Longitudinal Relationship between Cardiorespiratory Fitness and Academic Achievement

1,286 students in grades 5-7, aged 11-14 at baseline, from 14 Portuguese schools, followed 3 years

Portuguese schoolchildren aged 11-14 at baseline

Three-year prospective cohort. Cardiorespiratory fitness measured by the FITNESSGRAM PACER shuttle test at baseline and follow-up; students classified fit-fit, unfit-fit, fit-unfit, unfit-unfit. Outcome is teacher-assigned school marks in Portuguese, mathematics, English and science. Ordinal regression. Graded D: the outcome is teacher marks rather than an independent standardized test, the exposure is self-selected, and there is no genetically informative or within-family component. Recorded here as the best-executed exemplar of the observational fitness-achievement literature, not as causal evidence.

The effect sizes here are enormous compared with anything a randomized trial produces, which is the point. Persistently fit students had 3.49 times the odds (95% CI 1.97-6.20) of high achievement in Portuguese and 2.41 (1.39-4.14) in English, versus persistently unfit students. Students who became fit had OR 2.52 and 2.13 on the same two subjects. Two things break the causal reading. First, the effects appear in mother-tongue and foreign language and NOT in mathematics or science — the reverse of the mathematics-specific pattern every intervention meta-analysis reports, so the two literatures do not even agree about which subject responds. Second, odds ratios of 2.4-3.5 are one to two orders of magnitude larger than the d=0.00-0.23 range randomized trials return for the same exposure. A causal effect does not shrink that far when you randomize; a confound does.

HIGH. Cardiorespiratory fitness is roughly 55-72% heritable with no detectable shared-environment component (Schutte et al. 2016), and school achievement is roughly 60% heritable by adolescence. Persistently fit and persistently unfit adolescents are not randomly assigned groups: they differ in genotype, body composition, family circumstance and conscientiousness. No within-family, twin or sibling-fixed-effects version of this analysis exists anywhere in this literature.