The Role of Vitamin D in the Human Body
Vitamin D is essential for life.It enables absorption of calcium and phosphate from the small intestine and their incorporation into bones and teeth. Vitamin D enhances immunity and reduces inflammation. Calcium and phosphate are, among other, responsible for proper function of nerves and muscles.
Children not getting enough of vitamin D may develop rickets (1) – a disease with bone deformities. Vitamin D deficiency in adults results in osteomalacia (2) – weak bones with pains in bones and bone fractures.
Vitamin D Sources
The main source of vitamin D is production of vitamin D in the skin after exposure to sunlight. Ultraviolet B rays from the sun convert a substance derived from cholesterol in the skin to cholecalciferol (vit D3), which, is in two steps (in the liver and then in the kidneys), converted into an active form of vitamin d – calcitrol (1,25OH-D).
Excessive vitamin D is dissolved in the fatty tissue that serves as a body store of vitamin D. Healthy people living at latitudes between 45° N and 45° S should produce enough vitamin D from their face, forearms and hands (or back) exposure to the sun lasting at least 10-15 minutes twice a week (on any sunny day in the year).
If their blood calcium and phosphate levels are within the normal range, they do not need any additional vitamin D from foods or supplements. That means vitamin D is actually not a true vitamin – by definition, vitamins are substances essential for life but human body cannot synthesize them, so we have to get them with food.
Vitamin D is rather a pro-hormone that after some chemical changes in the body becomes an active hormone. A hormone, by definition, is a substance synthesized on one site and taking effect on another site(s) of the body.
Animal Food Rich in Vitamin D
Fish, fish liver and fish oil are main natural animal sources of vitamin D3 – cholecalciferol. Egg yolk, beef liver and cheese also contain some vitamin D.
In some countries, including United States, vitamin D is added to milk – vitamin D3 fortified milk -but not to other dairy products, like cheese or ice cream. Amount of vitamin D in fortified milk should be clearly noted on a product label.
Plant Food Containing Vitamin D
Only some plants contain (a small amount of) vitamin D2- ergocalciferol. Vitamin D3 fortified margarine, orange juice and morning cereals are available in some countries, including US and UK.
NOTE: People living above 45° N or below 45° S, having their body completely covered with clothes, using sunscreens with SPF above 8, or being home-bound would possibly need to eat some vitamin D fortified foods or vitamin D supplements to maintain sufficient levels of calcium and phosphate in the blood.
Foods High in Vitamin D
High amount of vitamin D is naturally present in fish, fish liver and oil, and egg yolk.
Table 1. Examples of Natural Foods Rich in Vitamin D
|Food||IUs per serving*||Percent DV**|
|Cod liver oil, 1 tablespoon||1,360||340|
|Salmon, cooked, 3.5 ounces||360||90|
|Mackerel, cooked, 3.5 ounces||345||90|
|Tuna fish, canned in oil, 3 ounces||200||50|
|Sardines, canned in oil, drained, 1.75 ounces||250||70|
|Egg, 1 whole (vitamin D is found in yolk)||20||6|
|Liver, beef, cooked, 3.5 ounces||15||4|
|Cheese, Swiss, 1 ounce||12||4|
*IU = International Unit. DV = daily value.
Some countries add vitamin D to fortified milk, soy milk, orange juice, margarine or “morning cereals”.
Table 2. Examples of Fortified Foods with Added Vitamin D
|Food||IUs per serving*||Percent DV**|
|Milk, nonfat, reduced fat, and whole, vitamin D-fortified, 1 cup||98||25|
|Margarine, fortified, 1 tablespoon||60||15|
|Ready-to-eat cereal, fortified with 10% of the DV for vitamin D
, 0.75-1 cup (more heavily fortified cereals might provide more of the DV)
*IU = International Unit. DV = Daily Value.
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