Out of the 53 living supercentenarians – people over 110 years old –
51 are women. That’s more than 95 percent! In fact, you might say that
no factor comes closer to predicting the likelihood of reaching such an
exceptional age than simply being female.
In most mammals the two sexes age differently, and even though we’ve
noticed this in our own species since the mid-18th century, researchers
still haven’t been able to understand exactly why. But we do know that
one of the most telltale signs of aging is the functional decline of
stem cells. So perhaps it’s time to look at how stem cell behavior
differs in men and women, Stanford’s Ben Dulken and Anne Brunet argue in a Cell Stem Cell paper
published this week. “As the search continues for ways to ameliorate
the aging process and maintain the regenerative capacity of stem cells,”
they write, “let us not forget one of the most effective aging
modifiers: sex.”
Adult stem cells are regulated in a sexually dimorphic manner (like
flashy peacock feathers, for example), and they respond to sex hormones.
By looking at the differences in regenerative decline, we might figure
out how hormones like estrogen and testosterone might modify lifespan.
In female mice, estrogen’s effects on stem cell populations is pretty
direct: It increases the number of blood stem cells, which helps during
pregnancy, and it enhances the regenerative capacity of brain stem cells
at the height of estrus (that is, when she’s in heat). So far, no one’s
looked into the impact these changes might have on lifespan. However,
recent work with mice reveals that estrogen supplements appear to
increase the lifespan of males, and a study of human eunuchs found that
these men lived on average about 14 years longer than non-castrated men.
Furthermore, knocking out various genes in mice has sometimes
resulted in longevity advantages for one sex but not the other. And a
twin study suggested that there might be a difference in telomere length
between males and females: these little caps at the ends of chromosomes
help to signify the lifespans of cells. Further research is needed in
order to understand how genetics might have an impact on stem cell aging
in males and females and at the very least, the duo concludes, future
research should emphasize the importance of controlling for sex in
studies in which age is a variable.
Sex likely plays a role in defining not only lifespan but also the
specific portion of a lifespan that’s healthy, or “healthspan”. The
effects of sex might not necessarily be the same for the two.