Skip the vitamin C and pop a D. Vitamin D may effectively boost immunity and help prevent colds, a Harvard study shows. People with the lowest vitamin D levels were 36% more likely to have upper respiratory infections, compared with those with the most D. (Asthma patients with low levels of D were nearly 6 times as likely to get sick as those with the greatest amounts.) Adequate amounts of D help produce cathelicidin, a protein with virus-killing qualities. Since it's tough to get enough D from sunlight or diet (fish and fortified dairy are the best sources), you'll need a supplement to attain optimal levels, say study author Carlos A. Camargo Jr., MD. Aim for at least 1,000 IU daily. - LG