People who are waiting for a kidney transplant are often told to lose weight if they have a body mass index (BMI) that exceeds 30, the definition of obesity.

But a new study shows that those who lose significant weight — 10 percent of their body weight — are at a higher risk for a poor outcome following the surgery, including failure of the new organ and even death.

The study, recently published in the American Journal of Kidney Diseases, also constitutes a more general and cautionary tale about weight loss. As lead author Meera Harhay, an associate professor in Drexel’s College of Medicine and Dornsife School of Public Health, said, not all weight loss is the same.

Harhay worked with other researchers from Drexel and Johns Hopkins University in Baltimore. She recently spoke to us about the study and its implications.

Weight loss can be a sign of many different health trajectories. When patients without obesity lose weight before a kidney transplant, we automatically worry that they might be getting sick, that they might be losing nutrition. When patients with obesity lose weight, it’s not always clear what is happening.

When obese people with kidney disease get evaluated for a kidney transplant, they’re often advised to lose weight first. For the most part, that’s because doctors are worried that performing surgery on people with obesity can lead to more complications. Many transplant centers say that if your body mass index is over a certain number — 40, for example — they will turn you down.

But losing weight in a healthy way when you’re also struggling with organ failure is no small thing.

I wanted to see how the way that someone’s body weight changed while they were waiting for a transplant could predict their health outcomes after the transplant.

We used a large registry database — the Organ Procurement and Transplantation Network’s database — that has information on all transplant recipients in the U.S. We included over 94,000 kidney transplant recipients that received their transplants over a 10-year period and examined how body weight changes that occurred before a kidney transplant affected post-transplant outcomes.

One of the reasons we focused on those who got kidney transplants instead of, say, all organ transplant recipients, is that kidney transplant recipients are unique. Instead of being prioritized for transplant by how sick they are, kidney transplant candidates are prioritized by how long they wait on the waiting list. We have people on the waiting list for five to 10 years. That waiting time, for a lot of people, is a time when health is changing. Some gain weight and some lose weight. Some people get stronger. Some people get sicker. Dialysis itself is really hard on the body. Patients can have a hard time keeping up with their nutrition. So changes in body weight can really be telling you a lot of different stories.

There was a relationship between losing a substantial amount of weight — 10 percent of your body weight, or 20 pounds for a person who weighs 200 pounds — with longer hospitalization after the transplant. Those patients also were more likely than patients with stable weight to have the transplanted organ fail. And their risk of death was 18 percent higher than those with stable weight.

We found that losing 10 percent body weight or more signaled higher risk across the board, regardless of whether the patient was underweight, an optimum weight, or obese.

Again, these patients have organ failure, so many of them may lose weight because of their illness. What surprised me is that the signal was the same for people who had an obese BMI or a morbidly obese BMI. That surprised me because those are the people who are most likely being told to lose weight. It worried me because I wondered what was actually happening when those patients were told that they were successful. What was actually going on as the numbers on the scale went down?

We have some theories. When you step on the scale and see the weight go down, you could be losing fat, you could be losing muscle, you could be losing water. There’s a whole lot of research that says if you lose muscle, you are getting weaker, so you would be more likely to die. Even obese and morbidly obese people can lose weight in this unhealthy way. I think that’s part of the signal that we picked up in this study.

The same person with obesity could lose 20 pounds in two very different ways. They may make healthier eating choices and exercise, or they could become malnourished, losing muscle. The number on the scale would go down in both instances. We, as physicians, need to know what we’re really seeing when we observe weight loss in our patients.

Now I feel like I have to be more thoughtful when I prescribe weight loss for my patients who are kidney transplant candidates. Even when I think a patient should lose weight to improve their mobility or health conditions, when I see that they are losing weight, I will take a second look and see if that person is actually getting healthier.

We have known for a long time that when people lose a lot of weight without really trying — unintentional weight loss — it’s a warning sign. Where it gets harder to disentangle is when you have someone who is obese and told to lose weight. Because the way they lose weight is probably very important for their health, too.

We have options to be more precise when assessing weight and weight loss. The number on the scale is helpful, but there are other tools that can tell us even more about a person’s health, such as waist circumference and waist-to-hip ratio. Newer technologies can directly measure a person’s body composition — percent fat, percent muscle.

There is a well-recognized syndrome called the physical frailty syndrome. It encompasses all the subtle signs of vulnerability — exhaustion, weakness, unintentional weight loss — that could help us determine who is at risk of a bad health outcome. After all, a 70-year-old person could be more fit than a 60-year-old person. A person with obesity could be stronger than a person without obesity.

Weight really is just a number. We’re recognizing that more and more. Age is just a number, as well. Neither paints a complete picture.