Overview
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.
In children, a vitamin D deficiency can cause rickets, a condition of soft, weak bones. In adults, many people may not be getting enough vitamin D, especially those who work in offices 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 of childbearing women in the Northern U.S. found that 54% of African-American women and 42% of white women had 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.
Uses
Getting the proper amount of vitamin D may help prevent several serious health conditions.
Osteoporosis
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.
Other Bone Disorders
Vitamin D protects against rickets and osteomalacia, softening of the bones in adults. Seniors who do not get direct sunlight for at least 45 minutes per week should make sure they get enough vitamin D through food supplements.
Prevention of Falls
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 – 1000 IU daily) may reduce that risk. In seniors, vitamin D may reduce falls by 22%.
Parathyroid Problems
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.
High Blood Pressure (Hypertension)
In population studies, people with low levels of vitamin D seem to have a high risk of developing high blood pressure than those with higher levels of vitamin D. However, there’s no proof that low levels of vitamin D cause high blood pressure in healthy people.
Evidence about vitamin D and blood pressure has been mixed. When people with kidney disease or an overactive parathyroid gland (hyperparathyroidism) develop high blood pressure, low levels of vitamin D may play a role. But another large study found that taking vitamin D and calcium supplements did not lower blood pressure significantly or reduce the risk of developing high blood pressure in postmenopausal women. If you have high blood pressure, you should follow your optimal health guide’s advice about whether vitamin D might help.
Cancer
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. This evidence is based mostly on studies of large groups of people
Some research suggests that postmenopausal women who take calcium and vitamin D supplements may have a lower risk of developing cancer of any kind compared to those who don’t take these supplements.
Seasonal Affective Disorder (SAD)
SAD is a type of depression that happens during the long dark winter months. 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 directly.
Diabetes
Population studies find 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. But there is no evidence that taking vitamin D can help prevent or treat type 2 diabetes.
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.
Heart Disease
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. Other population studies show that people with lower levels of vitamin D are more likely to have risk factors for heart disease, including high blood pressure, type 2 diabetes, obesity, and high cholesterol.
Obesity
Population studies have found that people who have lower levels of vitamin D are more likely to be obese compared to people with higher levels of vitamin D. One high-quality study also found that postmenopausal women who took 400 IU vitamin D plus 1,000 mg calcium daily for 3 years were less likely to gain weight than those who took placebo, although the weight difference was small. Women who were not getting enough calcium to start with (less than 1,200 mg per day) saw the most benefit.
Overall Mortality
Population studies suggest that people with lower levels of vitamin D have a higher risk of dying from any cause.
Dietary and other natural sources
Vitamin D has been described as a sunshine-dependent vitamin. Some dietary vitamin D2 comes from plants, but the largest contribution to dietary intake of vitamin D is the vitamin D3 in fish liver oils, eggs, milk, and liver. Milk is commonly fortified with 10 µg (400 IU) of vitamin D3 per quart. Extremely high potency (40,000 to 50,000 IU) products—sold as prescription or sometimes over the counter—often consist of vitamin D2. Most vitamin D dietary supplements contain vitamin D3. The conversion of international units to metric weights is extremely simple for both vitamin D2 and D3: 1 µg equals 40 IU; 0.025 µg equals 1 IU.
Vitamin D3 (or D2) from foods, or vitamin supplements is inert and must undergo two hydroxylation reactions in the body for activation. The first occurs in the liver and the second in the kidneys, which converts the vitamin D into calcitrol As calcitriol, vitamin D is fundamentally involved in the formation of bone, and so its deficiency can lead to rickets in children or osteoporotic changes in adults.
Although the vitamin can be synthesised in the body with sufficient exposure to sunlight or another ultraviolet (UV) light source, most people are not exposed to such UV light in consistent and sufficient quantities. No extra vitamin D is required when skin exposure to UV light is ample; but without such exposure, a person is completely dependent on ingested vitamin D.
Although adequate UV light exposure can provide sufficient vitamin D, many elderly persons have limited sunlight exposure, inadequate dietary sources, and a decreased ability to activate vitamin D, making them susceptible to vitamin D deficiency (Gloth et al. 1995; Holick 1999). Elderly people are likely to have substantially increased needs for dietary vitamin D because of their decreased mobility and exposure to sun and decreased activation in the liver and kidneys.
Thus, the nutritional need for dietary vitamin D depends on the biosynthesis in the skin, which in turn is influenced by time of exposure to sunlight, season (sun intensity and clothing), latitude, skin pigmentation, and the use of sunscreens.
Safety Considerations and limits
The formation of vitamin D in the skin is slowed once dietary vitamin D intakes are sufficient and blood levels of the activated forms are high. Therefore, excess exposure to sunlight does not lead to vitamin D toxicity (Holick 1999; Hathcock et al. 2007).
Dietary vitamin D can, however, produce toxic effects when consumed in very large quantities, especially over an extended period of time. Studies have shown that subjects with abnormally high levels of vitamin D intake can suffer from a wide range of signs and symptoms, from dehydration to permanent mineral deposits in soft tissues, including muscle, heart, kidney, and cartilage. Continued intake of toxic levels can have severe and persistent adverse consequences.
The traditional—but not data-based—conservatism of vitamin D recommendations is rapidly being corrected to evidence-based assessments. These assessments indicate that larger amounts are now considered safe for most persons. The Council for Responsible Nutrition recommends upper limits for vitamin D supplementation of 250 µg (4,000 IU) per day.