Introduction
The most significant achievement of the twentieth century was the near doubling of human life expectancy from 47 to 76 years of age. Although life expectancy is a demographic value, meaning it is an indicator of population aging, nevertheless, it reveals important aspects relevant to individual aging. This blog will explain life expectancy with a specific focus on: its determination and underlying assumption, the reasons for the dramatic increase since the 1900s and its relevance to healthy longevity.
What exactly is life expectancy?
“Life expectancy at birth represents the average number of years that a group of infants would live if the group was to experience throughout life the age-specific death rate present at the year of birth” (Murphy et al., 2013). The unmet assumption lies right in the definition which states that the age-specific death rate at birth remains unchanged throughout life. But since age-specific death rates change constantly throughout life, life expectancy, at best, represents an estimate of one’s possible lifespan. Despite the unmet assumption, life expectancy also denotes the health of a population. Accordingly, the US ranks 29th out of 38 similar developed countries in life expectancy (Organization for Economic Co-Operation and Development, 2019). Sadly, the US spends almost twice as much on healthcare compared to the average spending of the top ten countries with higher life expectancies (e.g. Norway, Australia, UK, Switzerland). (https://www.commonwealthfund.org/publications)
Given its less than rigorous premise, life expectancy, however, consistently reveals an unexplained gender gap in which life expectancy of females exceeds that of males by approximately 4-5 years (topic of a future blog). As of 2021 (Arias et al., 2022), life expectancy was 79.1 years for females, 73.2 years for males and 76.1 years average of both genders. Secondly, in recent years, life expectancies of subpopulations based on Hispanic origin and race show significant differences. Specifically, life expectancy (average of both genders) is highest for Asians (83.5 years), followed by Hispanics (77.7 years), Whites (76.4 years), Blacks (70.8 years) and American Indians/Alaska Native (65.2 years).
Doubling of life expectancy and recent changes
First Half of Twentieth Century
The greatest improvement in life expectancy occurred in the first half of the twentieth century. This resulted from societal advancements in the handling of sewage, sanitation, and clean water which came about after acceptance of the Germ Theory of Infections proposed by Louis Pasteur, Robert Koch and others. The Germ Theory of Infections also opened the way for the development of vaccines for diphtheria, whooping cough and tetanus and the launch of sulfa drugs and antibiotics e.g. penicillin. Together, these changes decreased the mortality rate of infants and children, allowing them to survive to older ages, pushing life expectancy up.
Second Half of the Twentieth Century
The second half of the twentieth century also contributed to increasing life expectancy but in different ways and to a lesser extent. There was a decrease in infant mortality with acceptance of hospital births favored over home births. However, more significantly, there was a decline in mortality rates among older individuals due to several developments. First, protocols for long term management of chronic diseases were established, especially for cardiovascular disease, the major killer of the elderly. Specifically, identification of more effective drugs, implantable medical devices (pacemakers, stents, defibrillators) and safer surgical procedures added years and increased life expectancy in the US population. Secondly, access to Medicare and Medicaid provided affordable health care for many. Thirdly, a scientific focus on aging ushered in effective lifestyle choices to maintain the healthspan and decrease mortality.
2014 to 2017
Life expectancy peaked in 2014 at 78.9 years (average of both genders). A slight decrease in life expectancy occurred from 2015-2017 and might not be significant except for the fact that even this small decline was, unprecedented and due, unfortunately, in large part to an increased mortality rate in young individuals from drug overdoses (opioids), suicides, homicides, and an uptick in deaths in older adults due to Alzheimer’s and cardiovascular disease (Harper et al., 2021).
2020-2021
Life expectancy experienced a sizeable (0.9-1.8 year) decline in 2020 (77 years) and 2021 (76.1 years), considered the biggest two-year decline in life expectancy since 1921-1923 (CDC National Center for Health Statistics). This decrease in life expectancy is no surprise. It resulted from the spike in deaths due to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), better known as the COVID-19 pandemic. Sadly, many of the negative factors identified in 2015-2017 such as drug overdoses remained an additional contributing factor to the decline in life expectancy in 2020 and 2021.
Significance of life expectancy to aging
From the discussion above, it is evident that many incredible societal and medical advances have removed environmental hazards, thereby, permitting individuals to reach older ages and in some cases approach the true life span (~119 years of age) of man. Despite this, the rate of aging in humans has not changed. In fact, if the life expectancy graphs are transformed to yield a value called the mortality rate doubling time for humans, it is apparent that from the 1900s onward, the mortality rate doubling time remains constant at 8 years beginning at puberty (lowest mortality rate). Since the mortality rate at puberty is extremely low, doubling that every 8 years gets one to about 119 years of age, which is the oldest validated age reached by humans.
Life expectancy and lifespan
Life expectancy relates to the characteristic human rate of aging. The rate of aging relates to survival, hence lifespan and longevity. While the human-specific mortality rate signifies inevitable physiological deterioration, it does not mean that aging is biologically programmed. Far from it, no master “age-directing” programs (genetic or otherwise) are known to exist. Aging is highly variable from one person to another exactly because it results primarily from environment factors (e.g. lifestyle choices) and the complex interaction of the environment with our genes (see Insight 1).
Gerontological investigations show that age changes accrue due to the slow loss of stress resistance, also referred to as loss of essential maintenance and repair mechanisms. Lifestyle choices that optimize those systems of maintenance and repair minimize aging. Previous blogs discuss many of these lifestyle choices, in particular the 4-prong exercise program (aerobics, resistance, balance, stretch)(see Insight 2; Insight 3: Ways to retard skeletal muscle aging ; Insight 4: Anti-aging benefits of aerobic and stretch exercises; Insight 5 – Optimizing Balance) enhance these maintenance and repair mechanisms to optimize stress resistance.
Expansion or Compression of Healthspan
Does the increase in life expectancy (longevity) evident over the last 100 years come with extra years of health (absence of disease and disability) or with an extension of disease and disability (extended senescent span)? At present there is no consensus on an answer. This is because there is no agreement on a) exactly what is an age-related disease, b) whether disease is more important than disability and c) how to quantify the social/psychological impact of disease/disability on quality of life. However, if one considers the ongoing effort of the scientific community to precisely define biological pathways involved in aging and to validate, through clinical trials, effective ways to minimize aging, it seems that each individual should be able to achieve extra years with health rather than with disease and disability.
References
Arias E, Tejada-Vera, Kochanek KD, hmad FB. Provisional Life Expectancy Estimates for 2021. Vital Statistics Surveillance Report, August 2022. https://www.cdc.gov
Harper S, Riddell CA, King NB. Declining Life Expectancy in the United States: Missing the Trees for the Forest. Annu. Rev. Public Health 42:381–403, 2021
Murphy SL, Xu J, Kochanek KD. Deaths: final data for 2010. Natl Vital Stat. Rep 61: 1-118, 2013.