Comments on the critical metabolic mass and the age of menarche.

Data from Frisch and Revelle (1971) and Frisch et al. (1973) are used to show that some of their biological conclusions relating body composition to age at menarche follow directly from arithmetical operations on the two observed items of information: weight and height. This emphasizes the dangers of drawing biological conclusions from derived, as opposed to observed, data. The hypothesis that an age-unrelated critical body weight triggers off menarche is examined in relation to preliminary data from a longitudinal study of adolescent girls. It is unacceptable because: (a) mean weights at menarche showed an upward trend with increasing age; (b) for given body weights the proportion of girls starting to menstruate increased with age; (c) at all ages the variation of body weight at menarche was as large as that among non-menstruating girls: (d) only 41 per cent of girls started to menstruate at weights 48 +/- 5 kg; and (e) eleven of the 20 girls whose weight exceeded 48 kg at the age 10-5-11 years did not start to menstruate although 1-1-5 years had elapsed since they attained the "critical" weight. Although no direct body composition data are presented the behaviour of skinfold suggests that body composition is unlikely to be the primary trigger of menarche.