Best Practice & Research Clinical Endocrinology & Metabolism
6Sexual dimorphism of body composition
Section snippets
Dimorphism in size and physique
Differences in adult body size are best illustrated by stature, which averages around 7% greater in males but also varies significantly between populations.1, 2 This dimorphism emerges primarily during postnatal growth, with male neonates only 1% longer than females at birth.3 During childhood, size dimorphism remains relatively minor until the onset of puberty. Here the two sexes take increasingly divergent pathways, with girls passing through puberty earlier and ceasing to grow at a younger
Shape dimorphism
Simple indices of shape indicate that the regional distribution of body weight likewise differs considerably between the sexes, although the magnitude of such differences varies in relation to age. The close association between pubertal development and regional body composition accounts for the emergence during adolescence of significant shape dimorphism. By early adulthood, sex differences in body shape are maximal, with females tending towards an hourglass body shape whilst males tend towards
Dimorphism in total body composition
The simplest model of body composition divides body weight into two components: the fat and lean masses. Adjusting these components for size can be undertaken in different ways, many of which fail to achieve their purpose.9 They are now conventionally adjusted for height by dividing by height squared, producing two indices: the lean mass index (lean mass/height2) and the fat mass index (fat mass/height2).10 These indices have the advantage that they provide discrete indices for the two
Lean
The increased lean mass of males can be attributed disproportionately to muscle mass. Based on magnetic resonance imaging (MRI) measurements of healthy adults, the ratio of muscle mass to total lean mass was 0.53 in males and 0.47 in females42, with this sex difference declining with age. However, this dimorphism is strongly influenced by age. Dissection studies demonstrate that around 40% of total musculature is located in the head and trunk at birth, but only 25–30% by adulthood.*30, 43 In
Hormonal contributions
The major contribution of pubertal development to the aetiology of sexual dimorphism in body composition can be attributed to the effects of sex hormones acting during this period. The endocrine event termed adrenarche represents an increase in the secretion of adrenal androgen hormones which produce an acceleration in growth in height, a transient acceleration of bone maturation, and the development of secondary sexual characteristics and fat distribution.4 Sex steroid hormones likewise play
General health implications
Sex differences in body composition have various implications for health and the risk of disease. In many cases both sexes illustrate consistent relationships between body composition phenotype and health outcome. Abdominal fat is generally considered the most harmful adipose tissue depot. Recent studies have attributed high metabolic activity to visceral fat in particular, suggesting that such activity is associated with cardiovascular damage and perturbations in blood sugar control.
Sex chromosome defects and other disorders of sex development
Individuals with sex chromosome defects or other disorders of sex development show significant abnormalities of body composition. These are due in part to abnormalities in sex hormone profile, but may also be influenced by other consequences of chromosomal abnormality.
Individuals with Klinefelter syndrome have an extra X chromosome but live as men. The extra X chromosome is associated with reduced testosterone levels during puberty, resulting in low levels of lean mass and bone density, and a
Evolutionary perspective
Sexual dimorphism in any trait poses the question of what selective pressures might account for the differences. In the case of body size, for example, we might ask whether selection has favoured bigger males or smaller females. In general there is very little difference between males and females in any given species in birth weight.83 Typically, dimorphism emerges just before sexual maturity, with the larger sex demonstrating either a more rapid growth, or an extension of the growth phase
Conclusions
Humans demonstrate significant sexual dimorphism in body composition in relation to lean, fat and bone masses, a scenario that appears relatively unusual in the animal kingdom. The sex differences can be attributed to profound differences in reproductive strategy, which in turn stem from the disproportionate contribution of females to the nutritional requirements of the large brain in human offspring. In contemporary populations, these sex differences have implications for the relationship
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