Oranges, grapefruits and other vitamin C-loaded foods have many health benefits. But study after study has shown that the vitamin does little—if anything—to cure, prevent or even shorten the duration of the common cold.

The most recent roundup of vitamin C research, published this spring in the Cochrane Database of Systematic Reviews, evaluated several decades of studies that included more than 11,000 subjects taking 200 or more milligrams of vitamin C each day. (The government's recommended daily allowance is 60 milligrams.) The research found that vitamin C did little to reduce either the length or severity of colds among the general population. However, studies have found that it may lower the risk of catching a cold among people whose bodies are under high physical stress—think marathon runners or soldiers on subarctic exercises. They were 50 percent less likely to catch a cold if they took a daily dose of vitamin C.