Current BMI Metrics Miss Obesity in Disabled Teenagers
June 19, 2015Metrics for assessing body fat percentage in youths with physical disabilities include body mass index (BMI) and an assortment of skin-fold measurements. All of these measures correlate well with dual-energy X-ray absorptiometry (DXA) for assessing percentage of body fat, but current cut points need to be refined for teenagers with disabilities, according to a small exploratory study.
Brooks C. Wingo, PhD, from the Department of Occupational Therapy, University of Alabama at Birmingham, and colleagues’ evaluation of surrogate measures was published online June 5 in the American Journal of Physical Medicine & Rehabilitation.
“In adolescents with some form of paralysis or paresis, there are significant alterations in body composition. Therefore, the cut point of BMI used to identify obesity in the general population may not be appropriate for patients with physical disability who have proportions of lean, fat, and bone tissue that vary considerably from the norm,” the authors write.
According to the authors, youths with physical disabilities have obesity rates that are 38% higher than those of youths who are not disabled, which could lead to costly health complications later in life. But there are few simple, cheap, and accurate measures of excess body fat and weight among youth with physical disabilities. Although techniques such as weighing people underwater provide accurate measures of body composition, that and other direct measurement strategies are prohibitively expensive and complicated, the researchers note. In addition, BMI does not accurately gauge obesity in youths with disabilities, who can register BMIs in the normal range despite having high body fat levels.
To identify alternate, accurate measures, Dr Wingo and colleagues recruited 29 adolescents aged 14 to 17 years with spinal cord injury, cerebral palsy, or spina bifida from Shriners Hospital for Children in Chicago, Illinois. They measured the patients’ height on a flat surface while straightening their legs and used an electronic wheelchair scale to record weight. They measured BMI; waist, arm, and leg circumferences; triceps skinfolds; and total and regional fat through DXA. Mean BMI among the group (22.43 kg/m2) was on the low end of the range reported by the National Health and Nutrition Examination Survey (NHANES), but the mean body fat percentage (33.43%), waist circumference, and triceps skinfolds were at the high end or above the range of means reported by NHANES.
“Results indicated that all clinical measures were significant predictors of body fat percentage, with BMI and waist circumference displaying the strongest associations with DXA. This indicates that these clinical measures are not inappropriate for estimating adiposity-related risks in children with mobility limitations, but that disability-specific cut points are needed to accurately classify risk of obesity in this group,” Dr Wingo and colleagues write.
They identified waist circumference at 83 cm for boys and at 78 cm for girls as cut points that maximize sensitivity and specificity for youths with physical disabilities. BMI cut points of 20 kg/m2 for boys and 19 kg/m2 for girls identified 58.8% of the boys and 58.3% of the girls in the sample as obese, a closer fit to the DXA identification than current BMI cut points.
“This study identified cut points of BMI, as well as four other measures that may be of clinical value,” the authors conclude. “More research is needed to further validate these findings and refine these cut points for the development of disability-specific obesity classifications, which will allow clinicians to better identify children at risk of adiposity-related diseases and offer parents preventive strategies to improve the health and quality-of-life of their children.”
Financial support for the study was provided by the Department of Education. The authors have disclosed no relevant financial relationships.
Am J Phys Med Rehabil. Published online June 5, 2015. Abstract