Effect Of Reduced Kidney Function Or Disease On Hyperuricemia And Gout
The ability of the kidney to eliminate urate is reduced by any abnormality of the kidney in the form of a primary kidney disease. However, kidney disease also reduces the ability of the kidney to perform all its other excretory and regulatory functions.
Some of the early features of kidney disease are the presence of protein or blood in the urine, or the inability of the kidney to concentrate urine so that urine is passed more frequently. In some cases kidney disease may only be recognize by an increased concentration in the blood of urea or creatinine, two other substances which are eliminated by the kidney.
Primary kidney disease reduces the excretion of all substances eliminated by the kidney, so their concentration in the body will rise. This will include some elevation of the serum urate concentration, with some varieties of kidney ‘disease having a greater effect than others. Whether kidney disease will produce actual hyperuricemia symptoms will depend in part upon the serum urate concentration prior to the development of the kidney disease. If the original serum urate was low and there was only a modest rise with the kidney disease, the serum urate may remain within the normal range. If, however, the original serum urate was toward the upper limit of normal, kidney disease may push the serum urate into the hyperuricemia range.
Some of the factors causing kidney disease are reversible and if kidney function returns to normal the hyperuricemia may revert to its former level. Nonetheless, it is important to remember that kidney disease and the kidney’s ability to excrete urate is only one factor in determining the serum urate concentration.
Some of the varieties of kidney disease which tend to produce a disproportionate degree of hyperuricemia include kidney disease due to childhood lead poisoning, kidney disease due to excessive consumption of analgesics, and polycystic kidney disease.
Effect Of Body Weight
Some of the information regarding how body weight affects gout was obtained from the My Gout Diet Foods site. All studies into the differences in urate concentrations between different individuals and different populations have confirmed that, whatever index is used, body weight is the most important determinant of the serum urate concentration. However, there is some disagreement as to which aspect of body weight is the most important. Is it the bulk of the muscle? Is it the lean body mass? To what extent do fat stores contribute? Is weight gain after maturity important, or is it best expressed as a measure of weight for a particular height? All of these measures have in common the patient’s body weight and, whichever adaptation or modification of body weight is used, weight always comes out as the top determinant of the serum urate concentration. This seems to apply to all races; for example, Japanese school children between the ages of 13 and 18 show the same high correlation between body weight and serum urate concentration as is found in Caucasian races. However, a high correlation between two things does not mean that there is a cause and effect relationship between them and it may be that they are both secondary to something else. So how could body weight have such a major role in determining the serum urate concentration?
Studies of urate production and excretion in obese people and in the same people after they have lost weight suggest strongly that there are several different mechanisms involved.
When obese people lose weight, the ability of their kidneys to eliminate urate improves, and this seems to be the main mechanism. However, some also show a reduction in urate production and a reduction in the severity of high blood pressure (hypertension), and this would also facilitate kidney excretion of urate. Thus weight loss reduces the production of urate and also improves renal elimination of urate, thereby giving a two-pronged attack on any associated hyperuricemia. Another factor may be the extent to which ATP break-down in muscle contributes to urate production in muscular patients whose high body weight is due to a high muscle mass.