Relationship between body mass index, cigarette smoking, and plasma sex steroids in normal male twins.

Smoking has been observed to affect plasma sex hormones and body mass index. The relationship between smoking, body mass index, and plasma concentration of sex hormones was studied in normal adult male twins. The analyses were performed for between 150 and 159 twin pairs for whom hormonal data were available on both twins. With bivariate analysis, neither body mass index nor smoking affected estrone, luteinizing hormone, follicle-stimulating hormone, ratio of testosterone to estradiol, or ratio of testosterone to dihydrotestosterone. Body mass index significantly (P less than 0.05) affected sex hormone binding globulin, whereas smoking had no effect. The plasma contents of testosterone and dihydrotestosterone and the luteinizing hormone/testosterone ratio were affected by both body mass index and smoking, although, after allowing for body mass, smoking was less significant (0.05 less than P less than 0.10). A path model was formulated to examine the relationship of body mass and sex steroid levels. Our results suggest that body mass index affects sex steroids, since common environmental factors do not account for the strength of the relationship. The bivariate analysis suggests that the smoking effect on sex hormones (except perhaps for dihydrotestosterone) is secondary to an effect on body mass index.