What is Vitamin D?
Vitamin D is a fat-soluble vitamin that plays a role in many important body functions. It is best known for working with calcium in your body to help build and maintain strong bones. Vitamin D is also involved in regulating the immune system and cells, where it may help prevent cancer.
Your body stores vitamin D and makes vitamin D when your skin is exposed to sunlight. Vitamin D is also found in some foods, mostly ones like milk that have been fortified with vitamin D. There are two forms of vitamin D: ergocalciferol (vitamin D2) and cholecalciferol (vitamin D3). Some research suggests that cholecalciferol is better at raising levels of vitamin D in the blood.
In children, a vitamin D deficiency can cause rickets, a disease that results in soft, weak bones. In adults, many people may not be getting enough vitamin D, especially those who live in northern areas (like the northern half of the U.S.) and the elderly. People with dark skin do not absorb sunlight as easily as those with light skin, so their risk of low vitamin D is even higher. One study suggests that three quarters of adults in the U.S. have low levels of vitamin D.
That’s important because researchers are beginning to find that low levels of vitamin D may be linked to other diseases, including breast and colon cancer, prostate cancer, high blood pressure, depression, and obesity. The evidence doesn’t prove that too little vitamin D causes these conditions, but that people with higher levels of vitamin D are less likely to get these diseases.
Benefits of Vitamin D
“Vitamin D helps your body absorb and use calcium, which you need for strong bones. Getting enough vitamin D throughout your life is important, since most bone is formed when you are young. For post-menopausal women who are at higher risk of osteoporosis, taking vitamin D along with calcium supplements can reduce the rate of bone loss, help prevent osteoporosis, and may reduce the risk of fractures.”
“Vitamin D protects against rickets and osteomalacia, softening of the bones in adults. Seniors who live in northern areas, and people who do not get direct sunlight for at least 45 minutes per week, should make sure they get enough vitamin D through fortified milk and dairy products. Alternatively, they can take a vitamin D supplement or a multivitamin with vitamin D.”
“People who have low levels of vitamin D are at greater risk of falling, and studies have found that taking a vitamin D supplement (700 to 1000 IU daily) may reduce that risk. In seniors, vitamin D may reduce falls by 22%.”
“The four parathyroid glands are located in the neck. They make parathyroid hormone (PTH), which helps the body store and use calcium and phosphorus. Vitamin D is often used to treat disorders of the parathyroid gland.”
“In population studies, people with low levels of vitamin D seem to be at greater risk of developing high blood pressure compared to those with higher levels of vitamin D. However, there is no proof that low levels of vitamin D cause high blood pressure in healthy people.”
“There is some evidence that getting enough vitamin D may lower your risk of certain cancers, especially of the colon, breast, prostate, skin, and pancreas.”
“Seasonal Affective Disorder (SAD) is a type of depression that happens during the winter months when there’s not much sunlight. It is often treated with photo (light) therapy. A few studies suggest that the mood of people with SAD improves when they take vitamin D.”
“Population studies suggest that people who have lower levels of vitamin D are more likely to develop type 2 diabetes than people who have higher levels of vitamin D. One study found that giving infants doses of 2,000 IU per day of vitamin D during the first year of life may help protect them from developing type 1 diabetes when they are older.”
“Population studies suggest that people with low levels of vitamin D have a greater risk of developing heart disease, including heart attack, stroke, and heart failure compared to people with higher levels of vitamin D. Low levels of vitamin D may increase the risk of calcium build up in the arteries. Calcium build up is part of the plaque that forms in arteries when you have atherosclerosis and can lead to a heart attack or stroke.
However, one large clinical study found that taking 200 IU of vitamin D along with 500 mg of calcium twice per day did not reduce the risk of heart attack or stroke.”
“Scientists think vitamin D has immune boosting effects. Preliminary research suggests vitamin D supplementation may help prevent and treat respiratory infections.”
Sources of Vitamin D
There are two dietary forms of vitamin D:
- Cholecalciferol (D3)
- Ergocalciferol (D2)
Some studies suggest that cholecalciferol (vitamin D3) is better absorbed by the body than ergocaliciferol (vitamin D2). Both forms are naturally found in foods and are added to milk. Not all yogurt and cheese are fortified with vitamin D.
Your body makes vitamin D when your skin is exposed to the sun. The color of your skin affects the production of vitamin D. A fair-skinned person may need only about 45 minutes of sunlight a week to get enough vitamin D while a person with dark skin may need up to 3 hours.
In northern areas, it is hard to get enough vitamin D from sunlight during the winter, so people living there may need to take vitamin D supplements. In the U.S., people who live above a line running from Los Angeles to South Carolina may not get enough vitamin D in winter.
Vitamin D is included in many multivitamins. It can be found alone as softgel capsules, tablets, and liquid in over-the-counter strengths from 50 to 1,000 IU. Higher doses are also available, but it is best to ask your doctor to recommend the safest, most effective dose. For those who have trouble digesting fat, vitamin D injections are also available by prescription.
Recommended dietary allowances for vitamin D are listed below. Seniors and people who do not get exposed to much sunlight may need to take supplements. Seniors may be at risk of developing vitamin D deficiency because, as we age, the body does not make as much vitamin D from sunlight, and it has a harder time converting vitamin D into a form it can use.
If you are concerned about your vitamin D levels, ask your doctor whether you should take a supplement, and how much.
- Infants 0 to 12 months: 400 IU (adequate intake)
- Children 1 to 18 years: 600 IU (recommended dietary allowance)
- 19 to 50 years: 600 IU (recommended dietary allowance)
- 70 years and older: 800 IU (recommended dietary allowance)
- Pregnant and breastfeeding females: 600 IU (recommended dietary allowance)
Because of the potential for side effects and interactions with medications, you should take dietary supplements only under the supervision of a knowledgeable health care provider.
Taking too much vitamin D can cause several side effects. However, scientists disagree about how much is too much. The National Institutes of Health has set the maximum tolerable upper limit at:
1,000 IU daily for infants 0 to 6 months
1,500 IU daily for infants 6 months to 1 year
2,500 IU daily for children 1 to 3 years
3,000 IU daily for children 4 to 8 years
4,000 IU daily for anyone over 9.
Side effects may include:
Being very thirsty
Metal taste in mouth
A frequent need to urinate
You cannot get too much vitamin D from sunlight, and it would be very hard to get too much from food. Generally, too much vitamin D is a result of taking supplements in too high a dose.
People with the following conditions should be careful when considering taking vitamin D supplements:
- High blood calcium or phosphorus levels
- Heart problems
- Kidney disease
Vitamin D refers to a group of fat-soluble secosteroids responsible for increasing intestinal absorption of calcium, iron, magnesium, phosphate, and zinc. 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. Very few foods contain vitamin D; synthesis of vitamin D (specifically cholecalciferol) in the skin is the major natural source of the vitamin. Dermal synthesis of vitamin D from cholesterol is dependent on sun exposure (specifically UVB radiation).
Vitamin D from the diet or dermal synthesis from sunlight is biologically inactive; activation requires enzymatic conversion (hydroxylation) in the liver and kidney. Evidence indicates the synthesis of vitamin D from sun exposure is regulated by a negative feedback loop that prevents toxicity, but because of uncertainty about the cancer risk from sunlight, no recommendations are issued by the Institute of Medicine (US) for the amount of sun exposure required to reach vitamin D requirements. Accordingly, the Dietary Reference Intake for vitamin D assumes no synthesis occurs and all of a person’s vitamin D is from food intake. As vitamin D is synthesized in adequate amounts by most mammals exposed to sunlight, it is not strictly a vitamin, and may be considered a hormone as its synthesis and activity occur in different locations.[misleading] Vitamin D has a significant role in calcium homeostasis and metabolism. Its discovery was due to effort to find the dietary substance lacking in rickets (the childhood form of osteomalacia).