Should We Use BMI or Waist Circumference for Kids With CKD?

Columbus, OH — May 2018

Debate over the best way to measure obesity stems from mixed results in various patient populations and clinical studies. In some, waist circumference seems to add value above and beyond the common body mass index (BMI). In others, its added value is minimal to nonexistent. The first-ever study examining the prevalence of obesity among children with chronic kidney disease (CKD) based on both measures, as well as the relative strength of each measure in predicting health indicators, found that the puzzle is not yet solved for this population.

“There was reason to believe that waist circumference might be a more valid measure of obesity in children with CKD,” says Hiren Patel, MD, chief of Nephrology at Nationwide Children's Hospital and first author on the study, which appeared in The Journal of Pediatrics. “CKD in children is associated with reduced muscle mass. In addition, waist circumference may better reflect central obesity, which is associated with metabolic risk factors that affect children with CKD.”

Using data from the ongoing Chronic Kidney Disease in Children (CKiD) cohort study, Dr. Patel and colleagues (including the cohort’s overall principal investigators) from several other participating institutions examined waist circumference, BMI and a range of health markers for 737 children with CKD ages 5 to 19.

The measures agreed on obesity classifications in 92 percent of children with nonglomerular CKD and in 87 percent in the glomerular CKD group. Overall, 71 children (nearly 10 percent) were categorized differently by the two measures; for example, 9.4 percent of children with glomerular CKD were classified as obese by waist circumference criteria but not BMI. More than 19 percent of the nonglomerular CKD group and 34 percent of the glomerular CKD group were classified as obese by at least one measure.

“Obesity is common in children with CKD,” says Dr. Patel. “Even though we have nutritional concerns with some patients, there are many who are still overweight or obese, which is a known risk factor for kidney disease progression and complications. Thankfully, obesity is a potentially modifiable risk factor, so even though these children have lots of other issues they’re dealing with, it’s something we can possibly impact to improve their outcomes.”

Waist circumference and BMI were also examined for associations with markers of renal, metabolic and cardiovascular health — quantified using lipid measures, insulin resistance, blood pressure, left ventricular mass index, proteinuria and estimated glomerular filtration rate (eGFR). BMI had a significantly stronger association with eGFR than waist circumference, although both measurements had similar associations with the health indicators overall and did not statistically differ on any other measure. Waist circumference did trend toward a stronger association with proteinuria than BMI, however.

“Based on what we know at this point, physicians should probably just stick with BMI as the primary clinical measurement of obesity, since it is less subject to variability in technique,” says Dr. Patel, who is also the medical director of the Kidney Transplant Program at Nationwide Children’s.

Despite overall similarities in the two measures, however, Dr. Patel and his CKiD colleagues say that the proper nuance for interpreting the relationship among central abdominal fat, BMI and health — including potentially influential patient-level factors such as duration of illness, wasting, lean body mass and use of corticosteroids or blood pressure medications — wasn’t fully accessible in the cross-sectional study. The clinician-researchers intend to use the cohort data in the future to compare the two obesity measures using longitudinal evaluation.

“There are still questions we have about waist circumference in kids with CKD,” says Dr. Patel. “We hope to find out whether it matters that 10 percent of children are classified differently by the two measures, as well as whether one measure outperforms the other in terms of predicting the adverse effects of obesity, which is one of our primary concerns.”

Reference:

Patel HP, Saland JM, Ng DK, Jiang S, Warady BA, Furth SL, Flynn JT. Waist Circumference and Body Mass Index in Children with Chronic Kidney Disease and Metabolic, Cardiovascular, and Renal Outcomes. The Journal of Pediatrics. 2017 Dec; 191:133-139.