Used to be, a little milk gave you all the vitamin D you needed. Now you'd have to chug a lot of it to get the daily dose suggested by the Institute of Medicine. But many docs say the new guidelines, which rose from 200 international units (IU) to 600 IU, still fall short. "It's a step in the right direction," says Michael F. Holick, Ph.D., M.D., director of the Vitamin D, Skin, and Bone Research Laboratory at the Boston University School of Medicine. "But essentially every tissue in your body needs vitamin D; 600 IU is just too low." The nutrient affects some 2,000 genes and could amp up your immunity to fight off everything from depression to cancer. Plus, many women are D-deficient, says Sarfraz Zaidi, M.D., author of The Power of Vitamin D. Based on new research, you should get 1,000 to 2,000 IU a day. WH waded through the science to find out how taking enough of the so-called super vitamin may help cut your disease risk.