Muscle and Fitness Levels Linked to Dialysis Patients’ Quality of Life

Higher levels of muscle mass contribute to higher quality of life for dialysis patients and could partially explain the “obesity paradox,” in which dialysis patients with a higher body mass index (BMI) have greater survival times than those with a lower BMI, according to a new study. The longer survival of heavier patients has long confused researchers because obese dialysis patients generally have lower levels of physical function, and better physical function is also associated with better survival as well as better quality of life.

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A new study in the Clinical Journal of the American Society of Nephrology used MRIs and other measures to better define body composition to tease out the relative effects of fat and muscle in these patients. Greater muscle mass was associated with better physical function among patients with similar BMIs. The results highlight the limitations of relying on BMI alone and imply that patients could benefit from interventions to increase muscle mass.

The study involved 105 maintenance hemodialysis patients at the University of Utah and at Vanderbilt University Medical Center. BMI is an easy but crude measure of body composition that does not differentiate how much of a person’s weight is due to fat compared to muscle. So in addition to the patients’ BMI, the researchers measured their waist circumference. To get a better idea of each patient’s personal make-up, they used MRI to gauge the mid-thigh muscle level and amount of intra-abdominal fat. They assessed physical function by testing how far the patients could walk in six minutes and quality of life through questionnaires about physical function and mental health status. The patients were tested at the start of the study, after six months, and after 12 months.

The patients with higher BMI, greater waist circumference, and more abdominal fat had poorer scores on the walking test as well as worse scores on the quality-of-life questionnaires. Underweight patients also performed worse than patients with a normal BMI, a detail that could also reflect the effects of muscle mass.

“Because survival rates are better among dialysis patients with higher BMI, some people have argued that obesity is good in dialysis patients. This study is showing that it’s much more nuanced than that,” said lead study author Srinivasan Beddhu, MD, of the University of Utah School of Medicine in Salt Lake City. “Higher fat mass is associated with worse physical function, and more muscle mass is associated with better physical function. So from the point of view of physical function and quality of life, obesity is not good for dialysis patients.”

Beddhu told ASN Kidney News that the relationship between mortality and BMI is complicated, with somewhat higher BMIs having a survival advantage even in the general population. In the U-shaped association, it seems bad to have either too much or too little fat. In the dialysis population, the curve shifts, with even higher levels of BMI having lower death risks. A higher BMI also seems to confer a survival advantage in patients with other chronic conditions such as heart failure and lung disease. “But that does not mean that fat is better than muscle,” Beddhu said.

“This is a well-designed study where they have made careful measurements to try to separate out the influences of fat and muscle,” said Kirsten Johansen, MD, professor of medicine at the University of California, San Francisco, School of Medicine and director of dialysis at the San Francisco VA Medical Canter, who was not involved in the study. Johansen said that the findings do not surprise her, but add important new data about the roles of fat and muscle.

“I’ve become convinced that, from a purely survival point of view, having a reserve of fat is good in case you get sick. But it isn’t good for physical function,” Johansen said. “I get nervous when people say patients who have higher body fat have better outcomes, so we should just let them be like that. These patients have really low levels of physical functioning and physical activity, so something that is having a negative impact on that may be having an impact on their quality of life, even if it is not negatively associated with survival. These data are highlighting once again that survival isn’t everything.”

The study’s findings are consistent with other recent reports looking at the role of muscle mass in dialysis patients, according to Kamyar Kalantar-Zadeh, MD, MPH, PhD, chief of the division of nephrology and hypertension and professor of medicine at the University of California Irvine School of Medicine. Kalantar-Zadeh was the lead author of a similar study of almost 800 maintenance hemodialysis patients published in the Clinical Journal of the American Society of Nephrology in 2010. That study used mid-arm muscle circumference as a surrogate for lean body mass and triceps skinfold as a surrogate for fat mass. The study found that patients with more mid-arm muscle not only ranked higher on a mental health, quality-of-life scale but also had greater five-year survival rates. That finding of greater survival adds important information, considering that the study by Beddhu and colleagues was cross-sectional, giving a snapshot in time of the association of the patients’ baseline levels with better quality of life.

Kalantar-Zadeh said that more studies are needed to go beyond associations and establish causation. “Dialysis patients have very high mortality and they have very poor quality of life. We need more studies to show whether doing something to increase muscle mass improves the outcomes in physical function and quality of life. If these associations are causal, interventions to increase muscle mass may improve patient outcomes,” he said.

Johansen agreed that “we need longitudinal studies and intervention studies to see what happens if we have patients lose weight and build up muscle. Can we improve patients’ functioning by doing these things?”

Beddhu echoed this assessment: “Because this study shows that higher muscle mass is associated with better physical function and quality of life in dialysis patients, interventions such as increased physical activity that decrease fat mass and increase muscle mass are likely to improve physical function, quality of life, and survival in dialysis patients. Such interventions need to be tested in clinical trials.”