High BMI is associated with increased filtration fraction in the remaining kidney, researchers find.
Live kidney donors who are overweight or obese have increased filtration fraction (FF) in their remaining kidney. This could explain the previously found increased risk of renal damage, according
to researchers.

“With the shortage of organs for donation, people who are older or obese are nowadays more readily accepted as living kidney donors, which may expose them to a higher risk of kidney damage,” said lead investigator Mieneke Rook, MD, a resident at the University Medical Center Groningen, The Netherlands.

“To ensure donor safety, it is important to identify the mechanisms through which obesity may be harmful to the remaining kidney after donation.” She reported findings during Renal Week 2006 in San Diego.

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Dr. Rook and her colleagues studied 200 consecutive kidney donors who had a mean age of 48 years. Thirty-eight percent of the group was male and the mean BMI was 25.6 kg/m2.

Four months before and two months after donation, investigators determined glomerular filtration rate (GFR) by the clearance of constantly infused 125I-iothalamate and measured effective renal plasma flow (ERPF) simultaneously by 131I-hippuran. FF was calculated as GFR/ERPF x 100. Mean arterial pressure (MAP) was measured at both time points.

Before donation, the mean GFR was 113 mL/min, the mean ERPF was 431 mL/min, and FF was 26.6%. After donation, the mean GFR was 72 mL/min, the mean ERPF was 282 mL/min, and FF was 25.7%. MAP, however, rose significantly from 92 mm Hg before donation to 95.9 mm Hg afterwards. Higher BMI and older age, but not MAP, independently predicted higher FF. Compared with normal weight donors, increases in FF occurred even in donors who were only modestly overweight.
In previous studies, coworkers from Dr. Rook’s research group found that hyperfiltration predicted increased rates of transplant loss and an increased risk of early death in kidney transplant recipients.
The new findings suggest that overweight or obese donors need to be monitored long term, she said. Moreover, elevated filtration pressure can be treated.
“Filtration pressure can be reduced by weight loss and proven drug treatments, such as ACE inhibitors and AT1 receptor blockers,” Dr. Rook said. “Moreover, we recently discovered that a low-sodium diet can also correct overweight induced hyperfiltration, and thus might be able to protect the kidney from the harmful effects of obesity.”