Researchers at the University of California,
San Francisco have determined that there is a strong relationship
between being obese and developing end-stage renal disease,
or kidney failure.
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.
