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 19th
century, 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 18th
century, 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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