Oct. 2, 2023 – Here’s a better reason to work out than simply losing weight.

Higher physical fitness and maintaining body weight lowered the risk of chronic kidney disease in adults with obesity, according to a study published Thursday in Obesity. But losing weight did not lower that risk. 

“We need to know more about the optimal strategies to reduce kidney disease risk in adults with obesity,” said study author Meera Harhay, MD, the medical director of clinical research at Drexel University in Philadelphia. 

Obesity is a well-established risk factor for kidney disease, a leading cause of death in the United States. Hormonal changes related to extra body fat can increase body fluid volume, raise blood pressure, and promote insulin resistance. All that forces your kidneys to work harder to filter your blood, leading to damage and scarring. 

“The kidneys can only compensate so much before there is permanent damage,” said Harhay, an epidemiologist and a kidney transplant expert. 

In the study, Harhay and colleagues used data from the Multi-Ethnic Study of Atherosclerosis, a National Institutes of Health initiative that followed 6,814 middle-aged adults in six U.S. cities, tracking their weight and other health measures over 10 years. From that pool, the researchers zeroed in on 1,208 adults who fit the medical definition of obese but did not start the study with kidney disease or diabetes (which can cause kidney disease).

They found that for every 11 pounds people gained, their risk of kidney disease shot up by 34%. But losing weight did not decrease the risk. This suggests that preventing weight gain may be more important than weight loss. 

Using the participants’ self-reported walking pace as a measure of fitness, the researchers found that those who walked slower than 2 miles per hour were 57% more likely to get kidney disease than faster walkers.

Once scar tissue forms in the kidneys, that damage can’t be undone. But regular exercise may help the body survive that damage, the researchers surmised.

Harhay said the benefit could be related to the anti-inflammatory effects of regular exercise and/or better health in the heart and blood vessels. “The mechanisms by which exercise and higher fitness are protective to the kidneys represent an important knowledge gap,” she said.

Beyond Body Weight

The study reflects a growing trend in obesity research away from a focus on just body weight to looking into other things that may explain the link between obesity and disease.

“For about 2 decades now, there has been in-depth research into what is the stronger factor for our longevity and disease risk,” said Matthew Ahmadi, PhD, a postdoctoral researcher at the University of Sydney, in Australia. “Is ‘fitness’ the primary driver, ‘fatness,’ or a balanced combination between the two?”

In a 2022 study co-authored by Ahmadi, people who walked at a faster pace had a 36% lower risk of dying during the study period (7 years) than those who walked more slowly. A 2021 study, from researchers at the University of Arizona, found that starting a regular exercise routine – whether it resulted in losing weight or not – countered the risk of early death linked to a high body mass index. 

It’s important to note that heart-pounding workouts can pose complications for patients with kidney damage. Kidney disease is linked to muscle loss (sarcopenia) and the loss of muscle strength (dynapenia). As such, resistance training may be a good option. In fact, recent research suggests that resistance training may be better at improving walking speed in patients with sarcopenia than programs that also use other types of training.

“A person with kidney disease should consult with their physician about their exercise goals,” said Harhay, who intends to explore how people with obesity and kidney disease can lose weight while maintaining muscle mass.