The pool of available kidneys is shrinking because one in four patients are too fat to donate.

Even patients who were put on a diet were unable to lose enough weight to donate, according to a study by Mala Sachdeva of North Shore-LIJ Health System Transplant Center on Long Island, New York, Medpage Today reports.

Patients with a Body Mass Index over 35 generally aren't allowed to donate because studies have shown complications for both the donor and recipient. A healthy BMI is under 25.

The impact of obesity could be devastating to the field: there are 92,000 people in the U.S. waiting for a kidney, according to The Daily Mail.

And a recent study showed that obesity costs the U.S. more than smoking.

Here's a chart showing obesity by region in the U.S.: