WHEN women work hard physically, it is the toil itself, not a lack of energy,
that stops menstruation, say researchers in the US. They鈥檝e found that the
reproductive cycle can stop even if women keep up their energy levels with extra
food.
It鈥檚 long been known that heavy manual labour may cause progesterone levels
to drop, and ovulation and periods to stop. However, it was unclear whether this
was a response to the work or to the body鈥檚 resulting lack of energy.
To resolve this, Grazyna Jasienska and Peter Ellison of Harvard University
studied 20 women in a Polish agricultural community who work hard during the
summer months. Even though they compensated by eating more in the summer, their
progesterone levels still dropped. This suggests that the work itself suppresses
the reproductive cycle.
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The researchers offer two possible explanations for this. First, it鈥檚
unlikely that extra food was freely available during our evolutionary past, so
increased workloads would almost always produce a shortage of energy. 鈥淪witching
off鈥� reproductive systems would spare women the extra burden of pregnancy and
give a woman an evolutionary advantage. Alternatively, a high turnover of energy
expenditure and intake may somehow constrain a woman鈥檚 ability to allocate
energy to reproduction (Proceedings of the Royal Society B, vol 265, p
673).
Exercise is known to lower the risk of breast cancer, probably by lowering
sex hormone levels. The researchers suggest that this may also be why the
incidence of breast cancer is lower in the developing world. 鈥淚f we are right,
we can predict that as lifestyles change in the developing world and physical
workloads decrease, the risk of breast cancer will increase,鈥� says Ellison.
Alan McNeilly, a reproductive biologist at the University of Edinburgh, says
the study is thought provoking. The link between lifestyle and the rate of
reproductive cancer is tenuous, he says. 鈥淏ut it鈥檚 not unreasonable,鈥� he adds.