Vitamin D

Vitamin D is a group of fat-soluble secosteroids responsible for enhancing intestinal absorption of calcium and phosphate. In humans, the most important compounds in this group are vitamin D3 (also known as cholecalciferol) and vitamin D2 (ergocalciferol). Cholecalciferol and ergocalciferol can be ingested from the diet and from supplements. The body can also synthesize vitamin D (specifically cholecalciferol) in the skin, from cholesterol, when sun exposure is adequate (hence its nickname, the "sunshine vitamin").

Although vitamin D is commonly called a vitamin, it is not actually an essential dietary vitamin in the strict sense, as it can be synthesized in adequate amounts by most mammals exposed to sunlight. A substance is only classified as an essential vitamin when it cannot be synthesized in sufficient quantities by an organism, and must be obtained from its diet. In common with other compounds commonly called vitamins, vitamin D was nevertheless discovered in an effort to find the dietary substance lacking in a disease, namely rickets, the childhood form of osteomalacia. Additionally, like other compounds called vitamins, in the developed world, vitamin D is added to staple foods, such as milk, to avoid disease due to deficiency.

Vitamin D is a fat-soluble vitamin. Fat-soluble vitamins are stored in the body's fatty tissue.

Function

Vitamin D helps the body absorb calcium. Calcium and phosphate are two minerals that are essential for normal bone formation.

Throughout childhood, your body uses these minerals to produce bones. If you do not get enough calcium, or if your body does not absorb enough calcium from your diet, bone production and bone tissues may suffer.

Vitamin D deficiency can lead to osteoporosis in adults or rickets in children.

Food Sources

The body makes vitamin D when the skin is directly exposed to the sun. That is why it is often called the "sunshine" vitamin. Most people meet at least some of their vitamin D needs this way.

Very few foods naturally contain vitamin D. As a result, many foods are fortified with vitamin D. Fortified means that vitamins have been added to the food.

Fatty fish (such as tuna, salmon, and mackerel) are among the best sources of vitamin D.Beef liver, cheese, and egg yolks provide small amounts.

Mushrooms provide some vitamin D. The vitamin D content is also being boosted by exposure to some commercially available mushrooms to ultraviolent light. Most milk in the United States is fortified with 400 IU vitamin D per quart. 

It should be noted that foods made from milk, such as cheese and ice cream, are usually not fortified. Vitamin D is added to many breakfast cereals and to some brands of soy beverages, orange juice,  yogurt, and margarine. 

It can be very hard to get enough vitamin D from food sources alone. As a result, some people may need to take a vitamin D supplement. Vitamin D found in supplements and fortified foods comes in two different forms:D2 (ergocalciferol)D3 (cholecalciferol)

Side Effects

Too much vitamin D can make the intestines absorb too much calcium. This may cause high levels of calcium in the blood. High blood calcium can lead to:
Calcium deposits in soft tissues such as the heart and lungs
Confusion and disorientation
Damage to the kidneys
Kidney stones
Nausea, vomiting, constipation, poor appetite, weakness, and weight loss

Recommendations

Ten to 15 minutes of sunshine three times weekly is enough to produce the body's requirement of vitamin D. The sun needs to shine on the skin of your face, arms, back, or legs (without sunscreen). Because exposure to sunlight is a risk for skin cancer, you should use sunscreen after a few minutes in the sun.

People who do not live in sunny places may not make enough vitamin D. Skin that is exposed to sunshine indoors through a window will not produce vitamin D. Cloudy days, shade, and having dark-colored skin also cut down on the amount of vitamin D the skin makes.

Because vitamin D can come from sun, food, and supplements, the best measure of one’s vitamin D status is blood levels of a form known as 25-hydroxyvitamin D. Blood levels are described either as nanograms per milliliter (ng/mL) or nanomoles per liter (nmol/L), where 0.4 ng/mL = 1 nmol/L. Levels below 30 nmol/L (12 ng/mL) are too low for bone or overall health, and levels above 125 nmol/L (50 ng/mL) are probably too high. Levels of 50 nmol/L or above (20 ng/mL or above) are sufficient for most individuals.

The Recommended Dietary Allowance (RDA) for vitamins reflects how much of each vitamin most people should get on a daily basis.
The RDA for vitamins may be used as goals for each person.How much of each vitamin you need depends on your age and gender. Other factors, such as pregnancy and your health, are also important.

Infants (adequate intake of vitamin D)0 - 6 months: 400 IU (10 micrograms (mcg) per day)7 - 12 months: 400 IU (10 mcg/day)

Children1 - 3 years: 600 IU (15 mcg/day)4 - 8 years: 600 IU (15 mcg/day)

Older children and adults9 - 70 years: 600 IU (15 mcg/day)

Adults over 70 years: 800 IU (20 mcg/day)Pregnancy and breast-feeding: 600 IU (15 mcg/day)

In general, people over age 50 need higher amounts of vitamin D than younger people. Ask your health care provider which amount is best for you.

Vitamin D toxicity almost always occurs from using too many supplements.The safe upper limit for vitamin D is:1,000 to 1,500 IU/day for infants2,500 to 3,000 IU/day for children 1 - 8 years4,000 IU/day for children 9 years and older, adults, and pregnant and breast-feeding teens and womenOne microgram of cholecalciferol (D3) is the same as 40 IU of vitamin D.

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