Very obese people who need a kidney transplant are far less likely to get one than people of normal weight, a new U.S. study finds.

And when they do, they wait on average a year to 18 months longer.

The reason seems to be both economic and medical, according to researchers at John Hopkins University in Baltimore. Very obese people have a greater risk for complications, and the transplant centres often must bear the additional cost of treating those problems. 

Patients might be turned down because of the higher costs and less rosy outcomes that are associated with overweight patients who undergo transplants.

"Being overweight should not be a disqualifying and discriminating factor against these patients," Johns Hopkins transplant surgeon Dorry Segev, who led the study, said in a release.

The researchers analyzed kidney disease data from 132,353 men and women between 1995 and 2006. All of the study participants were on the United Network for Organ Sharing deceased donor kidney transplantation list.

A person's weight was calculated using body mass index, which factors in height and weight to come up with a number that acts as a guide regarding whether a person is underweight, of normal weight or overweight. A BMI of 18.5 to 25 is normal, while a BMI of 25 to 30 means a person is overweight. An obese BMI is between 30 to 35, a severely obese one is between 35 to 40 and morbidly obese one is between 40 to 60.

The study showed that morbidly obese patients — those who averaged about 100 pounds over their ideal weight — were 44 per cent less likely to get a transplant than normal-weight patients. Those just slightly less obese were 28 per cent less likely to get a transplant.

The results indicate that very obese patients are more likely to die while waiting for a transplant.

The study was published online Wednesday in the Journal of the American Society of Nephrology.

With files from the Associated Press