Vitamin D helps prevent osteoporosis, or softening of the bones, by promoting the absorption of calcium, states WebMD. Some research indicates that an adequate intake of vitamin D also reduces the risk of multiple sclerosis, diabetes, high blood pressure, rheumatoid arthritis and cancer.
In a study done at Boston University, people with high blood pressure were exposed to ultraviolet rays for a period of 90 days. Their vitamin-D levels increased by 100 percent, and their blood pressure also normalized, states WebMD. In another study reported in the Journal of the American Medical Association in 2003, veterans reduced their risk of developing precancerous colon polyps by 40 percent by taking more than 645 international units of vitamin D daily and eating a high-fiber diet. Another study from 2004 indicated that women who took only 400 international units of vitamin D daily, the amount in most multivitamins, reduced their risk of developing multiple sclerosis by 40 percent.
As of 2010, the recommended daily allowance of vitamin D is 600 international units for people between the ages of 1 and 70, including pregnant and breastfeeding women, and 800 international units for those 71 and older, according to Mayo Clinic. However, some experts believe that a dose of 1,000 international units is necessary to promote optimal health, states WebMD. Since it is difficult to get this much vitamin D from food alone, most people need to take a vitamin-D supplement or get a daily dose of the sun.