Validity of body mass index as a measurement of adiposity in infancy

Authors: BELL, KATHERINE - Brigham & Women'S Hospital; WAGNER, CAROL - University Of South Carolina; PERNGW, WEI - University Of Michigan; FELDMAN, HENRY - Boston Children'S Hospital; SHYPAILO, ROMAN - Children'S Nutrition Research Center (CNRC); BELFORT, MANDY - Brigham & Women'S Hospital

Submitted to: Journal of Pediatrics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/10/2018
Publication Date: 3/15/2018
Citation: Bell, K.A., Wagner, C.L., Perngw, W., Feldman, H.A., Shypailo, R.J., Belfort, M.B. 2018. Validity of body mass index as a measurement of adiposity in infancy. Journal of Pediatrics. http://doi.org/10.1016/j.peds.2018.01.028.

Interpretive Summary: Human body size and proportion are widely used as proxy measures for body composition. For example, body mass index (BMI, kg/m2) is often used to represent body fatness. In infants, high levels of BMI and BMI z-score (BMIZ), and rapid increases in BMI are all associated with obesity in early childhood or adulthood. However, the extent to which these measurements truly represent body fatness is not well understood. Our aim was to see how well BMI and BMIZ performed as indicators of body fat levels in infants. Body fat levels are characterized as body fat percentage, body fat mass, and fat mass index (kg/m2). We studied data collected during a randomized trial of maternal vitamin D supplementation. 447 full-term infants were measured at 1, 4, and 7 months of age. Infant body fat and body fat percentage were measured using DXA. Infant weight and length were also measured. We looked at associations among BMI, BMIZ, and body fat levels at each time point. We also looked at associations between changes in BMIZ and changes in body fat levels between time points. BMIZ was moderately associated with body fat percentage, and more strongly associated with fat mass and fat mass index. Changes in BMIZ were moderately associated with body fat changes from 1 to 4 months of age, but poorly associated with the changes from 4 to 7 months. BMIZ change is not a good indicator of body fat change in infants, especially later in infancy. BMI and BMIZ are limited as proxy measures of body fatness in infants.

Technical Abstract: To assess the validity of body mass index (BMI) and age- and sex-standardized BMI z-score (BMIZ) as surrogates for adiposity (body fat percentage [BF%], fat mass, and fat mass index [kg/m2]) at 3 time points in infancy (1, 4, and 7 months) and to assess the extent to which the change in BMIZ represents change in adiposity. We performed a secondary analysis of 447 full-term infants in a previous trial of maternal vitamin D supplementation during lactation. Study staff measured infant anthropometrics and assessed body composition with dual-energy x-ray absorptiometry at 1, 4, and 7 months of age. We calculated Spearman correlations (rs) among BMI, BMIZ, and adiposity at each time point, and between change in BMIZ and change in adiposity between time points. Infants (N = 447) were 52% male, 38% white, 31% black, and 29% Hispanic. The BMIZ was moderately correlated with BF% (rs = 0.43, 0.55, 0.48 at 1, 4, and 7 months of age, respectively). BMIZ correlated more strongly with fat mass and fat mass index, particularly at 4 and 7 months of age (fat mass rs = 0.72-0.76; fat mass index rs = 0.75-0.79). Changes in BMIZ were moderately correlated with adiposity changes from 1 to 4 months of age (rs = 0.44 with BF% change; rs = 0.53 with fat mass change), but only weakly correlated from 4 to 7 months of age (rs = 0.21 with BF% change; rs = 0.27 with fat mass change). BMIZ is moderately correlated with adiposity in infancy. Changes in BMIZ are a poor indicator of adiposity changes in later infancy. BMI and BMIZ are limited as surrogates for adiposity and especially adiposity changes in infancy.