Scientists have long suspected that osteoarthritis, a painful condition of the joints that affects over 20 million Americans, occurs more often in people who don't get all the nutrients that they need. Recently, researchers at the University of North Carolina–Chapel Hill found a possible link between lower amounts of selenium—a mineral found in a variety of foods, from brazil nuts to noodles to tuna—and osteoarthritis of the knee.

Researchers clipped the toenails of 940 volunteers and measured the amount of selenium in each sample. After adjusting for age, race, gender, and body mass—all of which affect the incidence of osteoarthritis—they discovered that the people with the condition (28 percent of the group) had lower amounts of selenium in their bodies. Each increase of 0.1 part per million of selenium in the toenail correlated with a 15 to 20 percent lower risk of osteoarthritis in one or both knees. While it's unclear how low levels of selenium might contribute to osteoarthritis, one theory is that the nutrient aids in the formation of antioxidant proteins that keep cartilage in the joints healthy. Or it's possible that low selenium levels indicate a general lack of nutrition that may increase risk.

Lead author Joanne Jordan, a UNC associate professor of medicine and orthopedics, started the study after she noticed that people in areas of China with selenium-poor soils and diets had an abnormally high risk for the early onset of joint problems. The results, published by the American College of Rheumatology, don't mean that people should start taking selenium supplements, she says, since selenium can become toxic pretty quickly. Better to just pay attention to diet.

See our in-depth guide on osteoarthritis