Studies have shown that the skin can produce about 10,000IU of vitamin D a day with moderate sun exposure, a much higher dosage than recommended by USDA.  More recent studies have found low D levels are associated with higher rates of MS. Benefits of D includes calming the immune system, preventing a number of cancers and decreases risks of developing MS and osteoporosis.  The American Journal of Clinical Nutrition demonstrated that only vitamin D-3 was effective.  The RDA dose of 400IU is far too low.  New recommendations are from 2,000 to 5,000IU a day.