Add one more unexpected consequence of Americans' expanding waistlines: A growing number of potential organ donors are ineligible to donate because of their weight. "It's an ongoing struggle that I think every transplant center has to deal with," said Dr. Sandra Taler, a nephrologist at the Mayo Clinic who studies the health of living transplant donors. Rising obesity has spurred a small but growing effort to pay closer attention to the health of obese donors, whose risks are still incompletely understood. There is no binding donor weight limit, but a little more than half of transplant centers cap donor body mass index at 35.