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Hormones, offspring sex ratios, and weekly training distances of male runners: implications for overtraining
  1. William H James
  1. The Galton Laboratory, University College London, Wolfson House, 4 Stephenson Way, London NW1 2HE, UK

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    Editor,—Mackelvie et al1 report that, within a group of male runners, free testosterone levels correlate negatively and highly significantly with the weekly training distance (p<0.005). Crawford et al2 reported that the offspring sex ratio (proportion male) of a sample of male runners varied significantly by their weekly training distance (table 1). Inspection of these data suggests that the regression of sex ratio on training mileage is U shaped (χ2 = 16.7, df 4, p<0.005).

    There is now abundant evidence that the sex ratio of offspring of mammalian (including human) parents is partially dependent on the hormone levels of both parents around the time of conception.3

    The relevant hormones almost certainly comprise testosterone, oestrogen, and the gonadotrophins, and probably progesterone. However, others—for example, growth hormone and thyroid hormones—may be implicated.

    These data on sex ratios need confirmation, but, if confirmed, they would suggest endocrine involvement in overtraining. While the right arm of the U of the regression does not imply pathology, it is certainly suggestive of a process that is associated with pathology. So I suggest that hormone assays should be performed on male runners, particularly those who run more than 50 miles a week. One may wonder whether their established testosterone deficit is accompanied by an even more extreme gonadotrophin deficit.

    Table 1

    Numbers of sons and daughters sired by male runners, by their weekly training mileage around the time of conception (data of Crawford et al2)

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