News flash: Vitamin D is the latest wonder supplement. We already know that it may improve mood, ease chronic pain, and heart health (studies have linked low levels of D with heart disease and heart attacks). It may even reduce fracture risk in the elderly. But — wait for it — it could also help treat, or even prevent, your rheumatoid arthritis. “The influence of vitamin D on the immune system is just being understood,” says Robert Hylland, M.D., a rheumatologist in Muskegon, MI. Here's what the experts know now.