BMI trajectories after primary school-based lifestyle intervention: Unravelling an uncertain future. A mixed methods study

Abstract

This mixed methods study aimed to examine plausible body mass index (BMI) trajectories after exposure to a primary school-based lifestyle intervention to aid in estimating the long-term intervention benefits. BMI trajectories for children at control schools (mean 7.6 years of age) were modelled until 20 years of age through extrapolating trial evidence (N = 1647). A reference scenario assumed that the observed 2-year effects of the 'Healthy Primary Schools of the Future' (HPSF) and 'Physical Activity Schools' (PAS) were fully maintained over time. This was modelled by applying the observed 2-year BMI effects until 20 years of age. Expert opinions on likely trends in effect maintenance after the 2-year intervention period were elicited qualitatively and quantitatively, and were used for developing alternative scenarios. Expert elicitation revealed three scenarios: (a) a constant exposure-effect and an uncontrolled environment with effect decay scenario, (b) a household multiplier and an uncontrolled environment with effect decay scenario, and (c) a household multiplier and maintainer scenario. The relative effect of HPSF at 20 years of age was -0.21 kg/m2 under the reference scenario, and varied from -0.04 kg/m2 (a) to -0.06 kg/m2 (b), and -0.50 kg/m2 (c). For PAS, the relative effect was -0.17 kg/m2 under the reference scenario, and varied from -0.04 kg/m2 (a, b), to -0.21 kg/m2 (c). The mixed methods approach proved to be useful in modelling plausible BMI trajectories and specifying uncertainty on effect maintenance. Further observations until adulthood could reduce the uncertainty around future benefits. This trial was retrospectively registered at Clinicaltrials.gov (NCT02800616).

Keywords: BMI, body mass index; Body mass index; Child; HPSF, the Healthy Primary School of the Future; Health promotion/economics*; PAS, the physical activity school; SES, socioeconomic status; Trajectory.

Conflict of interest statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

© 2021 The Authors.

Figures

Fig. 1

Notes: BMI = body mass index. Solid black line: observed median BMI values. Solid green line: predicted median BMI values based on Model 1 (linear mixed model, see Box 1) Solid red line: predicted median BMI values based on Model 2 (piecewise mixed model, see Box 1) Dashed lines: median – interquartile range; median + interquartile range. * Where separate lines are not visible, the values of Model 1 and Model 2 do overlap. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 2

Title: Projected median BMI values for children at control schools at age 12–20 years [average SES]. Panel 1: Boys – model 2 (piecewise mixed model, see Box 1). Panel 2: Girls – model 1 (linear mixed model, see Box 1). Notes: BMI = body mass index, FDGS = Fifth Dutch Growth Study. Solid red line: median projected BMI values. Dashed red lines: median – interquartile range; median + interquartile range. Dashed blue line: median BMI values as observed in the Fifth Dutch Growth Study (Schönbeck et al. (2011)). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 3

Title: Combination of uncertainty scenarios and corresponding BMI-effects [95% confidence interval]. Notes: HPSF = Healthy Primary School of the Future, PAS = Physical Activity School * statistically significant effect (p ≤ 0.05). The BMI-effects during the primary school period in the reference scenario are based on Bartelink et al. (2019) (Bartelink et al., 2019a).