Basics & Facts
Vitamin D is not in fact a vitamin. It differs from others because it is produced in the body, it’s not found in many foods, and food sources of vitamin D must be chemically changed by the body to become active. Like other vitamins, however, it is necessary for many functions including facilitating calcium absorption for healthy bones, strengthening muscles and supporting the immune system.
Sources of Vitamin D
Not many foods contain vitamin D unless they are fortified, with the exception of some fatty fish, liver and egg yolks. Americans usually get vitamin D from fortified milk and some dairy products except most cheeses and ice cream, which are not typically fortified. Breakfast cereals are another common source.
Vitamin D is found in many vitamin supplements in two forms—D2 (ergocalciferol) and D3 (cholecalciferol). Most manufacturers have switched from D2 to D3 because research shows that D3 is more easily absorbed and used by the body.
The human body becomes a vitamin D factory when skin is exposed to sunlight. This is the most efficient way to obtain vitamin D. Except for some groups of people, such as those living in northern latitudes and people with heavily pigmented skin, 10 to 15 minutes a day three times a week in direct sunlight is sufficient for most healthy people to get adequate levels of vitamin D, according to the National Institutes of Health.
Vitamin D has been in the spotlight in recent years because of emerging research about a number of health benefits and conditions including the following:
Osteoporosis: The use of calcium supplements has long been considered a first-line treatment for this disease (which causes bone thinning and weakening). Over the years, however, vitamin D has been shown to be just as essential to maximize calcium’s absorption into bones. Supplements containing both have been proven to reduce bone loss and decrease the risk of osteoporosis-related fractures.
Cancer: Some research indicates that people with higher levels of vitamin D in their blood have a lower risk of colon cancer, breast cancer and prostate cancer. At the same time, studies have linked high vitamin D levels to an increased risk of pancreatic cancer. Overall, although scientists think there may be a link between vitamin D and cancer risk, more research is needed to make absolute conclusions.
Autoimmune diseases: Autoimmune means the body’s immune system attacks its own tissues. Several conditions are associated with this mechanism including type 1 diabetes, rheumatoid arthritis and multiple sclerosis. According to Oregon State University’s Linus Pauling Institute, a number of studies on animals and human public health have shown a link between adequate vitamin D and a lower risk of developing autoimmune diseases.
High blood pressure: Some studies indicate that higher levels of vitamin D may be associated with a decreased risk of high blood pressure. This link is more apparent with regard to vitamin D obtained through sunlight exposure as opposed to taking supplements.
According to the National Institutes of Health, it is difficult for the average American to get enough vitamin D from food sources alone. With the increasing use of sunscreen, spending time outdoors doesn’t guarantee adequate levels, either. Most people need supplements to have sufficient vitamin D as measured by a lab test.
Mild deficiency rarely produces symptoms, but severe or prolonged deficiency can result in osteomalacia and rickets. Osteomalacia is a disease limited to adults in which bones become weak because they can’t absorb calcium. Symptoms include:
- Muscle weakness
- Bone pain
- Spontaneous fractures
- Numbness around the mouth
- Numbness and tingling in the arms and legs
Rickets is a rare childhood disease that occurs for the same reason as osteomalacia, but because bones are still growing, they become soft and deformed. Occasional cases of rickets are still reported in the U.S. according to the Centers for Disease Control and Prevention.
Those most at risk for vitamin D insufficiency are:
- Infants that are exclusively breastfed
- Older adults
- People who have limited sun exposure
- People with darkly pigmented skin
- Obese people
- People with fat malabsorption conditions
Vitamin D is fat soluble and therefore stored in fatty tissues. It can build up to toxic levels, especially if too much is taken in supplements. It is difficult to reach toxic levels through vitamin-D rich food, and practically impossible to overdose from too much sunlight. Symptoms of toxicity are:
- Nausea and vomiting
- Lack of appetite
- Abnormal heart rhythm
Long-term buildup of vitamin D can cause kidney and heart damage.
Vitamin D has the potential to interact with several medications. Some examples:
- Steroids, such as prednisone
- Weight-loss drugs
- Cholesterol lowering drugs
- Anticonvulsive medications
In most instances, these drugs interfere with the body’s ability to absorb or break down vitamin D. The safe daily upper limits of vitamin D (measured in international units, or IUs) have been set by the Institute of Medicine’s Food and Nutrition Board:
- Infants: 1,000 to 1,500 IUs/daily
- Children 1-8 years: 2,500 to 3,000 IUs
- Children 9 years and older through adulthood: 4,000 IUs
- Adolescents and women who are pregnant or breastfeeding: 4,000 IUs
People with health conditions or pregnancy should always check with their physician before starting vitamin D supplements.
The daily amount of vitamin D needed by most healthy people is considered the Recommended Dietary Allowance (RDA). These amounts may vary due to age, the presence of health conditions or other individual circumstances.
- Infants up to 1 year: 400 IU (considered an adequate amount)
- Children 8 years old up to adults 70 years old: 600 IU
- Adults over 70 years: 800 IU
- Pregnant and breastfeeding women: 600 IU
Most supplements have vitamin D3 listed as IUs, however, some may use micrograms (mcg). One microgram of D3 is equal to 40 IUs.
Because most people in the U.S. do not get enough vitamin D through diet or sunlight exposure, many may need additional supplements, especially infants, children and the elderly. Too much vitamin D, however, may result in symptoms of toxicity, so always discuss supplement use with a healthcare provider.
- Medline Plus
- Medline Plus
- Medline Plus
- Oregon State University Pauling Institute
- National Institutes of Health Office of Dietary Supplements – Health Professional
- National Institutes of Health Office of Dietary Supplements – Quick Facts
- Institutes of Medicine Food and Nutrition Board
- Institutes of Medicine Food and Nutrition Board – Dietary Reference Intakes
- Harvard Health Publications
- National Institutes of Health PubMed
- American Journal of Clinical Nutrition
- Centers for Disease Control and Prevention