Some doctors recommend that people with osteoarthritis (the regular kind of arthritis—not rheumatoid) take vitamin C, in hopes its antioxidant properties will help protect their joints. But this study from Duke University suggests that vitamin C might be exactly the wrong thing for your joints.

What the researchers wanted to know: Does vitamin C help people with osteoarthritis?

What they did: The scientists used a strain of guinea pig that's particularly prone to arthritic knees. Guinea pigs, like humans, can't make their own vitamin C, so they have to get it in their diet. The 46 male guinea pigs were divided into three groups, to get low, medium, or high doses of vitamin C in their daily chow. All of the animals got enough of the vitamin to ward off scurvy. When the guinea pigs were one year old, they were euthanized and their knee joints were examined.

What they found: Not only did vitamin C not help—it actually made the guinea pigs' arthritis worse. The higher the dose, the worse the arthritis.

What it means to you: The levels of vitamin C in the guinea pigs' blood were comparable to human levels, so there's a good chance this applies to people, the authors say. An earlier study of vitamin C in humans found less risk of cartilage loss in people who ate more vitamin C.

Caveats: Guinea pigs still aren't people.

Find out more: From the Arthritis Foundation: http://www.arthritis.org

Read the article: Kraus, V.B., et al. Ascorbic Acid Increases the Severity of Spontaneous Knee Osteoarthritis in a Guinea Pig Model. Arthritis & Rheumatism. June 2004, Vol. 50, No. 6, pp. 1822–1831.