Why do women live longer than men? That is the million-dollar question that no one has yet been able to answer.
But two researchers are looking more closely at this age-old riddle. Steven Austad, Ph.D., and Kathleen Fischer, Ph.D., of the University of Alabama at Birmingham explored this phenomenon in a perspective piece published in Cell Metabolism.
“Sex differences in longevity can provide insights into novel mechanisms of aging, yet they have been little studied,” the study wrote. “Humans are the only species in which one sex is known to have a ubiquitous survival advantage.”
While other species (roundworms, fruit flies) show lifespan differences among sexes, it shows longevity in the advantage of the other sex. But in humans it appears that it’s females all the time.
Austad said it’s still unknown why women live longer, but “it’s amazing that it hasn’t become a stronger focus of research in human biology.”
“Sometimes when things are so well known or universal (“everybody knows that”), they escape the interest of researchers,” Austad, distinguished professor & chair of the UAB Department of Biology, told R&D Magazine in an exclusive interview.
Data that points to a longer lifespan for women:
- The Human Mortality Database, which possesses entire lifespan tables for both sexes from 38 countries that goes back to 1751 for Sweden and 1816 for France. “Given this high-data quality, it is impressive that for all 38 countries for every year in the database, female life expectancy at birth exceeds male life expectancy,” per study.
- Gerontology Research Group data for the oldest of the old reveal that females make up 90 percent of the supercentenarians—those alive 110 years or more. Longer female survival expectancy is seen across the lifespan, at early life (birth to age 5) and at age 50.
- The birth cohorts from the mid-1800s to the early 1900s for Iceland—the small, homogenous country, which underwent disasters such as famine, flooding, volcanic eruptions and disease epidemics, shows a vivid example of the fairer sex’s survival, per study. Over that period, life expectancy at birth fell to as low as 21 years during catastrophes and rose to as high as 69 years during steady times. “Yes in every year, regardless of food availability or plagues. Women at the start of life and near its end survived better than men.”
- Resistance to most of the major causes of death. “Of the 15 top causes of death in the United States in 2013, women died at a lower age-adjusted rate of 13 of them, including all of the top six causes,” the researchers wrote.
Cell Metabolism reached out to Austad to contribute this perspective paper, “Sex differences in lifespan.”
The researcher first became interested in the topic when Georgetown University asked him to lecture on it in 2003. While lab models like the roundworm C. elegans, the fruit fly Drosophila melanogaster and the mouse Mus musculus are frequently used in scientific studies, people in those fields are not aware of how longevity patterns by sex can vary according to genetic backgrounds, differences in diet, housing or mating conditions, according to Austad.
These variables led to different results in longevity research. A survey of 118 studies of lab mice by Austad and colleagues in 2011 found that 65 studies reported that males outlived females, 51 showed that females outlived males, and two showed no sex difference.
But if variables are carefully monitored, mice might prove to be a useful model to study sex differences in the cellular and molecular physiology of aging, according to the paper. This research will be helpful in drug development for human use that affect aging, according to Austad.
These differences are sometimes attributed to hormones, even as early as the surge in testosterone during male sexual differentiation in the uterus. Longevity may also relate to immune-system differences, stress, mitochondrial fitness or even the fact that men have one X chromosome, while women have two X chromosomes.
But here’s a big twist in the female advantage. While women may survive longer, they, on average, appear in poorer health than men throughout adult life, according to the study. Women are more prone to joint and bone problems, such as osteoarthritis and back pain, than are men. Also back and joint pain tend to be more severe in women, which could lead to chronic sleep deprivation and stress. Therefore, the sex differences in morbidity could be due to connective-tissue maladies in women, and connective tissue in humans is known to respond to female sex hormones.
This is just one of several plausible hypotheses for the mystery surrounding why women live longer, on average, than men, but not for long, according to Austad.
“We will certainly figure this out. Once science turns its real focus onto something like this, the answers soon begin to appear,” he told R&D. “One of the most interesting research strategies might be to study species in which this isn’t true or in which males live longer than females to determine what key differences are seen between those species—and people.”