Glucosamine

# Overview
Glucosamine (C6H13NO5) is an amino sugar and a prominent precursor in the biochemical synthesis of glycosylated proteins and lipids. Glucosamine is found as a major component of the exoskeletons of crustaceans and other arthropods, in fungi and many higher organisms and is one of the most abundant monosaccharides. It is produced commercially by the hydrolysis of crustacean exoskeletons. Glucosamine is commonly used as a treatment for osteoarthritis, although its acceptance as a medical therapy varies.

# Biochemistry
Glucosamine was first identified in 1876 by G. Ledderhose, but the stereochemistry was not fully defined until 1939 by the work of Walter Haworth.  D-Glucosamine is made naturally in the form of glucosamine-6-phosphate, and is the biochemical precursor of all nitrogen-containing sugars. Specifically, glucosamine-6-phosphate is synthesized from fructose-6-phosphate and glutamine as the first step of the hexosamine biosynthesis pathway.  The end-product of this pathway is UDP-N-acetylglucosamine, which is then used for making glycosaminoglycans, proteoglycans, and glycolipids.
As the formation of glucosamine-6-phosphate is the first step for the synthesis of these products, glucosamine may be important in regulating their production.  However, the way that the hexosamine biosynthesis pathway is actually regulated, and whether this could be involved in contributing to human disease, remains unclear.

# Health effects
Oral glucosamine is commonly used for the treatment of osteoarthritis. Since glucosamine is a precursor for glycosaminoglycans, and glycosaminoglycans are a major component of joint cartilage, supplemental glucosamine may help to rebuild cartilage and treat arthritis. Its use as a therapy for osteoarthritis appears safe, but there is conflicting evidence as to its effectiveness.

### Use
A typical dosage of glucosamine salt is 1,500 mg per day.  Glucosamine contains an amino group that is positively charged at physiological pH.  The anion included in the salt may vary.  Commonly sold forms of glucosamine are glucosamine sulphate and glucosamine hydrochloride. The amount of glucosamine present in 1500 mg of glucosamine salt will depend on which anion is present and whether additional salts are included in the manufacturer's calculation. Glucosamine is often sold in combination with other supplements such as chondroitin sulfate and methylsulfonylmethane.
Glucosamine is a popular alternative medicine used by consumers for the treatment of osteoarthritis. Glucosamine is also extensively used in veterinary medicine as an unregulated but widely accepted supplement.

### Safety
Clinical studies have consistently reported that glucosamine appears safe. Since glucosamine is usually derived from shellfish, those allergic to shellfish or who have kosher concerns may wish to avoid it. However, since glucosamine is derived from the shells of these animals while the allergen is within the flesh of the animals, it is probably safe even for those with shellfish allergy. Alternative sources using fungal fermentation of corn are available. Another concern has been that the extra glucosamine could contribute to diabetes by interfering with the normal regulation of the hexosamine biosynthesis pathway, but several investigations have found no evidence that this occurs. The U.S. National Institutes of Health is currently conducting a study of supplemental glucosamine in obese patients, since this population may be particularly sensitive to any effects of glucosamine on insulin resistance.
In the United States, glucosamine is not approved by the Food and Drug Administration for medical use in humans.  Since glucosamine is classified as a dietary supplement, safety and formulation are solely the responsibility of the manufacturer; evidence of safety and efficacy is not required as long as it is not advertised as a treatment for a medical condition.
In Europe, glucosamine is approved as a medical drug and is sold in the form of glucosamine sulphate.

### Clinical studies
There have been multiple clinical trials of glucosamine as a medical therapy for osteoarthritis, but results have been conflicting.  The evidence both for and against glucosamine's efficacy has led to debate among physicians about whether to recommend glucosamine treatment to their patients.
Multiple clinical trials in the 1980s and 1990s, all sponsored by the European patent-holder, Rottapharm, demonstrated a benefit for glucosamine. However, these studies were of poor quality due to shortcomings in their methods, including small size, short duration, poor analysis of drop-outs, and unclear procedures for blinding. Rottapharm then sponsored two large (at least 100 patients per group), three-year-long, placebo-controlled clinical trials of the Rottapharm brand of glucosamine sulfate.  These studies both demonstrated a clear benefit for glucosamine treatment.  There was not only an improvement in symptoms but also an improvement in joint space narrowing on radiographs. This suggested that glucosamine, unlike pain relievers such as NSAIDs, can actually help prevent the destruction of cartilage that is the hallmark of osteoarthritis.  On the other hand, several subsequent studies, independent of Rottapharm, but smaller and shorter, did not detect any benefit of glucosamine.
This situation led the National Institutes of Health to fund a large, multicenter clinical trial studying reported pain in osteoarthritis of the knee, comparing groups treated with chondroitin sulfate, glucosamine, and the combination, as well as both placebo and celecoxib. The results of this 6-month trial found that patients taking glucosamine HCl, chondroitin sulfate, or a combination of the two had no statistically significant improvement in their symptoms compared to patients taking a placebo. The group of patients who took celecoxib did have a statistically significant improvement in their symptoms.  These results suggest that glucosamine and chondroitin did not effectively relieve pain in the overall group of osteoarthritis patients.  However, exploratory analysis of a subgroup of patients suggested that the supplements may be effective in patients with pain classified as moderate to severe (see testing hypotheses suggested by the data).
In an accompanying editorial, Dr. Marc Hochberg also noted that "It is disappointing that the GAIT investigators did not use glucosamine sulfate ... since the results would then have provided important information that might have explained in part the heterogeneity in the studies reviewed by Towheed and colleagues" But this concern is not shared by pharmacologists at the PDR who state, "The counter anion of the glucosamine salt (i.e. chloride or sulfate) is unlikely to play any role in the action or pharmacokinetics of glucosamine". Thus the question of glucosamine's efficacy will not be resolved without further updates or trials.
A subsequent meta-analysis of randomized controlled trials, including the NIH trial by Clegg, concluded that hydrochloride is not effective and that there was too much heterogeneity among trials of glucosamine sulfate to draw a conclusion.