ABSTRACT
Differences in body size and shape between males and females are referred to as “sexual dimorphism.” Physical differences in stature and shape are the rst indicators that allow us to distinguish between men and women, and we are biologically programmed to be acutely sensitive to these markers. In addition to size differences, at any given body mass index (BMI), women generally have higher levels of total body fat and lower levels of fat-free mass, and hence they weigh less and are shorter.1 So-called “essential” body fat, the lowest level of body fat compatible with health, is often said to be 4%–6% in men and 8%–12% in women. This has been empirically veried in men in the Minnesota Starvation Experiment2 and more recently by Friedl et al.,3 but to our knowledge, there have been no parallel experiments in women. These differences appear to be quasi-universal in human populations. In a study of 96 preindustrial populations, Wells4 found that women had a greater mean triceps skinfold thickness in all but one population group and a greater mean subscapular skinfold in all but two. Similar sexual dimorphism in body composition is found in many animal species, including mammals, snakes, and waterfowl. The evolutionary driver of these differences appears to be
sexual selection. Men with a higher fat-free mass and women with an adiposity-dened female body shape have higher reproductive success.5,6
In spite of having lower levels of body fat, health risk is greater for men than for premenopausal women at any given BMI because of the differential risk associated with different patterns of fat distribution. Vague7 initially compared android (apple-shaped) and gynoid (pear-shaped) patterns. Women have greater deposits of gluteo-femoral fat that is cardioprotective,8 less abdominal fat, and hence lower waisthip ratios. Extremity skinfolds (calf, front thigh, and triceps) are especially large in women relative to men and generally increase with overall levels of body fat (Figure 7.1). Central skinfolds (supraspinale, subscapular, and abdominal) are more similar in men and women and across weight-status categories. The ratio of extremity fat (e.g., triceps skinfold) to central fat (e.g., subscapular skinfold) is therefore higher in women.
