The Institute of Medicine's current recommended intake of vitamin D is 5 micrograms (200 IU) up to age 50, 10 micrograms (400 IU) between the ages of 51 and 70, and 15 micrograms (600 IU) after age 70. Optimal intakes are much higher, though, with at least 25 to 50 micrograms (1,000 to 2,000 IU) recommended for those over age 2.
If the multivitamin you take does not have 1,000 IU of vitamin D, you may want to consider adding a separate vitamin D supplement, especially if you do not spend much time in the sun. In extremely high doses—hundreds of thousands of IU or more—vitamin D is toxic and can even cause death; but in adults, taking up to 2,000 IU per day as a supplement is safe. In fact, some people may need 3,000 or 4,000 IU per day for adequate blood levels, particularly if they have darker skin, spend winters in the northern U.S., or have little exposure to direct sunlight. If you fall into these groups, ask your physician to order a blood test for vitamin D.
Food sources: Very few foods naturally contain vitamin D. Good sources include dairy products and breakfast cereals (both of which are fortified with vitamin D), and fatty fish such as salmon and tuna. For most people, the best way to get the recommended daily intake is by taking a supplement, but the level in most multivitamins (400 IU) is too low; encouragingly, some manufacturers have begun adding 800 or 1,000 IU of vitamin D to their standard multivitamin preparations.
Getting vitamin D from the sun: Correctly applied sunscreen reduces our ability to absorb vitamin D by as much as 90 percent. At higher latitudes (in the northern U.S., for example), exposure to the sun in winter will not form vitamin D (due to the angle of the sun's rays), so a supplement is advisable.
If you live north of the line connecting San Francisco to Philadelphia, odds are you don't get enough vitamin D. The same holds true if you don't or can't get outside for at least a 15-minute daily walk in the sun. African-Americans and others with dark skin, as well as older individuals, tend to have much lower levels of vitamin D, due to less formation of the vitamin from the action of sunlight on skin. Worldwide, an estimated 1 billion people have inadequate levels of vitamin D in their blood, and deficiencies can be found in all ethnicities and age groups. (1) A study of teens in Boston, for example, found that 42 percent had insufficient levels of vitamin D, while an international study of post-menopausal women with osteoporosis found that more than 60 percent had inadequate levels of vitamin D. (2, 3)
Vitamin D's Role in Bone and Beyond
Vitamin D helps ensure that the body absorbs and retains calcium and phosphorus, both critical for building bone. Laboratory studies also show that vitamin D keeps cancer cells from growing and dividing, and plays a critical role in controlling infections (see more about , below).
Studies indicate that insufficient intake of vitamin D is associated with an increased risk of fractures, and that vitamin D supplementation may prevent them, especially when vitamin D is taken in conjunction with calcium. (4, 5) It may also help increase muscle strength, which in turn helps to prevent falls, a common problem that leads to substantial disability and death in older people. (6, 7)
Other early studies suggest an association between low vitamin D intake and increased risks of breast, colon, and other cancers. (1, 8) A recent randomized trial among postmenopausal women showed significant reductions in cancer incidence among those randomized to vitamin D. ()
Vitamin D and Immune Function
Vitamin D’s role in regulating the immune system has led scientists to explore two parallel research paths: Does vitamin D deficiency contribute to the development of multiple sclerosis, type 1 diabetes, and other so-called “autoimmune” diseases, where the body’s immune system attacks its own organs and tissues? And could vitamin D supplements help boost our body’s defenses to fight infectious disease, such as tuberculosis and seasonal flu?
Multiple Sclerosis: Multiple sclerosis (MS) rates are much higher far north (or far south) of the equator than in sunnier climes, and researchers suspect that chronic vitamin D deficiencies may be one reason why. One prospective study that looked at this question found that among white men and women, those with the highest vitamin D blood levels had a 62 percent lower risk of developing MS than those with the lowest vitamin D levels. () The study didn’t find this effect among black men and women, most likely because there were fewer black study participants and most of them had low vitamin D levels, making it harder to find any link between vitamin D and MS if one exists. No randomized controlled trials have been conducted on vitamin D and MS, and more research is needed.
Type 1 Diabetes: Type 1 diabetes is another disease that varies with geography—a child in Finland is about 400 times more likely to develop it than a child in Venezuela. () Evidence that vitamin D may play a role in preventing type 1 diabetes comes from a 30-year study that followed more than 10,000 Finnish children from birth: Children who regularly received vitamin D supplements during infancy had a nearly 90 percent lower risk of developing type 1 diabetes than those who did not receive supplements. () Other European case-control studies, when analyzed together, also suggest that vitamin D may help protect against type 1 diabetes. () No randomized controlled trials have tested this notion, however, so more research is needed.
The Flu and the Common Cold: The flu virus wreaks the most havoc in the winter, abating in the summer months. This seasonality led a British doctor to hypothesize that a sunlight-related “seasonal stimulus” triggered influenza outbreaks. () More than 20 years after this initial hypothesis, several scientists published a paper suggesting that vitamin D may be the seasonal stimulus. () Among the evidence they cite:
- Vitamin D levels are lowest in the winter months. ()
- The active form of vitamin D tempers the damaging inflammatory response of some white blood cells, while it also boosts immune cells’ production of microbe-fighting proteins. ()
- Children who have vitamin D-deficiency rickets are more likely to get respiratory infections, while children exposed to sunlight seem to have fewer respiratory infections. ()
- Adults who have low vitamin D levels are also more likely to report having had a recent cough, cold, or upper respiratory tract infection. ()
But don’t skip your flu shot just yet: No large-scale randomized controlled trials have been done on vitamin D supplements and the flu in general, or on vitamin D and the H1N1 flu virus strain ("swine flu"). Until such studies provide more evidence, it is too early to recommend high doses of vitamin D to prevent or treat colds or flu. ()
Tuberculosis: Before the advent of antibiotics, sunlight and sun lamps were part of the standard treatment for tuberculosis (TB). () More recent research suggests that the “sunshine vitamin” may be linked to TB risk. Several case-control studies, when analyzed together, suggest that people diagnosed with tuberculosis have lower vitamin D levels than healthy people of similar age and other characteristics. () Such studies do not follow individuals over time, so they cannot tell us whether vitamin D deficiency led to the increased TB risk or whether taking vitamin D supplements would prevent TB. There are also genetic differences in the receptor that binds vitamin D, and these differences may influence TB risk. () Again, more research is needed.
Vitamin D and Risk of Premature Death
A promising report in the Archives of Internal Medicine suggests that taking vitamin D supplements may even reduce overall mortality rates: A combined analysis of multiple studies found that taking modest levels of vitamin D supplements was associated with a statistically significant 7 percent reduction in mortality from any cause. () The analysis looked at the findings from 18 randomized controlled trials that enrolled a total of nearly 60,000 study participants; most of the study participants took between 400 and 800 IU of vitamin D per day for an average of five years. Keep in mind that this analysis has several limitations, chief among them the fact that the studies it included were not designed to explore mortality in general, or explore specific causes of death. More research is needed before any broad claims can be made about vitamin D, chronic disease prevention, and mortality. () (For more information on vitamin D and chronic disease prevention, see Ask the Expert—Vitamin D.)
References
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