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BMC Med Res Methodol. 2012; 12: 38.
Published online Mar 29, 2012. doi:  10.1186/1471-2288-12-38
PMCID: PMC3375197
Childhood body mass index trajectories: modeling, characterizing, pairwise correlations and socio-demographic predictors of trajectory characteristics
Xiaozhong Wen,corresponding author1 Ken Kleinman,1 Matthew W Gillman,1 Sheryl L Rifas-Shiman,1 and Elsie M Taveras1
1Obesity Prevention Program, Department of Population Medicine, Harvard Medical School and Harvard Pilgrim Health Care Institute, Boston, MA 02215, USA
corresponding authorCorresponding author.
Xiaozhong Wen: xiaozhongwen/at/hotmail.com; Ken Kleinman: ken_kleinman/at/hms.harvard.edu; Matthew W Gillman: matthew_gillman/at/harvardpilgrim.org; Sheryl L Rifas-Shiman: sheryl_rifas/at/harvardpilgrim.org; Elsie M Taveras: elsie_taveras/at/harvardpilgrim.org
Received August 3, 2011; Accepted March 29, 2012.
Abstract
Background
Modeling childhood body mass index (BMI) trajectories, versus estimating change in BMI between specific ages, may improve prediction of later body-size-related outcomes. Prior studies of BMI trajectories are limited by restricted age periods and insufficient use of trajectory information.
Methods
Among 3,289 children seen at 81,550 pediatric well-child visits from infancy to 18 years between 1980 and 2008, we fit individual BMI trajectories using mixed effect models with fractional polynomial functions. From each child's fitted trajectory, we estimated age and BMI at infancy peak and adiposity rebound, and velocity and area under curve between 1 week, infancy peak, adiposity rebound, and 18 years.
Results
Among boys, mean (SD) ages at infancy BMI peak and adiposity rebound were 7.2 (0.9) and 49.2 (11.9) months, respectively. Among girls, mean (SD) ages at infancy BMI peak and adiposity rebound were 7.4 (1.1) and 46.8 (11.0) months, respectively. Ages at infancy peak and adiposity rebound were weakly inversely correlated (r = -0.09). BMI at infancy peak and adiposity rebound were positively correlated (r = 0.76). Blacks had earlier adiposity rebound and greater velocity from adiposity rebound to 18 years of age than whites. Higher birth weight z-score predicted earlier adiposity rebound and higher BMI at infancy peak and adiposity rebound. BMI trajectories did not differ by birth year or type of health insurance, after adjusting for other socio-demographics and birth weight z-score.
Conclusions
Childhood BMI trajectory characteristics are informative in describing childhood body mass changes and can be estimated conveniently. Future research should evaluate associations of these novel BMI trajectory characteristics with adult outcomes.
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