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Female fat and fertility

Female fat and fertility[1]

For most females, fat is a nuisance. Like a bad guest, it arrives apparently uninvited, lingers far too long and is hard to get rid of. There is a certain level of body fat that each woman would like and it is that level of fat that makes her feel and look good. Whereas today we view female fat in terms of style and fashion, in times gone by, female fat dominated the subject of fertility. Most of the ancient symbols of fertility, such as the 30,000BC Venus of Willendorf, were of seriously fat women.  The conclusion of a successful pregnancy requires about 50,000 calories over and above normal daily living and exclusive breast feeding, the only option in times gone, by requires about 500 to 1000 additional calories every day. Unless women have some fat reserves to take this task on, bearing in mind that food might become in short supply during pregnancy, then it is not wise to travel down this reproductive road. It isn’t as though women have to make a difficult decision themselves. Nature does it for them.

The first road to fertility begins with the onset of menstruation or menarche.  In the middle of the 19thcentury, the mean age for menarche was 17 years. A century later that had fallen to 14.5 years and in the US it now stands at 12.8 years[2]. These changes are due to improved nutrition resulting in young girls reaching the magic number at an earlier age. That magic number relates to body weight and the actual magic number is 46.7 kg (103 pounds). During their growing up period, children grow in a fairly linear manner (not withstanding the comments of aunts and uncle’s that “My, hasn’t she just shot up”). In today’s terms, at or around 9.5 years on average, a major growth spurt occurs in young girls. In addition to the laying down of the female reproductive tissues, there is a sudden spurt in the laying down of fat. About 2 years later, this dramatic growth spurt slows down (no one knows why) and about 6 to 8 months later, menstruation begins. Most of these data are based on detailed longitudinal studies of growth in US girls in Berkeley, Boston and Denver between 1940 and 1950. Some girls reach menarche earlier and some later, that being a feature of normal distribution. Irrespective of when they reach menarche, the magic figure of 103 pounds or 46.7 kg also applies. In fact this figure really translates into an average of 22% body fat and it is this fat content that determines whether or not menarche begins.

In today’s world with an obsession about body fat and fitness, many women fail to menstruate because their body fat falls below the critical level. That ancient safety valve that spared women with inadequate nutrient and energy reserves from becoming pregnant still kicks in. Now we know the biochemistry a bit better. Fat contains an enzyme aromatase, which converts a weak male androgen into estrogen, a key female reproductive hormone. Prior to menopause, one third of all estrogen in circulation in females is thus derived. With the onset of the menopause, this rises to 100%. The second key biochemical force to be reckoned with is the protein leptin, secreted from fat. Leptin suppresses appetite and thus the fatter we are, the more leptin we produce and thus the greater the suppression of appetite. We now know from human genetics studies, that a deficiency, relative or absolute of leptin in young girls, totally negates any sexual development. Leptin and related hormones secreted from the adipose tissue form the signal between fatness and fertility sending signals to that part of the brain most involved in sexual development.

The power of body fat levels to shape the female body can reach quite disturbing levels. Young girls, who show a precocious talent for ballet, are known to follow a restricted diet during their strict training to keep their weight as low as possible. Normally, in such young girls, their bone growth occurs at the tips of the long bones where bone is soft and where few blood vessels penetrate this part of the bone. When young girls reach the age of menarche, their bone marrow turns from red bone marrow (making red cells) to a fatty bone marrow and this fat uses the aromatase enzyme to produce estrogen which cause the soft end of the bones to engage with a blood supply, thus ending bone growth. Among female ballet dancers in training, failure to reach the menarche due to excessively low body fat levels, allows the long bones to grow for longer which is why many great ballerinas have spider-like limbs, far longer than would be predicted by their height.

Fat is uniquely important in female biology. And yet, for the most part it remains a very poorly understood tissue. Perhaps a better understanding of the fat and fertility link might rehabilitate fat from simply embarrassing flab to a banner of femininity.

[1]For this blog I draw heavily on a book entitled “Female fertility and the body fat connection” (Chicago University Press) by Professor Rose Frisch, formerly of the Harvard School of Public Health who was a pioneer in this area

[2] In the early 18thcentury, the mean age of males undergoing voice change was 18 years in the choir of John Sebastian Bach. Today, it too has fallen to 13.5 years


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