The long-range study found that the obese have up to a seven times greater risk of kidney failure than normal weight people, suggesting that obesity should be considered a risk factor for the condition, and that kidney failure is yet another consequence of obesity.
"There are more and more people with kidney failure, but it hasn't been appreciated much that kidney failure can be a consequence of obesity," said Chi-yuan Hsu, MD, UCSF assistant professor of medicine and lead author of the study. "We think this study is important because it demonstrates quite convincingly that people who are obese or overweight are at much higher risk of kidney failure."
The study, published in the January 3 issue of the Annals of Internal Medicine, was conducted jointly with Kaiser Permanente of Northern California Division of Research.
Research findings showed that being even moderately overweight nearly doubles the risk of developing the condition, which is a complete failure of the kidneys to process waste so that dialysis or transplantation become necessary.
"If you are mildly overweight, not even frankly obese, you are roughly 90 percent more likely to develop end-stage renal failure," Hsu said, with the risk reaching over 700 percent greater for the morbidly obese.
The research is based on data derived from over 320,000 Northern California Kaiser members whose height and weight were measured during health checkups between 1964 and 1985. A total of 1,471 cases of end-stage renal disease occurred among study participants during an average follow-up period of about 26 years. Cases of end-stage renal disease were determined using the U.S. Renal Data System, a comprehensive national registry that collects and disseminates information on end-stage renal disease.
Researchers calculated the body mass index (BMI) of study participants and found that those with a higher BMI were at greater risk of kidney failure.
BMI is weight in kilograms divided by height in meters squared. A BMI of over 25 defined a person as overweight, and BMI of over 30 moved a person into class I obesity. Class II obesity was defined as BMI of 35 to 39.9, and class III obesity (or morbid obesity) was defined as a BMI of 40 or above. A five-foot-ten-inch man weighing 160 pounds would have a BMI of 23. If he weighed 190 pounds he would be overweight with a BMI of 27.3. At 220 pounds he would be obese with a BMI of 31.6.
Of the study participants, 58 percent were of normal weight and 39 percent had a BMI of 25 or greater. The risk of kidney failure among "overweight" study participants was 1.87 times that of normal weight participants, or nearly 90 percent greater. The most obese study participants, with a BMI of 40 or above, had over seven times the risk of kidney failure.
More than 400,000 Americans receive long-term kidney dialysis and more than 20,000 have a functioning transplanted kidney. The number of people who suffer from end-stage renal disease is projected to increase to more than 650,000 by 2010, with associated Medicare expenditures of $28 billion. Kidney failure is ninth among the leading causes of death in the United States, with an annual death rate of about 20 percent a year.
Traditional risk factors for end-stage renal disease are high blood pressure and diabetes, Hsu said. But the study found that obesity remained a risk factor, even after adjustment for blood pressure and diabetes status. Most kidney doctors don't think of weight loss as a potential way of reducing kidney failure, he added. The study suggests that kidney doctors should calculate their patients' BMI when evaluating their risk of kidney failure.
Hsu said one reason for the higher rate of kidney failure among obese patients might be that they are more likely to develop diabetes and hypertension. Another reason is that obesity places more metabolic demand on the kidneys, forcing them to work harder. "As the person gets bigger, hyper-filtration occurs and this over filtration is what tears the kidneys down."
It has long been appreciated that obesity is linked to heart disease, high blood pressure, and diabetes, among other ailments. Now, kidney failure should be added to that list, Hsu said.
"Kidney failure is yet another bad thing that is likely to happen to you, if you are obese. This is a bad thing that was not previously recognized," he said.
Study co-authors are Charles E. McCulloch, PhD, of UCSF, and Carlos Irabarren, MD, MPH, PhD; Jeanne Darbinian, MPH, and Alan S. Go, MD, of Kaiser.
The research was supported by the National Institutes of Health
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Journal
Annals of Internal Medicine