Adjustment of directly measured adipose tissue volume in infants

BACKGROUND: Direct measurement of adipose tissue (AT) using magnetic resonance imaging is increasingly used to characterise infant body composition. Optimal techniques for adjusting direct measures of infant AT remain to be determined. OBJECTIVES: To explore the relationships between body size and direct measures of total and regional AT, the relationship between AT depots representing the metabolic load of adiposity and to determine optimal methods of adjusting adiposity in early life. DESIGN: Analysis of regional AT volume (ATV) measured using magnetic resonance imaging in longitudinal and cross-sectional studies. SUBJECTS: Healthy term infants; 244 in the first month (1–31 days), 72 in early infancy (42–91 days). METHODS: The statistical validity of commonly used indices adjusting adiposity for body size was examined. Valid indices, defined as mathematical independence of the index from its denominator, to adjust ATV for body size and metabolic load of adiposity were determined using log-log regression analysis. RESULTS: Indices commonly used to adjust ATV are significantly correlated with body size. Most regional AT depots are optimally adjusted using the index ATV/(height)(3) in the first month and ATV/(height)(2) in early infancy. Using these indices, height accounts for<2% of the variation in the index for almost all AT depots. Internal abdominal (IA) ATV was optimally adjusted for subcutaneous abdominal (SCA) ATV by calculating IA/SCA(0.6). CONCLUSIONS: Statistically optimal indices for adjusting directly measured ATV for body size are ATV/height(3) in the neonatal period and ATV/height(2) in early infancy. The ratio IA/SCA ATV remains significantly correlated with SCA in both the neonatal period and early infancy; the index IA/SCA(0.6) is statistically optimal at both of these ages.