Compared with normal weight individuals, those who were overweight were nearly twice as likely to develop kidney failure, and the extremely obese were seven times more likely, said Chi-yuan Hsu, M.D., of the University of California here.

Previous studies have shown that the overweight and obese are at increased risk for diabetes and hypertension, which are traditional risk factors for kidney failure. But Dr. Hsu and colleagues found that obesity remained a risk factor even after adjusting for diabetes and hypertension at baseline.

The long-term study examined data on more than 320,000 Northern California Kaiser Permanente members whose height, weight, blood pressure, and diabetes status were assessed during health checkups between 1964 and 1985.

Through December of 2000, researchers identified cases of end-stage renal disease using the U.S. Renal Data System, a national registry that collects and disseminates information on end-stage renal disease. Average follow-up was about 26 years.

A total of 1,471 cases of end-stage renal disease occurred among study participants. When researchers looked for a link between kidney failure and body-mass index, they found the following increases in risk compared with normal weight individuals:

· For those who were overweight (BMI=25-29.9) the adjusted relative risk was 1.87 (95% confidence interval=1.64-2.14).

· For those with class I obesity (BMI=30-34.9) the adjusted relative risk was 3.57 (95% CI=3.05-4.18).

· For those with class II obesity (BMI=35-39.9) the adjusted relative risk was 6.12 (95% CI=4.97-7.54).

· For those with extreme obesity (BMI>40) the adjusted relative risk was 7.07 (95% CI=5.37-9.31).

These results were adjusted for baseline diabetes and hypertension status, although the researchers acknowledged that one limitation of the study was that ascertainment of diabetes status was not uniform among the cohort and that a single baseline blood pressure measurement, as used in this study, is prone to inaccuracy.

The researchers also acknowledged the possibility that individuals with elevated BMIs at baseline were more likely to develop new cases of diabetes and hypertension during the study period, which could have been the causal pathway to end-stage renal failure. The study did not account for this possibility.

But the researchers argued that elevated BMI can independently lead to kidney failure through other mechanisms. Dr. Hsu noted that obesity places more metabolic demand on the kidneys, forcing them to work harder. "As the person gets bigger, hyperfiltration occurs, and this over filtration is what tears the kidneys down," he said.

"In summary, we have identified overweight or obese as a strong and potentially modifiable risk factor for the development of end-stage renal disease," the study authors said. "Conversely, kidney failure should be added to the list of adverse consequences of obesity."

Dr. Hsu put it in more patient-friendly terms. "Kidney failure is yet another

Hsu C et al. Body-mass index and risk for end-stage renal disease.

Source: Diabetes In Control: Annals of Internal Medicine. 2006; 144(1).