Having children plays a complicated role in the rate we age

For thousands of years we have tried to understand why we age. The ancient Greek philosopher Aristotle suggested that this occurs simultaneously with gradual aging. drying of internal moisture necessary for life.

In modern times, a leading idea known as the disposable soma hypothesis suggests that aging this is the price we pay for reproduction, and evolution the priority of gene transmission comes first. This creates a fundamental trade-off: the enormous amount of energy expended in bearing and raising offspring is spent on repairing DNA, fighting disease, and keeping organs in good shape.

This may especially apply to women who invest more in reproduction than in men during pregnancy and lactation. However, when scientists tested this hypothesis by testing whether women with more children lived shorter lives, the results were mixed: some studies supported the idea, while others found no effect.

“It's very difficult to make sense of something that is just a correlation. [between having more children and a shorter life] and what is the root cause, unless you have a good, large data set spanning multiple generations,” says Elizabeth Bolund from the Swedish University of Agricultural Sciences, who was not involved in the study.

Yuan Yang from the University of Groningen in the Netherlands and colleagues suggested that the discrepancy between studies exists because the cost of reproduction is not fixed – it depends on the mother's environment. “In good times, this trade-off is not really noticeable. The trade-off only becomes apparent in difficult times,” says Young.

To explore this idea, the researchers analyzed parish records of more than 4,500 Finnish women over 250 years. These include the Great Finnish Famine from 1866 to 1868, which provides an opportunity to assess how hard times affect reproduction and life expectancy, Young says.

They found that among women who lived before or after the famine, or who did not have children during it, there was no significant association between the number of children they had and their life expectancy. However, women who had children during the famine saw their life expectancy reduced by six months per child.

The study is based on research published last year, which used a dataset of the pre-industrial population of Quebec, Candé, monitored over two centuries, showed this trade-off in mothers who were likely to live in poor conditions. health or under severe stress, but has not examined how specific environmental conditions influence this.

Instead, Young's team points to a specific catastrophic event as the driving force that exposes the tradeoff for mothers. “This very large data set allows us to take into account confounding factors. [such as genetics and lifestyle factors]”,” says Bolund. “This study is the closest we can get to identifying a cause-and-effect relationship without doing a controlled experiment in the laboratory.”

The study also confirms the energy demands of pregnancy and breastfeeding, which require hundreds of additional calories per day. When women are hungry, they can't get this energy from food, so their bodies pay for it by “reducing their basal metabolism.” [the minimum number of calories your body needs to function at a basic level] It also explains why previous studies sometimes found the trade-off only in lower socioeconomic groups, who in fact always lived in relatively resource-poor environments, he says.

The fact that this trade-off appears to occur in particularly difficult circumstances, where women tend to have many children, may partly explain why women today typically live longer than men, with girls born in the UK between 2021 and 2023, Bolund said. he is expected to live four years longer than their male counterparts.

Reproduction costs are now quite low in Western societies, where average the number of children women give birth to has dropped significantly for centuries, Bolund says. As a result, few women today are likely to reach the threshold where the cost to their lives becomes apparent. Bolund and her colleagues” research for example, in the historical population of Utah, found that it only appeared when women had more than five children—a significantly lower rate 1.6 births the average woman in the United States is expected to give birth. during her lifetime.

Therefore, other environmental factors may become more important in explaining the difference in life expectancy between men and women. Men tend to smoke more often than women, and also drink more alcoholwhich affect life expectancy, Bolund says. The current gap in life expectancy between men and women is likely a combination of the latter's lower reproductive costs compared to other periods in history and differences in lifestyle between the sexes.

Research also shows that sex chromosomal differences involved. “The sexes differ for a variety of reasons beyond reproductive costs, so we need to do more research into how different factors contribute to sex-specific aging,” Young says.