Vitamin D is found in cells throughout the body. A study (Journal of Clinical Rheumatology, 2009) of 1,248 people age 55 and older looked at how their knee health changed over a period of about seven years. Those with a low amount of vitamin D in their diet were at increased risk for worsening knee osteoarthritis. Fatty fish — such as salmon, tuna and mackerel — are good sources of this vitamin. It’s also added to vitamin D fortified milk, yogurt, orange juice and cereal.