Muscle Dissatisfaction and Muscle-Enhancing Substance Use: A Population-Based Twin Study in Young Adult Men

AbstractIn the population-based FinnTwin16study, proportions of genetic and environmental factors contributing to muscle dissatisfaction and muscle-enhancing substance use were assessed in 319 pairs of twin brothers: 141 monozygotic (MZ) and 178 dizygotic (DZ) pairs. In addition there were 86 twin individuals from pairs in which only one co-twin responded. Of all respondents, 30% experienced high muscle dissatisfaction. The corresponding proportion of muscle-enhancing substance use was 10%. The subjects were similar in age (23.8 years, 95% confidence interval [CI] 23.76?23.84), body mass index (23.7, 95% CI 23.5?23.9), and waist circumference (84.5 cm, 95% CI 83.7?85.2), independent of their muscle dissatisfaction or muscle-enhancing substance use status and independent of their zygosity. The MZ polychoric correlation for muscle dissatisfaction was .39 (95% CI .17?.58) and .27 for DZ pairs (95% CI .07?.46). The MZ tetrachoric correlation for muscle-enhancing substance use was .65 (95% CI .28?.87) and .56 for DZ pairs (95% CI .26?.78). The AE model, where additive genetic factors (A) accounted for 42% (95% CI .23?.59) and unique environmental factors (E) 58% (95% CI .41?.77) of the liability, provided the best fit for muscle dissatisfaction. The CE model, where common environmental factors (C) accounted for 60% (95% CI .37?.77) and unique environmental factors (E) 40% (95% CI .23?.63) of the liability, provided the best fit for muscle-enhancing substance use. Both genetic and unique (nonfamilial) environmental factors are involved in muscle dissatisfaction in the population. Nongenetic factors (both familial and non-familial) appear to best explain the use of muscle-enhancing substances.