Abstract
This study aimed to identify patterns of change in muscle-building behaviors from adolescence to emerging adulthood and determine what adolescent factors predict new-onset muscle-building behavior in emerging adulthood. Prospective cohort data from a diverse sample of 1,535 participants followed from adolescence (baseline, M (age) = 14.4 ± 2.0 years) to emerging adulthood (follow-up, M (age) = 22.1 ± 2.0 years) from the population-based EAT 2010-2018 (Eating and Activity over Time) study were analyzed. Changes in muscle-building behavior were identified (starting, stopping, persistent use, or never use). Log-binomial regression models examined adolescent predictors of starting (i.e., new-onset) muscle-building behaviors in emerging adulthood. Prevalence of any use in adolescence (EAT 2010) and/or emerging adulthood (EAT 2018) was 55.1% (males) and 33.0% (females) for protein powder/shakes, 6.7% (males) and 5.4% (females) for steroids, and 19.4% (males) and 6.5% (females) for other muscle-building substances (e.g., creatine, amino acids). In particular, 22.6% (males) and 13.7% (females) started protein powder/shakes, 2.2% (males) and 1.0% (females) started steroid use, and 9.0% (males) and 2.0% (females) started other muscle-building substances during emerging adulthood. Adolescent protein powder/shake consumption was associated with starting steroids/other muscle-building substances use in emerging adulthood in males (adjusted risk ratio [ARR] 2.09, 95% confidence interval [CI] 1.29-3.39) and females (ARR 4.81, 95% CI 2.01-11.48). Adolescent use of protein powders/shakes may lead to a two- to five-fold higher risk of new use of steroids and other muscle-building products in emerging adulthood. Clinicians, parents, and coaches should assess for use of muscle-building behaviors in adolescents and emerging adults and discourage use of harmful products.