Evidence that vitamin D can do wonders for the body is mounting. While the National Academy of Sciences recently boosted its recommended daily allowance from 200 international units to 600 IU, a new study has concluded that dramatically higher levels are needed for the cancer- and chronic-disease-fighting powers of vitamin D to kick in. The study, published in the journal Anticancer Research, found that most people need to take 4,000 to 8,000 IU per day to keep blood concentrations at beneficial levels. Only 10 percent of Americans—those with outdoor jobs, mostly—have concentrations that high. If you’re among the other 90 percent, the best way to elevate your level is by taking supplements, like those from Nature Made ($10; naturemade.com), or just ditch work early for a ride. Twenty minutes of exposure three times a week will net you plently of vitamin D.