Glucosamine is produced within the body in small amounts in reactions involving glucose and glutamic acid. It is a small molecule (molecular weight = 179.17) that is easily absorbed in vivo. Humans may decrease production with aging. It is not found in any common foods and cannot be obtained externally. If the body is not synthesizing the substance, it needs to be taken as a supplement. It is found in abundance in cartilage, with small amounts measured in tendons and ligaments; it is an essential substrate matrix that is a component of cartilage.
It is still not clear what the actions are of glucosamine taken as a nutritional supplement. Purported effects are the promotion and maintenance of the structure and function of cartilage in the joints of the body. It has also been reported that glucosamine has anti-inflammatory effects. The biochemistry, however, has been known for quite some time. Glucosamine, a sugar and a sulfated amino monosaccharide, is involved in glycoprotein metabolism where it is found in proteoglycans as polysaccharide groups called GAGs. All GAGs contain derivatives of glucosamine or glactosamine. These polysaccharides comprise 95% of the ground substance in the intracellular matrix of connective tissue. One of the GAGs, hyaluronic acid, is essential for the function of articular cartilage and is responsible for shock absorbing and deformability functions.6 In vitro studies show that it can alter chondrocyte metabolism; it is not clear whether oral glucosamine can reach chondrocytes in vivo.8 Over 90% of the studies in glucosamine pharmacokinetics have used animal models. It has been shown that about 90% of the salt is absorbed from the small intestine and transported to the liver. The majority is then catabolized in the first pass; seldom is it detected in serum after oral ingestion. Free glucosamine is not usually detected in plasma.910 How much is taken into joints is not known for humans, while some uptake is seen in articular cartilage in animals.
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