Vitamin D - How Much We Need, Sources & Health Benefits
Author: Winston Craig, MPH, PhD, RD.
Few foods have it and most people donít get enough sun to make it. Skin cells can manufacture vitamin D when exposed to adequate sunlight. Ten to fifteen minutes of bright sunlight on the arms and face 2 to 3 times per week is usually sufficient time to manufacture enough vitamin D to meet one's needs. The UVB rays of ultraviolet light are the rays that are responsible for making the vitamin D precursor.
However, people who live north of Atlanta in the East and Los Angeles in the West donít get enough UVB from the sun in winter to make sufficient vitamin D. During the winter months when the sun's rays are of insufficient strength, people living in Southern Michigan cannot manufacture sufficient vitamin D from November through March.
While some vitamin D is stored in the body during the summer months, it is important that you get a good dietary source of vitamin D to help you out during the winter months. Many breakfast cereals, some dairy products, and some soy and rice beverages are fortified with 25 percent of the daily value (DV) for vitamin D. In addition, some orange juices are now fortified with both calcium and vitamin D, and Viactiv soft calcium chews contain similar amounts of vitamin D.
You can, however, get too much vitamin D, although it would be difficult to get too much from food and it is impossible to get too much from sun exposure. Vitamin D toxicity is possible from taking an excessive amount from vitamin supplements. One has to be careful since many calcium supplements are often fortified with vitamin D to help with calcium absorption.
The National Academy of Science recommends that people not consume in excess of 2,000 IU a day. Above that level vitamin D may be unsafe. If you consume too much vitamin D from food and pills you could absorb excessive levels of calcium. This can lead to kidney stones, kidney calcification, kidney failure, and calcification of soft tissue.
The recommended daily intake of vitamin D is 200 IU for adults up to age 50. The level jumps to 400 IU for people over 50 yrs age and 600 IU for people over 70 years age. The elderly tend to be deficient in vitamin D, due to impaired vitamin D metabolism. By age 70, vitamin D production is only about 30 percent of what it was at age 25, for exposure to similar levels of sunlight. The elderly at highest risk of vitamin D deficiency include those who are homebound or institutionalized, those living in the northern third of the country and who have a low vitamin D intake. Persons with dark skin need twice as much sunlight exposure as fair skinned persons since skin pigment-ation reduces vitamin D production. The use of sun screen is also known to reduce vitamin D production. Sunblock with an SPF factor of 8 or greater can block out 90 percent or more of the UVB rays.
Health Benefits of Vitamin D
Vitamin D increases the efficiency of calcium and phosphorus absorption from food. These 2 minerals are needed to form dense bone structure. Hence, a deficiency of vitamin D can increase the risk of bone fractures. If a person is vitamin D deficient and not getting enough calcium, then the body draws calcium from the bones, resulting in osteoporosis. The risk of bone fractures can be markedly reduced in post-menopausal women taking modest levels of vitamin D supplements.
New research is revealing that vitamin D may be involved with a diverse range of body processes beyond maintaining healthy bones. Many tissues in the body have a vitamin D receptor, suggesting a wide-spread function for vitamin D. Elderly women who took a vitamin D supplement reduced their risk of falls by 20 percent. Falls in the elderly can have severe consequences and are responsible for about 40 percent of nursing home admissions. Tufts researchers believe that vitamin D may promote better muscle function. Stronger muscles are better able to support the body, and reduce the risk of falls.
People who live close to the equator have lower rates of cancer. The further one lives from the equator, the less exposure to UVB rays, and the less vitamin D produced in the body. Vitamin D is a very potent hormone that inhibits cell proliferation. Studies have shown that vitamin D inhibits the growth of breast, colon, lung, and prostate cancer cells.
Children who received a daily vitamin D supplement during the first year of life experienced an 80 percent reduction in risk of type 1 diabetes compared with those children who took no supplement. Preliminary evidence also suggests that adequate vitamin D inhibits the development of multiple sclerosis and lowers blood pressure (through its action on renin production).
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Low dietary vitamin K intake has been associated with an increased risk of hip fracture. Now researchers at Tufts University have found that a low vitamin K intake (70 mcg/day) is associated with a 4% lower bone mineral density in the hip and spine of women (but not men) compared to those with a high vitamin K intake (310 mcg/day).