Serum IGF-I in middle age covaries with reproductive life-history traits in british men and women

Objectives: The growth hormone/insulin-like growth factor-I (GH/IGF-I) axis may be an important component of individual life-history variation. This study examined whether IGF-I levels covary with reproductive life-history traits in a long-term British cohort study (the National Child Development Study). Methods: Using data on up to 5,252 individuals (2,431 men and 2,821 women) born in March 1958, relationships of serum IGF-I at age 45 with pubertal age, time to pregnancy (TTP), age at first reproduction (AFR), and lifetime reproductive success (LRS) were analyzed using general linear models. Results: IGF-I showed modest non-linear associations with pubertal age, being low in women who first menstruated, and men whose voice broke, in the oldest respective age categories. In women, but not men, IGF-I was negatively related to TTP. IGF-I was positively associated with AFR in both sexes and tended to be negatively related to LRS in women only. Some, but not all, relationships were attenuated after controlling for multiple potential confounders. Conclusion: These results are consistent with the hypothesis that the GH/IGF-I axis partially underlies individual variance in reproductive outcomes, through either direct endocrine effects on reproductive function or behavior, indirect effects on other traits, or a combination of the above. Several limitations of this study include the late age at which blood samples were collected and the difficulty of disentangling social and biological factors contributing to the traits studied. Am. J. Hum. Biol., 2012. (C) 2012 Wiley Periodicals, Inc.