Researchers have found that daily intakes of vitamin D by adults in the range of 4000-8000 IU are needed to maintain blood levels of vitamin D metabolites in the range needed to reduce by about half the risk of breast cancer, colon cancer, multiple sclerosis, and type 1 diabetes.
While an Institute of Medicine committee has stated that 4000 IU/day is a safe dosage, their recommended minimum daily intake is only 600 IU/day.
According to the study in the journal Anticancer Research:
"The supplemental dose ensuring that 97.5 percent of this population achieved a serum 25(OH)D of at least 40 ng/ml was 9,600 IU/d ... Universal intake of up to 40,000 IU vitamin D per day is unlikely to result in vitamin D toxicity."
Meanwhile, during the winter, pre-teen girls especially may need more vitamin D to have healthy bones, according to a separate new study. The research found that teen girls need a vitamin D intake of about 750 IU per day to have levels in their blood that allow for healthy bone growth.
This amount is higher than the U.S. Institute of Medicine's recommendation of 600 IU per day.
"... [W]earing sun block and not spending time outside can cut down on the vitamin D we get from the sun."