Is the effect of testosterone on body composition modulated by the androgen receptor gene CAG repeat polymorphism in elderly men?

OBJECTIVE: The androgen receptor (AR) gene contains a CAG repeat polymorphism coding for a polyglutamine chain, the length of which is inversely correlated with AR transcriptional activity. We explored whether this polymorphism modulates the activities of testosterone (T) related to body composition in elderly men.
DESIGN: We performed cross-sectional analyses using data from a 4-year follow-up study in community-dwelling men aged 75-89 years (n=159).
METHODS: Body composition was assessed by dual-energy X-ray absorptiometry and its relation with T and the AR gene CAG repeat length was assessed by multiple linear regression analyses, adjusting for confounding and exploring effect modification.
RESULTS: AR gene CAG repeat length was not directly related to body composition, either with or without adjustment for confounding variables like age, weight, total T or sex hormone binding globulin (SHBG) levels. However, exploration of effect modification showed that CAG repeat length modulated the relation between T and body composition (standardized regression coefficients of interaction term: beta=0.12, P<0.01 and beta=-0.09, P<0.05 for fat-free mass and fat mass respectively). These results were confirmed using similar models and data of mean T, SHBG and weight of the 2 years' preceding body composition assessment instead of data of the same year (beta=0.09, P<0.05 and beta=-0.09, P<0.05 respectively).
CONCLUSION: These findings suggest that the AR gene CAG polymorphism contributes, albeit modestly, to the between-subject variation of T action on body composition in community-dwelling elderly men.