Women who have multiple children may age faster than others, new research suggests. Someone who has delivered three babies probably won’t look any older than a mother-of-one the same age, but the stress of repeated pregnancies may be damaging her cells in invisible ways.
Pregnancy and supporting the life of a developing fetus requires a lot of energy from a woman’s body and evolutionary biologists think that the energy devoted to this may sap energy away processes that help preserve lifespan.
Scientists at Northwestern University found that young women who had had multiple babies showed two different signs of accelerated aging.
‘One of the biggest mysteries for evolutionary biologists is explaining why we age,’ study co-author Dr Christopher Kuzawa told Daily Mail Online.
‘And one assumption is that we have finite energy available to allocate across functions, so one or the the other can get short-changed.’
In other words, the body is constantly negotiating energy and deciding what its top priority is, whether that be sleeping, eating or reproduction.
‘The trade-off viewed as most important has been [between] reproduction and maintaining lifespan,’ says Dr Kuzawa explains.
Dr Kuzawa, his team and other evolutionary biologists think that the female body is pre-programmed to choose supporting fetal life over lifespan-lengthening activities like tissue and cell repair.
There’s historical data that supports this idea,’ says Dr Kuzawa.
‘Analyses of historical records of royalty and the aristocracy finds that lifespans get shorter the more kids they had.’
Of course, that’s just correlation, and it is harder to establish when scientists look at data on more diverse contemporary populations, among which there are also more confounding variables.
To try to cut down on complicating factors like wealth and access to health care, the Northwestern scientists limited their study to a group of more than 3,200 Phillipino women between the ages of 20 and 22.
These women are ‘operating under a bit more nutritional stress and are more stretched in terms of resources,’ so their bodies might have a greater need to ration energy, Dr Kuzawa says.
A handful of women had had as many as four or even five children already, but the largest group had no children.
This study is not a slam dunk showing that having kids leads to aging…we may also see that these differences wash out with time, said Dr Christopher Kuzawa, Northwestern University professor and study co-author.
The Northwestern then looked at two very different measures of biological age in these women. First they examined telomeres, the protective end caps of DNA strands.
We’ve long known that these are good measures of biological age and lifespan.
Throughout our lives, our bodies constantly make new cells through a process called replication. Each time every chromosome has to copy itself, and a little bit of telomere wears away with each iteration.
‘Telomeres get shorter with each replication and if they get too short, the cell can no longer replicate,’ Dr Kuzawa says.
Despite the fact that every woman in his study was in her early 20s, ‘even at that young age, we saw shorter telomeres among women that have had more live births,’ he adds.
His team used blood samples to analyze the telomere lengths. The cells they gather from blood are immune cells, so really they were quantifying the age of the women’s immune systems.
‘We have no idea if you would see the same thing if you would look at the brain or liver or other tissues in the body, which of course we can’t look at in living people,’ says Dr Kuzawa.
‘But to the extent that these telomeres are getting shorter [as women have more children], this might limit the ability of the immune cells to fight infection and cancer many decades down the road.’
They also looked at the epigenetic ages of these women.The epigenome is a catalog of chemical tags that get added onto DNA throughout our lives, and dictates which genes get switched on and off (or expressed).
That in turn dictates which cells become what kind of tissue, and how they function.
These chemical tags can be affected, damaged and changed throughout our lives, which changes the expression of our genes. These changes can even be passed on to offspring.
So epigenetic age actually tells scientists a great deal about our cellular and genetic health and the markers of one process in particular, methylation, ‘is actually a really good biological clock,’ Dr Kuzawa says.
‘If age is accelerated according to these markers, compared to chronological age, it’s an indicator of mortality risk.’
That was exactly what he and his team found in their study: the more children a young woman had had, the older she was in epigenetic years.
‘We appear to have similar stories in really different biology,’ Dr Kuzawa says.
This kind of aging is important to lifespan, but really has nothing to do with outward aging, so ‘you’re not going to see a difference in wrinkles or appearance,’ he adds.
The exact causes of these effects on telomeres and the epigenome aren’t clear yet, but could be related to oxidative stress from the immune system changes a woman has to go through to carry a a baby, and the stress of lactation and pregnancy itself.
But, he insists: ‘This study is not a slam dunk showing that having kids leads to aging…these are subtle differences consistent with this evolutionary hypothesis.’
‘We don’t know if these effects will persist. It’s possible they will but we may also see that these differences wash out with time,’ Dr Kuzawa says.