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Methodology and Mathematical Modeling

A New Total Body Potassium Method to Estimate Total Body Skeletal Muscle Mass in Children^1,2

³ Obesity Research Center, St. Luke's-Roosevelt Hospital, Columbia University College of Physicians and Surgeons, New York, NY 10025; ⁴ Pediatric Unit, Verona University Medical School, Verona, Italy 37134; ⁵ Mel and Enid Zuckerman Arizona College of Public Health, University of Arizona, Tucson, AZ 85724; ⁶ Faculty of Human Movement, Technical University of Lisbon, Lisbon, Portugal 1495–688; and ⁷ Merck & Co., Inc., Rahway, NJ 08889

^* To whom correspondence should be addressed. E-mail: zw28{at}columbia.edu.

A whole body skeletal muscle [(SM); kg] mass estimation model, based on total body potassium [(TBK); mmol] measured by whole body ⁴⁰K counting (WBC) was developed (SM = 0.0082·TBK) and validated in adults in a previous study. It is unknown whether the adult TBK SM prediction model is applicable for pediatric use. The aim of this study was to derive and validate a pediatric TBK SM prediction equation. SM measured by MRI was used as the criterion and TBK was measured by WBC. The protocol was completed in 116 healthy children, 66 males and 50 females, 11.7 ± 3.5 y (mean ± SD, range = 5–17 y). A strong linear correlation was observed between TBK and SM (r = 0.984; P < 0.001). The SM:TBK ratio was 0.0071 ± 0.0008 kg/mmol in the children studied, much lower than the corresponding value of 0.0082 kg/mmol in adults. An empirical SM prediction equation was developed using TBK alone: SM = 0.0085·TBK – 2.83, r² = 0.97, SEE = 1.39 kg. Bland-Altman analysis did not disclose a significant bias in the prediction of SM. When biological factors entered along with TBK in the general linear model, another prediction equation was developed: SM = 5.52 + 0.001·TBK (mmol) + 0.081·weight (kg) – 0.049·height (cm) + 0.00004·TBK · height + race (–0.60 for Caucasian, 0.49 for African-American, and 0 for Hispanic). Because the adult TBK SM prediction model is not applicable for pediatric use, this study provides new empirical TBK SM prediction equations that should prove useful for studies on nutrition, growth, and development in children.