As we head into cold and flu season, many people swear by megadoses of vitamins C and D to stave off sickness. However there's more and more evidence that they won't do much good.

According to a BBC News story, "Scientists say they can find no convincing evidence to show that taking vitamin D supplements will fend off a cold. A New Zealand team did the 'gold standard' of tests — a randomized placebo-controlled trial — to see what impact the supplements would have. The 161 people who took daily vitamin D for 18 months caught as many colds as the 161 who took fake pills."

NEWS: Can Vitamins Harm Your Health?

The new study, "Effect of Vitamin D3 Supplementation on Upper Respiratory Tract Infections in Healthy Adults," was conducted at the University of Auckland, New Zealand, and concluded, "In this trial, monthly administration of 100,000 IU of vitamin D did not reduce the incidence or severity of URTIs (upper respiratory tract infections) in healthy adults."

The study was reported in the Journal of the American Medical Association.

It's not just vitamin D that's worthless in preventing colds, it's also the old standby vitamin C. The idea of vitamin C as a panacea got a huge boost in the 1970s from Nobel Prize winning chemist Linus Pauling, who took three grams of vitamin C daily to prevent colds, and later claimed that the vitamin could treat a variety of diseases including cancer.

WATCH VIDEO: Have you ever wondered why your food tastes so good? Learn about the scientist behind the flavor.

Though the claims were proven false, the basic idea that extra vitamin C will prevent colds remains. Though it won't help, it usually won't harm: unless a person is taking megadoses, excess vitamin C will simply be flushed out of the body. While certain people with vitamin deficiencies require supplements, most adults who eat a reasonably balanced diet get enough vitamins in their meals, and don’t require supplements.

One Australian study in 2004 reviewed dozens of clinical trials examining vitamin C's effect on colds. It found no significant difference in incidence of colds between people who took the vitamin every day and those who did not.

NEWS: New Sars-Like Bug: Pandemic Potential?

However there was a slight difference in duration of the colds: those taking vitamin C found their colds ended about half a day earlier than those who did not. It concluded, "The failure of vitamin C supplementation to reduce the incidence of colds in the normal population indicates that routine mega-dose prophylaxis is not rationally justified for community use…. Also, the consistent and statistically significant small benefits on duration and severity for those using regular vitamin C prophylaxis indicates that vitamin C plays some role in respiratory defense mechanisms."

The bottom line? A vitamin C or D pill won't keep colds away, but when you're sick, drink your orange juice.