Vitamin C
Illustrator: Richard Mia
During the winter months, millions of Americans reach for handfuls of pills and fizzy powders bursting with everyone's favorite immune booster: vitamin C. After all, if eating an orange is healthy, then getting the vitamin C equivalent of 16 oranges is even healthier, right? Certainly that's what the Nobel Prize–winning chemist Linus Pauling, PhD, thought. In the 1960s, Pauling developed a theory that taking large amounts of vitamin C (he reportedly took up to 18,000 milligrams a day—about 280 oranges' worth) warded off colds, the flu, and even cancer.

Research in the decades since has failed to substantiate Pauling's claims, but new science actually suggests that America's vitamin C habit may be harming our health. A recent study conducted at the University of Jena, in Germany, found that supplements of vitamins C and E can subvert some of the benefits of exercise. Physical activity produces chemical by-products called reactive oxygen species (ROS). While ROS can cause some tissue damage, they also spur the body to metabolize calories (helping you lose weight) and improve insulin response (helping you process sugar). This last perk explains why many doctors recommend that patients at risk for diabetes get more exercise. Yet vitamins C and E are antioxidants, meaning they work to eradicate ROS. Consuming the levels of antioxidants found in regular servings of fruits and vegetables destroys just some ROS; megadosing with the amounts found in supplements can extinguish nearly all of them before they have a chance to spark the body's metabolic and insulin responses. Excess vitamin C is also linked to an increased risk of osteoarthritis as well as gastrointestinal problems like diarrhea and indigestion. Looks like it's time to kick that Emergen-C habit.

Next: How to avoid catching a cold