Kidney Transplant: How Big Is Too Big?

Obese kidney transplant recipients may have worse outcomes but more research is needed

/ Author:  / Reviewed by: Joseph V. Madia, MD

(RxWiki News) Before kidney transplantation, heavier patients are often told to lose weight to reduce the risk of complications. However, it's still unclear if some obese patients are too heavy to get a kidney transplant at all.

Obesity is an epidemic in the United States and can play a role in the development of kidney disease. 

Recently, a team of researchers looked at past studies to gain a better understanding of how obesity affects kidney transplant patients. The researchers asked, "How big is too big for transplantation?"

Their analysis suggested that the answer is not entirely clear. Still, there was some evidence that obese patients may have worse outcomes after kidney transplantation.

Results showed that measures of body fat - including body mass index (BMI) and waist size - were associated with worse outcomes, including delayed function of transplanted kidney, failure of transplanted kidney, heart disease and high costs. However, researchers have been unable to pinpoint a limit at which obese patients should not be recommended for transplantation.

"Control your weight before kidney transplantation."

The investigation was led by Saint Louis University researchers Krista L. Lentine, MD, associate professor of internal medicine in nephrology, and Betsy Tuttle-Newhall, MD, director of abdominal transplant.

"Lifestyle alterations that seem reasonable to improve health outcomes should be encouraged," said Dr. Tuttle-Newhall in a press release. "Just as we require patients with alcoholic liver disease to stop drinking prior to transplant, it is reasonable to ask kidney transplant candidates to lose excess body fat and attempt to increase lean muscle mass by becoming more physically active and modifying their diet."

In addition to finding that patients with higher BMI did not do as well after kidney transplant, the researchers identified some additional items that need to be studied further in order to make reliable recommendations about kidney transplants in obese patients. These items included:

  • use of BMI and other measures of body fat as predictors of outcomes after transplant
  • cost effectiveness of transplant in heavier patients
  • impact of weight loss strategies before transplant surgery

"Current guidelines from the American Society of Transplantation recommend a supervised weight loss regimen including a low-calorie diet, behavioral therapy, and a physical activity plan to achieve a body mass index (BMI) of less than 30 prior to kidney transplantation," said Dr. Lentine in the same press release. "But, these guidelines also note that we don't have enough data to determine if some obese patients aren't appropriate candidates to receive kidney transplants at all.

"For this reason current acceptable BMI limits for kidney transplant candidates vary across transplant centers."

BMI is used to determine people's weight category. For adults, a BMI of 18.5 to 24.9 is considered normal. A BMI of 25 to 29.9 is considered overweight, while a BMI of 30 or more is considered obese.

According to the study's authors, future research should try to identify the point at which a patient is too heavy to be recommended for kidney transplantation. That is, researchers should pinpoint the BMI limit at which the risks and costs of transplantation outweigh the benefits.

The researchers also noted that BMI alone is not always the best measure of body fat. They suggest that future studies use other measures of body fat.

Even though study results showed that obese transplant recipients did not fare as well as normal weight recipients, the authors pointed out that many obese patients on dialysis (the alternative to transplantation) have better long-term survival after a transplant, compared to those who stay on dialysis. In other words, transplantation is linked to better survival rates than dialysis among obese patients.

The researchers also cited past research showing that kidney patients on dialysis with a higher BMI may live longer than those with a lower BMI - a finding known as the "obesity paradox." It remains unclear how excess fat is beneficial to dialysis patients, but the benefit does not appear to happen when it comes to transplantation.

In addition, research has yet to prove if losing weight before kidney transplant surgery is beneficial. More research is needed to see if planned weight loss through diet, exercise and weight loss surgery actually improves outcomes. Research on this topic has not distinguished between intentional weight loss and unplanned weight loss due to illness.

According to Dr. Lentine, "This review showed us where there are gaps in the existing research and where current data is too light to be able to draw solid conclusions.

"We've figured out some key questions to ask so that we can help our patients have successful transplants and healthy lives."

The study was published December 5 in the American Journal of Nephrology. Funding for this research was provided by the National Institutes of Health, Centers for Disease Control and Prevention, nonprofit agencies and corporate sponsors.

The authors reported no conflicts of interest.

Reviewed by: 
Review Date: 
January 28, 2013
Last Updated:
August 19, 2013