(July 2011) In the countries with the longest life expectancies, average life span has grown over the past two centuries at the "absolutely remarkable rate" of about 2.5 years per decade, three months per year, or six hours per day, according to demographer James Vaupel of Duke University and the Max Planck Institute for Demographic Research. Life expectancy in most industrialized countries has increased at this pace for the past 60 years, although in the United States there was a period of stagnation, especially for women, in the 1980s and 1990s, he noted.

While the future is uncertain, "it seems plausible that very long lives may be the probable destiny of younger people alive today," Vaupel said. "It is possible, if we continue to make progress in reducing mortality, that most children born since the year 2000 will live to see their 100th birthdays in the 22nd century."

Virtually all of the progress made in increasing life expectancy is the result of better medical care and rising living standards, he said. In the future, progress "will be fueled in part by interventions developed on the basis of deeper understanding of genetics and the root causes of aging in humans and other species."

Vaupel and other researchers in the growing field of biodemography are trying to unravel the often intertwined social and biological factors that contribute to longevity. With support from the U.S. National Institute on Aging, a group of U.S. and European scientists met last month at Duke's Population Research Institute to share findings from their ongoing work. The eventual goal is to identify interventions—public health policies, medical treatments, behavior advice—that prolong survival so more people lead longer, healthier lives.

Centenarians Shed Light on Longer Health Span

People in high-income countries are living in good health nearly 10 years longer than their parents did, not because aging has been slowed or reversed, but because people are reaching old age in better health, according to Vaupel. But when the aging process does begin, it is still taking place at about the same pace. "Deterioration, instead of being stretched out, is being postponed," he said.

As a group, centenarians—those who live to age 100—tend to be extreme examples of healthy aging. Researchers are studying populations in Japan, Denmark, and Hawaii that have unusually large shares of centenarians to understand what sets them apart.

Most of these " 'exceptional survivors' delay major clinical diseases and disability," which is evidence of an "extended health span," according to Dr. Bradley Willcox, a University of Hawaii gerontologist. He and colleagues have examined data on the physical and mental health of 8,000 Japanese American men in Hawaii tracked since 1965 and more than 1,000 centenarians in Okinawa, Japan, since 1975. Many centenarians in this group lived independently well into their 90s.

These scientists found a gene that if inherited from both parents triples a man's chances of reaching age 100. But they also identified a set of disease risk factors that if avoided in mid-life increased chances of healthy survival into old age. 

The study results suggest that men with fewer risk factors at age 50 are more likely to live to age 90 with no mental or physical impairment. Being overweight, high blood glucose, high triglyceride levels, high blood pressure, low "grip strength," having ever smoked, consuming more than three alcoholic drinks per day, not earning a high school diploma, and being unmarried increase the chances of impairment in old age.

Willcox is convinced that long life is influenced by good health habits including the low-calorie, nutrient dense diets of Okinawans and Japanese Americans, and what he has called "Mercedes-Benz genes vs. Ford Escort genes." Being born with certain genes may predispose a person to a longer life than people born with other genes, but Willcox argues that "if you treat that Ford Escort well, it could last longer than the Mercedes, so drive carefully."

To understand the distinct roles behavior and genes play in health at old age, Yi Zeng, a Duke and Peking University demographer, and colleagues compared the adult children of Chinese centenarians and their neighborhood peers. People who reach their 100th birthdays in developing countries are "extreme examples of healthy aging," he said. Compared to their Western counterparts, they are likely to be the "most robust, having survived severe living conditions and high mortality in the past."

The children of these centenarians had significantly better physical and mental health, and reported higher life satisfaction than their neighbors of similar age, gender, and socioeconomic status. But their findings suggest that life style and living conditions significantly interact with genes to contribute to health in old age, particularly for those without a genetic predisposition to long life.

"The effects of the environmental factors on health are much stronger among elders who have no family history of longevity compared to centenarians' children who likely carry genes that promote longevity," he explained. Some of those factors that interact with genes to affect health positively in later life included receiving adequate medical care in childhood, higher household income, and participating in social and leisure activities.

Danes, Prehistoric Skeletons, and Baboons May Help Explain Why Women Outlive Men Today

Compared with women, men are physically stronger, have fewer disabilities, and are much more likely to tell interviewers that they are in good health. On the other hand, men have higher mortality than women have at all adult ages (see Figure 1). This discrepancy between health and survival—known as the male-female health-survival paradox—has long puzzled demographers and other researchers.


Figure 1
Additional Years of Life Expectancy at Birth for Women Compared With Men in Selected Countries, 2010

 

Source: Carl Haub, World Population Data Sheet, 2010 (Washington, DC: Population Reference Bureau, 2010).


Both biology and behavior likely play a role, reported researchers from the University of Southern Denmark. Kaare Christensen and Anna Oksuzyan based their analysis on the extensive and detailed data available from health surveys that sampled the entire Danish population and incorporated data on nonrespondents. Their analysis shows that Danish men were not as physically disabled as women of the same age but had higher mortality. Fundamental biological differences between the sexes such as different hormones and disease patterns play a part in these outcomes, but Christensen and Oksuzyan's findings support the notion that men's shorter life expectancy is partially rooted in behavior. Men's greater risk-taking behavior and greater reluctance to seek medical treatment, to take part in health surveys, and to report disabilities and diseases may contribute to the survival gap, they suggested.

Svenja Weise and Jutta Gampe of the Max Planck Institute for Demographic Research pointed to interaction between biological and behavior to explain for survival differences between the sexes. They based their analysis on observations from 25,000 prehistoric human skeletons dating back as many as 11,000 years from sites in Scandinavia. Through most of history, women lived shorter lives than men and suffered from higher mortality rates, particularly during the reproductive years. This pattern continued until the late Middle Age when a shift to today's pattern of higher female survival occurred as populations became more urban and trade increased.

Weise pointed to a number of reasons for the shift. Later age at marriage led to fewer pregnancies, reducing women's risk of dying in childbirth. Access to food and health care within households became more equal. Men are more vulnerable to infections and parasites, and greater migration and trade increased the spread of disease, disadvantaging men. Finally, urban settings led to more competition for mates, causing increased male risk-taking behavior.

Males also tend to lead shorter lives than females among several primate species, reported Susan Alberts, a Duke University biologist. She described research on several thousand aging primates in the natural world by a team of biologists and demographers published in a recent issue of the journal Science. They found that the male-female mortality patterns of six species—sifaka, blue monkeys, mountain gorillas, capuchins, chimpanzees, and baboons—resemble one another and those of humans.

But mortality differences between the sexes in primates may be driven by intense competition among males for females, suggesting their life spans are shaped more by local social forces than evolutionary history or biology, according to Alberts. In baboons, she said, females remain in the groups they are born in, living with their mothers and other maternal kin, while males move between groups from adulthood and then every few years after that. These patterns avoid inbreeding and lead to strong relationships among females. As male baboons age, testosterone declines and they spend more time alone, findings that could have implications for their survival and potentially for understanding aging human males, she said.

History–Civil War POWs, 1940s African American Migrants–Informs Future Public Health Priorities

Historical data can help researchers explore influences on aging and mortality without "waiting 80 years to get answers," said Seth Sanders, a Duke University economist and director of Duke's Population Research Institute. Some historical events, such as changes in political regimes or famines and severe heat waves, create "natural mortality experiments," by forcing change on an entire population. Scientists from the Max Planck Institute for Demographic Research presented evidence of marked declines in mortality shortly after East and West Germany were reunified and modern medical care became widely available in the former East Germany.


Figure 2
Life Expectancy at Birth in the United States, 1900-2009

Source: Elizabeth Arias, "United States Life Tables, 2006," National Vital Statistics Reports 58, no. 21 (Hyattsville, MD: National Center for Health Statistics, 2010): table 11.


Analyzing historical data can guide future public health priorities. Sanders' own work has examined the toll urban life took on African Americans who left Mississippi for northern cities during the Great Migration. Overall, the urban migrants tended to have significantly higher incomes but also higher mortality than African Americans who remained in the rural South.

When Sanders examined the causes of death, he found migrants were more likely than their rural peers to die from diseases linked to smoking and drinking (emphysema, liver cirrhosis), underscoring the importance of lifestyle to later mortality. "Greater wealth did not necessarily translate into better health or longer lives," he explained. "Money does not go as far in cities. And more money appears to have brought greater access to alcohol and cigarettes."

Researchers are beginning to accept the idea that stressors at any point during life have lasting impacts on health and mortality. Using historical data, Dora Costa, a University of California Los Angeles economist, examined the extensive information kept on prisoners during the Civil War. She found that prisoners under age 30 were more likely to survive than older prisoners, but that POW camp survivors tended to die earlier than other veterans, suggesting they were permanently scarred. Her results and those of researchers studying POWs during World War II disprove the conventional wisdom that "what doesn't kill you makes you stronger."

There is also a growing consensus that living conditions in early life can influence elderly health. Costa has documented how deprivation suffered during gestation and early childhood reduced life expectancy in later adulthood. By comparing elderly Union Army veterans with Americans in their sixties and seventies between 1960 and 1980, she was able to quantify the growing impact that public health interventions and technological change—water filtration, sewage systems, vaccines, vitamins, and refrigeration—had on survival.

Her findings have implications for understanding future trends. "Baby boomers are a lucky generation," she said noting they were born after the "steep plunge" in infant and child mortality, and are likely to be "particularly long-lived" compared with previous generations.

But mortality may not fall as steeply for those born after 1955, she suggested (see Figure 2). Living conditions in the United Sates changed "much more dramatically" between 1915 and 1955 than between 1955 and 1995. Future improvements in old age mortality rates will likely reflect better medical care, disease prevention, and health habits, which may favor well-educated people who tend to follow health advice more precisely. "Public health interventions in the past—water filtration, sewers—tended to benefit the poorest most," she explained. Studies such as Costa's can help policymakers here and in developing countries identify where they can invest limited public health funds to have the greatest impact.

The First International Biodemography Network Meeting was held May 2011 at Duke University’s Population Research Institute. For more information: contactdupri@duke.edu.


Paola Scommegna is senior writer/editor at the Population Reference Bureau. Jerry Oster of Chapel Hill, N.C., contributed to this piece.