Gout is a painful and common type of arthritis. About 1 in 100 people develop gout. The condition is usually associated with a long-lasting, abnormally high amount of uric acid in the blood, called chronic hyperuricemia.
The rate of gout has increased in recent decades, not only in America but also in other developed countries. The increase is possibly due to dietary and lifestyle changes, greater use of medications that cause hyperuricemia, and aging populations. Gout is very uncommon in developing countries.
Metabolism of Purines. The process leading to hyperuricemia and gout begins with the metabolism of purines, nitrogen-containing compounds that are important for energy. Purines can be divided into two types:
The process of breaking down purines results in the formation of uric acid in the body. Most mammals have an enzyme called uricase, which breaks down uric acid so it can be easily removed from the body. Because humans lack uricase, uric acid is not as easily removed, and can build up in body tissues.
Uric Acid and Hyperuricemia. Purines in the liver produce uric acid. The uric acid enters the bloodstream, and most of it eventually goes through the kidneys and is excreted in the urine. The remaining uric acid travels through the intestines, where bacteria help break it down.
Normally these actions keep the level of uric acid in the blood plasma (the liquid part of the blood) at a healthy level, which is below 6.8 mg/dL. But under certain circumstances, the body produces too much uric acid or removes too little. In either case, concentrations of uric acid increase in the blood. This condition is known as hyperuricemia.
If concentrations of uric acid reach 7 mg/dL and above, needlelike crystals of a salt called monosodium urate (MSU) form. As MSU crystals build up in the joints, they trigger inflammation and pain, the characteristic symptoms of gout.
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