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The researchers analyzed data collected from nearly 3,000 black and white young adults who had normal kidney function.
The participants, who had an average age of 35, were grouped according to four ranges of body-mass index (BMI), a measurement of body fat based on height and weight. The groups were normal weight, overweight, obese and extremely obese.
Over time, kidney function decreased in all the participants, but the decline was much greater and quicker in overweight and obese people, and appeared to be linked solely with body-mass index.
"When we accounted for diabetes, high blood pressure and inflammatory processes, body-mass index was still a predictor of kidney function decline," study first author Dr. Vanessa Grubbs, an assistant adjunct professor of medicine at the University of California, San Francisco, said in a university news release.
"There was something unique about just being too large that in and of itself affected kidney function even before the onset of kidney disease," Grubbs said. "We're not able to tease out the [reason] for that just yet, but we're hoping to look at it in a future study."
The researchers also found that measuring blood levels of a protein called cystatin C is better than the more common method of measuring creatinine levels in detecting subtle changes in kidney function. This holds true even when kidney changes are still within what is considered the normal range.
"The fact that we were able to use this marker to see declines in kidney function long before patients would be deemed to have chronic kidney disease is good, in that it may allow us to detect problems earlier and hopefully intervene sooner," Grubbs said.
The findings, published online recently in the American Journal of Kidney Diseases, show the need for doctors to intervene early to prevent kidney disease in obese patients, the researchers said.
"We're getting larger and larger at younger and younger ages, so the problems we will see that are directly related to obesity are going to become more common, and they're going to start earlier in life," Grubbs said.
"Even before the level at which we can diagnose illnesses, decline in kidney function is happening," she said. "Is it reversible? We're not sure. Preventable? It stands to reason that it would be."
Although the study showed an association between obesity and increased risk of kidney disease, it did not prove a cause-and-effect relationship.
-- Robert Preidt
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SOURCE: University of California, San Francisco, news release, Dec. 13, 2013