Ken Ward, of Waikanae, asks :-

Why is it that as they age men’s bottoms shrink, while women’s expand?

Dave Grattan, an anatomist at Otago University's School of Medicine, responded.

The answer to this question is really in two parts: sex-differences in the distribution of fat, and age-related loss of muscle mass

Males and females are different in the distribution of fat in the body. Starting at puberty, the presence of female sex hormones promotes the female-specific pattern of fat deposition, for example in the breasts and buttocks, producing the more “rounded” form of women compared to men. As we age, most of us gain weight in the form of increased fat mass. Females tend to gain this weight in a female specific way (e.g. buttocks) while males typically gain more abdominal and visceral fat (pot belly). Hence, a woman’s “bottom” tends to expand with age.

At the same time, we all lose muscle mass as we get older. Particularly in very old individuals, this loss of muscle is what makes us frail and weak compared to when we were young. It is important to consider that the “bottom” is predominantly muscle. It is made up of several different muscles, the largest one known as the gluteus maximus. As there is proportionately less fat in men than women at this site, the muscle is the major component of their “bottom”. With decreasing muscle mass, the “bottom” shrinks in males as they age. Females are also losing muscle mass, but the proportionately higher amount of fat tissue at this site may mask this effect.