation cannot penetrate glass, therefore, exposure to sunshine in a room through a glass window will not produce vitamin D. As per experts, when all factors are in favor, the sunlight exposure to face, arms, trunk and legs for 5 minutes to 30 minutes are enough to get sufficient vitamin D.

Dietary Supplements 

The requirement of daily vitamin D can be fulfilled by taking dietary supplements that contain vitamins D2 or D3. Vitamin D2 is manufactured using UV irradiation of ergosterol in yeast, and vitamin D3 is produced with irradiation of 7-dehydrocholesterol from lanolin and the chemical conversion of cholesterol. Most evidence indicates that vitamin D3 increases serum 25(OH)D levels to a greater extent and maintains these higher levels longer than vitamin D2, even though both forms are well absorbed in the gut. Therefore, taking vitamin D3 is wiser than taking vitamin D2.

A variety of foods and their vitamin D levels per serving are listed in Table 3.

Table 3: Vitamin D Content of Selected Foods#

(mcg) per
Units (IU)
per serving
Percent DV*
Cod liver oil, 1 tablespoon34.01,360170
Trout (rainbow), farmed, cooked, 3 ounces16.264581
Salmon (sockeye), cooked, 3 ounces14.257071
Mushrooms, white, raw, sliced, exposed to UV light, ½ cup9.236646
Milk, 2% milkfat, vitamin D fortified, 1 cup2.912015
Soy, almond, and oat milks, vitamin D fortified, various brands, 1 cup2.5-3.6100-14413-18
Ready-to-eat cereal, fortified with 10% of the DV for vitamin D, 1 serving2.08010
Sardines (Atlantic), canned in oil, drained, 2 sardines1.2466
Egg, 1 large, scrambled**1.1446
Liver, beef, braised, 3 ounces1.0425
Tuna fish (light), canned in water, drained, 3 ounces1.0405
Cheese, cheddar, 1 ounce0.3122
Mushrooms, portabella, raw, diced, ½ cup0.141
Chicken breast, roasted, 3 ounces0.141
Beef, ground, 90% lean, broiled, 3 ounces01.70
Broccoli, raw, chopped, ½ cup000
Carrots, raw, chopped, ½ cup000
Almonds, dry roasted, 1 ounce000
Apple, large000
Banana, large000
Rice, brown, long-grain, cooked, 1 cup000
Whole wheat bread, 1 slice000
Lentils, boiled, ½ cup000
Sunflower seeds, roasted, ½ cup000
Edamame, shelled, cooked, ½ cup000

* DV = Daily Value. The FDA developed DVs to help consumers compare the nutrient contents of foods and dietary supplements within the context of a total diet. The DV for vitamin D on the new Nutrition Facts and Supplement Facts labels and used for the values in Table 3 is 20 mcg (800 IU) for adults and children aged 4 years and older. The new labels must list vitamin D content in mcg per serving and have the option of also listing the amount in IUs in parentheses. FDA required manufacturers to use these new labels starting in January 2020, but companies with annual sales of less than $10 million may continue to use the old labels that list a vitamin D DV of 400 IU until January 2021. Foods providing 20% or more of the DV are considered to be high sources of a nutrient, but foods providing lower percentages of the DV also contribute to a healthful diet.
** Vitamin D is in the yolk.

A more elaborated article along with MCQs on Vitamin D has been provided in Platinum Course.


#National Institutes of Health, Office of Dietary Supplements