title: Estimating trajectories of energy intake through childhood and adolescence using linear-spline multilevel models.
abstract: BACKGROUND
  Methods for the assessment of changes in dietary intake across the life course are underdeveloped.
  
  
  METHODS
  We demonstrate the use of linear-spline multilevel models to summarize energy-intake trajectories through childhood and adolescence and their application as exposures, outcomes, or mediators. The Avon Longitudinal Study of Parents and Children assessed children's dietary intake several times between ages 3 and 13 years, using both food frequency questionnaires (FFQs) and 3-day food diaries. We estimated energy-intake trajectories for 12,032 children using linear-spline multilevel models. We then assessed the associations of these trajectories with maternal body mass index (BMI), and later offspring BMI, and also their role in mediating the relation between maternal and offspring BMIs.
  
  
  RESULTS
  Models estimated average and individual energy intake at 3 years, and linear changes in energy intake from age 3 to 7 years and from age 7 to 13 years. By including the exposure (in this example, maternal BMI) in the multilevel model, we were able to estimate the average energy-intake trajectories across levels of the exposure. When energy-intake trajectories are the exposure for a later outcome (in this case offspring BMI) or a mediator (between maternal and offspring BMI), results were similar, whether using a two-step process (exporting individual-level intercepts and slopes from multilevel models and using these in linear regression/path analysis), or a single-step process (multivariate multilevel models). Trajectories were similar when FFQs and food diaries were assessed either separately, or when combined into one model.
  
  
  CONCLUSIONS
  Linear-spline multilevel models provide useful summaries of trajectories of dietary intake that can be used as an exposure, outcome, or mediator.
authors.affiliation: MRC Centre for Causal Analyses in Translational Epidemiology, University of Bristol, Bristol, United Kingdom.
authors.forename: Emma L
authors.initials: EL
authors.lastname: Anderson
authors.forename: Kate
authors.initials: K
authors.lastname: Tilling
authors.forename: Abigail
authors.initials: A
authors.lastname: Fraser
authors.forename: Corrie
authors.initials: C
authors.lastname: Macdonald-Wallis
authors.forename: Pauline
authors.initials: P
authors.lastname: Emmett
authors.forename: Victoria
authors.initials: V
authors.lastname: Cribb
authors.forename: Kate
authors.initials: K
authors.lastname: Northstone
authors.forename: Debbie A
authors.initials: DA
authors.lastname: Lawlor
authors.forename: Laura D
authors.initials: LD
authors.lastname: Howe
mesh_terms: D000293:Adolescent; D017195:Adolescent Nutritional Physiological Phenomena; D015992:Body Mass Index; D002648:Child; D002664:Child Nutritional Physiological Phenomena; D002675:Child, Preschool; D004034:Diet Surveys; D002149:Energy Intake; D005260:Female; D006801:Humans; D016014:Linear Models; D008137:Longitudinal Studies; D008297:Male; D055361:Multilevel Analysis