SITAR-d: extending the SITAR growth curve model to allow for variability in post-pubertal velocity

Background:
The SITAR growth curve model with its three random effects is useful for summarising height growth in individuals and groups. However SITAR performs less well for weight which unlike height continues to change post-puberty.

Aim:
To describe SITAR-d, an extended version of SITAR which adds a fourth random effect called “post-growth” to the standard model, allowing post-pubertal weight velocity to vary between individuals.

Subjects and methods:
SITAR and SITAR-d models in height and log weight were fitted to data (n = 128,157) for 3172 boys aged 9–20 from Christ’s Hospital School between 1936 and 1969. Weight and height measured in later life were predicted from the values at age 18. Secular trends in the mean growth curves were explored.

Results:
SITAR-d fitted better than SITAR for both height and weight (residual SD 0.67 cm vs 0.74 cm for height and 2.38% vs 2.52% for weight). There was an appreciable secular trend to earlier puberty for both height and weight, but no appreciable trend in mean height or weight.

Conclusions:
SITAR-d is useful for extracting early adult weight gain from growth data, potentially an important risk factor for obesity-related morbidity. Height also fits better with SITAR-d, reflecting a minority of individuals with delayed development.