Scientists have gained new insight into how vitamin D, once thought to be a mere calcium sidekick in building bone, might help prevent heart disease, cancer, diabetes, and even the flu. They're also noticing that up to half of us aren't getting the recommended daily allowance (up to 400 IU); even fewer attain the 1000 IU needed to reap the extra benefits. So when patients of NYC's Dr. Jeffrey Morrison complain of symptoms such as achy joints and fatigue, he'll run tests. If levels are low, he'll suggest vitamin D3 supplements and several edible sources: 3.5 ounces of cooked salmon offer 90 percent of the RDA; light tuna (in oil) in a 3-ounce can has 50 percent; four cups of fortified milk provide 100 percent. And for 350 percent in one fell swoop, suffer through a tablespoon of cod-liver oil.