Glucosamine and chondroitin occur naturally in cartilage, the flexible, fibrous connective tissue in joints and between bones. In this study, 605 adults with chronic knee pain and osteoarthritic joint-space narrowing took 1,500 mg of glucosamine, 800 mg of chondroitin, these two together, or a placebo. After two years, while there were no differences between the placebo group and those taking either one of the supplements by itself, those taking glucosamine and chondroitin together had 0.1 mm less joint-space narrowing.
Discussing their findings, doctors said both glucosamine and chondroitin contain sulfate, which the body needs to synthesize proteins in cartilage, and that prior studies have found reduced protein synthesis when sulfate is low. The superior results for glucosamine and chondroitin together in this study suggest a biological mechanism that helps restore cartilage.