Male and female fat distribution and fat cell reactivity differs. Male-type centrally stored fat cells (around the abdomen) are known to be different in size and function to gluteal tissue, being more responsive to lipolytic and less responsive to lipogenic stimuli. Males generally use more energy for a set amount of exercise because of the larger proportion of lean body mass, or muscle, for a given body mass. Gluteal fat stores in females are widely regarded as having a biological function as an energy reserve for reproduction and milk production during lactation, and these are relatively resistant to fat loss even with high energy output or low energy intake.
Women experience greater hormonal swings throughout life (menarche, menstrual cycle, pregnancy and menopause) and these can impact significantly on body fat levels. The hormone levels in men by contrast, change only gradually with time as testosterone levels gradually decline with age. Women who don’t gain excessive weight during pregnancy, and who remain active both during pregnancy and after parturition, don’t seem to be at risk for extra fat gain—at least for the first 1-2 pregnancies. Women who are overfat or obese and who gain excessive weight during pregnancy are more likely to have problems after delivery. Breast-feeding mothers (over 6 months of breast-feeding) have been shown to stay leaner for longer than non-breast feeding mothers. In general also, the more babies a woman has, the greater her chances of becoming overfat or obese. It’s not known, however, whether this is due to physiological factors or the reduced ability to exercise and increased opportunity for over-eating, or whether it’s simply an association with other socioeconomic factors. Large families and obesity are more common in lower socioeconomic groups and the social status may be the determining factor for both.
Body composition differences between the genders also favour males in terms of energy expenditure. Men, in general, have a higher lean body mass to total mass ratio, with 12-24 per cent of body composition in the form of adipose tissue. Females on the other hand carry 15-30 per cent of their body mass in the form of fat. Higher lean body mass, even given constant weight, ensures a higher resting metabolic rate and hence men tend to have a greater energy use at rest. During exercise it has been shown that a man utilises up to 40 per cent more energy walking a set distance than a similar sized female.
Finally, there are psychological and sociological differences between the sexes which, although potentially changeable, can have a profound impact on body fat levels. The idealised female shape, for example, is now one of thinness. This has changed throughout history, there being good evidence (in art and literature) that a more ample female form has been preferred in the past. When fatness indicated wealth, a well-fed female body was desired. In the 20th century in Western countries, where energy-rich food is generally plentiful, fatness is easy and leanness is more associated with wealth, status and influence. Anorexia nervosa is the pathological extreme of this obsession with a thin figure.
Dietitian Jenny O’Dea from Sydney University has shown that females generally idealise a much thinner body size for themselves than is preferred by most males. The social pressures to conform to an ideal shape is greater in females (even if it is largely females who apply that pressure). A pot belly in a male doesn’t attract the same social pressures as a similar degree of fatness in a female, even though the male’s fatness is more dangerous to health.
Sex differences in body fatness have very real implications for fat loss planning. It is totally unrealistic to expect similar results from a male and female placed on the same type of fat loss program, although this is often expected by partners who may undertake the same kind of program. To the extreme frustration (and often guilt and depression) of the female partner, it is usually the male who is able to lose fat more easily and faster, and to keep this off longer, than the female. More support is often needed for females to ensure adequate eating patterns to encourage satisfaction with a body shape which is less than ideal and to prevent discouragement with slow fat losses.
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