What are the main reasons that life expectancy has increased

Since the mid-1800s, the maximum human life expectancy has increased by about three months per year. Many people would like to see lifespans continue to increase, and scientists have been asking how easily it can be achieved.

A recent report by University of Minnesota College of Biological Sciences Professor Craig Packer, PhD, along with colleagues from many other institutions, presents evidence that nature has placed constraints on how much the underlying rate of aging can be slowed. This implies that the recent increase in life expectancy had nothing to do with slower aging.

"Essentially all of the extension in lifespan over the past century has resulted from improved nutrition and public health,” Packer said. “Any further increase by slowing the aging process will have to overcome powerful evolutionary constraints.”

The report is published in the journal Nature Communications.

New Data for an Age-Old Question

Major factors influencing life expectancy are infant and juvenile mortality rates, age-independent mortality (such as from accidents) and senescent mortality. Among these, senescent mortality—mortality among older individuals—is highly sensitive to the speed of aging. I.e., slow down aging, and older (senescent) members of the species will die off more slowly, resulting in a longer life expectancy.

The researchers asked if the rate of aging has been, in fact, the most important factor in determining life expectancies in related species, and if not, which ones have, whether among related species or in one species—humans—over time.

To answer this question, the researchers examined large data sets from six genera of nonhuman primates that together comprised 13 species, including chimpanzees, gorillas, baboons, and capuchin monkeys. Since humans are the only species in our genus—Homo—the team also examined nine data sets on populations that had not benefited from modern advances in medicine, public health, and standards of living. They took account of five parameters that reflected the age-dependent mortality rates for infants, juveniles, and older individuals, as well as for age-independent mortality rates.

Within each genus, they simulated the effects of variation in the parameters to see which, when varied, would yield the differences actually seen between the species in that genus (or between human populations). That is, which would reproduce the differences between real species in terms of life expectancy and also “lifespan equality.” Lifespan equality is high when most members of a species live long lives of comparable length, low when many members die in all age groups and few reach “a ripe old age.”

When the numbers had been crunched, variations in the pattern of early deaths best accounted for variation in life expectancy and lifespan equality actually seen within each primate genus. Age-independent mortality was also important, but “actuarial senescence” of adults and old individuals accounted for very little. In other words, the rate of aging had little effect.

Change-Resistant Aging Rates  

The study supports the idea that historically, when life expectancies were low, mortality improvements for infants, and reductions in age-independent mortality, “were the central contributors to the decades-long trend toward longer human life expectancies and greater lifespan equality,” the researchers concluded.

Most improvements were in social, economic, and public health advances. Since the mid-20th century, declines in the baseline level of adult mortality— through improvements in workplace safety, smoking cessation, and treatment of adult-onset diseases—have likely played an important role in industrialized societies, the researchers added.

But while these improvements can increase lifespans by eliminating early deaths, people who live longer will still age at the rate nature has set.

Thus, “continued improvements in nutrition and public health are unlikely to translate into a substantial further reduction in the rate of aging,” Packer said. “It remains to be seen if future advances in medicine can address the underlying cellular processes that currently limit maximum human lifespans that have largely been determined by the long, slow process of evolution.”

From Claire Ninde, APC Director

With National Public Health Week taking place April 3-9, I ask you to take a trip back in time and imagine yourself living 200 years ago in 1817. Living conditions then were vastly different from today due to poor sanitation, lack of proper sewage management, non-existent or inadequate treatment of drinking water, no food inspection or municipal garbage collection, crowded housing and no real understanding of nutrition.

People died painfully, mostly in infancy or childhood, primarily from diseases such as tuberculosis, pleurisy, typhus, tonsillitis, cholera and dysentery. With a lack of medical understanding of these ailments, a common treatment was bloodletting. The average lifespan at the time was around 35 years.

Over the last 200 years, U.S. life expectancy has more than doubled to almost 80 years (78.8 in 2015), with vast improvements in health and quality of life. However, while most people imagine medical advancements to be the reason for this increase, the largest gain in life expectancy occurred between 1880 and 1920 due to public health improvements such as control of infectious diseases, more abundant and safer foods, cleaner water, and other nonmedical social improvements.

This period is actually referred to as the “First Public Health Revolution” and it occurred before the medical interventions of antibiotics and advanced surgical techniques were in place. Historians have concluded that improved sanitation, public water treatment, sewage management, food inspection and municipal garbage collection almost eliminated the aforementioned causes of death. Also, other social advancements such as greater understanding of nutrition, better housing conditions, air quality improvements, child labor laws and higher literacy rates also greatly improved overall health and life expectancy.

This historical examination would not be complete without an acknowledgement of the impact of vaccines on improved health and life expectancy in the U.S., although widespread use of vaccines did not occur until the 1900s compared to some of the earlier life-extending population health improvements mentioned above.

Vaccines have been so effective at improving health and saving and extending lives that most people in the U.S. have no idea what it’s like to watch a child die a painful death from a tetanus infection or to witness a loved one experience brutal paralysis and death from polio. Providing and supporting vaccinations is a key tool in today’s public health toolbox.

The importance of highlighting public health’s role in extending life and improving health is crucial. Currently, medical health care expenditures amount to close to 18 percent of our GDP, whereas the more cost-effective public health initiatives have been vastly underfunded, despite the evidence that they greatly improve the health of the nation.

Health policy that aims to increase life expectancy and lower health care costs should demonstrate an investment in public health efforts that address multiple determinants of health, such as environmental factors, behavioral health and housing. Another important distinction between public health and individualized medical care is the foundational emphasis on health equity. Public health initiatives focus on serving populations, not just individuals, and improving everyone’s ability to reach their highest health potential, not just those who can afford it.

On a local level, San Juan Basin Public Health is launching our next Community Health Assessment that will result in a new Public Health Improvement Plan to be released within the next year. As we embark upon this process, that includes collecting and analyzing data related to chronic health indicators as well as quality-oflife factors like housing, education and mental health, our goal is to engage local community members in the effort to shape public health policy.

With your feedback, SJBPH will prioritize the public health issues unique to the communities we serve, and through the plan, create strategies to work with local partner organizations and providers to address these issues.

SJBPH will be publicizing opportunities to participate in the assessment, and you can visit sjbpublichealth.org/apc for more information. Don’t forget to thank your local public health practitioner for 30 additional years of life!