When his mother's kidneys began to fail three years ago, Ed Guillen knew what he had to do: donate one of his kidneys to her. But Guillen received a shock during a phone call with the Stanford Kidney Transplant Clinic, where his mother was being treated. He was ineligible to be a donor, even before tests to see if he was a genetic match.

At 5-foot-10 and around 280 pounds, Guillen, a 39-year-old software developer in Redmond, Wash., was considered too heavy to donate an organ without potentially endangering his own health. He would need to lose more than 70 pounds, he was told, before the clinic would consider him as a candidate.

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 the donor body mass index, or BMI, at 35. About 10 percent do not allow donors with BMIs over 30, generally considered the cutoff for obesity, while the rest allow some heavier people to donate.

"Part of the shortage of living donor kidneys is because of the stringent criteria we're placing on our living donor candidates who come in for evaluation," said Dr. Mala Sachdeva of the North Shore University Hospital Transplant Center on Long Island, where 23 of the 104 potential donors who contacted the center the past three years were morbidly obese, exceeding the center's BMI limit of 35. Three of them lost weight and donated. Twenty-four more potential donors had BMIs over 30, a group that some centers would have excluded.

National data on how many would-be donors are excluded because of obesity is scarce, but experts suspect the numbers are significant. This comes at a time when more than 92,000 people nationwide are waiting for a kidney transplant and the prevalence of kidney disease is growing — in many instances because of rising rates of diabetes and hypertension linked to obesity.

But even as some centers have become more concerned about the dangers, the percentage of accepted donors who are obese has risen, to nearly 20 percent in 2008 from around 14 percent in 2000. Donors with BMIs over 35 remained relatively uncommon, about 2 percent of all donors.

Weighing the risks

While obese donors have a higher risk of minor surgical complications, major complications are rare. Doctors worry most about donors' health in the years after the operation, because obesity increases the risk of kidney disease.

"The question is, if you have one kidney, are you at risk of getting kidney failure sooner than if you have two?" Taler said. "You'd think it would be an obvious answer, but it's not."

Healthy donors of normal weight, whose risks are best understood, have similar or even decreased risk of kidney failure compared with the general population. But there have been no studies following obese kidney donors over decades. Doctors fear that single kidneys in obese donors may become overworked, putting them at greater risk for disease.

Kidneys make up a vast majority of living-donor organs, but obesity is also an increasing problem among liver donors, who donate part of that organ. Obese potential liver donors, both living and deceased, often have nonalcoholic fatty liver disease, which makes donation risky for living donors and less effective for the recipient. And Type 2 diabetes, which often accompanies obesity, may preclude kidney donation in living and deceased donors.

While some transplant centers offer overweight potential donors access to a hospital nutritionist, patients frequently must turn elsewhere to lose weight if they wish to meet strict donation criteria. And acceptable weight at the time of donation is no guarantee that a donor will keep the weight off.

Transplant centers are required to follow up with donors for two years after surgery, but the follow-up is often conducted by phone, and many centers fail to collect complete information.

Some hospitals are making an effort to follow up in person and adding post-donation wellness programs.

The follow-through

Advocates for donor wellness hope that more transplant centers will begin to see kidney donation and donor follow-up as an opportunity to help donors commit to caring for their bodies, not just as a set of forms to fill out.

For example, Alysun Deckert, a University of Washington nutritionist and a marathoner, has volunteered since 2001 to lead Team Transplant, a running group made up of more than 100 transplant recipients, donors and medical workers.

Ed Guillen, who was hoping to donate a kidney to his mother, enrolled in a program called 20/20 Lifestyles, which offers nutritional counseling and personal training sessions. He lost 90 pounds over eight months, enough to be eligible for donation screening, and was found to be a good genetic match.

"Before, I would ride my bike five miles down the road to a local brew pub and have some beers and ride back, and that's what I considered a workout," he said. He started running up to an hour most days and lifting weights, a habit he kept until April, when he donated a kidney to his mother.

Both are doing well, although Guillen was not allowed to run for six weeks after the operation. He is now easing back into his exercise routine.

"For those who are very motivated to want to donate and make a meaningful change in their personal health, it can be a really great motivator," said Dr. Jane Tan of Stanford, who saw Guillen before and after his donation. "It's really a delight for a physician involved to see a health benefit in the donor or the recipient."