Recently, CNN reported that an alarming 70 percent of children in the U.S. may be seriously lacking in vitamin D and it's estimated even fewer U.S. adults may have adequate vitamin D levels to prevent osteoporosis, heart disease, dementia, and cancer. (Women, read up on the latest from Christiane Northrup on vitamin D and breast cancer protection.) Should you beef up on vitamin D–rich foods, get more sun, or pop a supplement? Harvard researchers say go ahead and take the vitamin D supplement—it's the surest way to know you're getting enough.