The Demography of Aging

What factors really determine our longevity?

Posted Dec 09, 2017

“He who loves practice without theory is like the sailor who boards ship without a rudder and compass and never knows where he may cast.” —Leonardo da Vinci

“It is a capital mistake to theorize before one has data.” —Sir Arthur Conan Doyle

A university professor went on a pilgrimage to visit a famous Zen master. While the master quietly served tea, the professor talked about Zen. The master poured the visitor's cup to the brim, and then kept pouring. The professor watched the overflowing cup until he could no longer restrain himself. "It's too full! No more will go in!" the professor blurted. "You are like this cup," the master replied, "How can I show you Zen unless you first empty your cup?" In the same way, we need to empty ourselves of myths and misinformation on aging, so that we can appreciate the reality of our situation.

A baby girl born today in parts of America, Europe, and the Pacific Rim has a better than 50-50 chance of living beyond the age of 80. To put this startling fact in context, during the Bronze Age (approximately 3000 BC), the average life expectancy was 18. In the days of the Roman Empire, it had risen to about 35. In early 20th-century America, the average life expectancy at birth was only 47. It took mankind 19 centuries to increase the lifespan by 12 years (from 35 to 47). That’s about three days of increased life expectancy each year over two millennia. In the last 100 years the average life expectancy at birth has nearly doubled from 47 to 80.

For the first time in human history, most of us can realistically expect to live into old age. Right now you have a 50-50 chance of living to the age of 80. If you are already 80, you have a 50-50 chance of reaching 90. Today people usually live well into their 70s and 80s, and before long, some cultures will have more than 25 percent of their population who are over age 65. So old age, once the privilege of the very few, has become the modern destiny for each of us. This is a monumental achievement of the 20th century that ranks with placing a man on the moon, the advances in telecommunication, splitting the atom, and unraveling our DNA. But where is the celebration? No one seems to appreciate this truly historic human accomplishment of longer life expectancy.

Another statistic that is relevant to our awareness of aging is the death rate. The death rate is one per person (it has remained remarkably constant for millennia). Each of us has a realistic chance of reaching 85, but living indefinitely is not an option. The implication of our inevitable mortality is that the nature of our journey becomes more important than its length. And the good news is that a wealth of scientific evidence shows that we can significantly influence the quality and possibly the rate of our aging. If this were not the case, there would be no secrets of aging, and we would be left to the winds of fate.

Our bodies are the future dwelling places of our own old age. What will this dwelling place (the cart in our metaphor) look like? There is no precise answer, but one truth we must face regarding our aging journey is that over time we will irreversibly change. For example, the thymus gland, which is large in children, withers away by adolescence. The human ovary ages so precisely that it fulfills its function by about age 55, leading to the changes associated with menopause. The physical manifestations of aging, the way we will look and function in old age, are highly individual. Life is always changing and evolving.

The Big Lesson From Life Course Epidemiology

Life Course Epidemiology is the study of the factors that influence our longevity. Several epidemiological studies have convincingly shown that 75 percent of our mortal hazard relates directly to our environment. Moreover, the observation appears to be culturally insensitive. A key environmental feature is socioeconomic status (SES) — more specifically, the income gap. However it is not just the difference between the wealthiest and poorest members of a society that seems to matter, but also how rich or poor you are relative to others around you. Education is clearly linked to SES. Another critical factor is job satisfaction. If one’s boss is a martinet, and our work environment is stressful, our longevity is compromised no matter how much money we make. For example, several studies document that being laid off or experiencing loss of job security is associated with increased mortality, often from heart disease.

Living with a loving partner extends our longevity. Caring for a pet also confers a salutary effect. Moderate regular exercise and a healthy diet tend to slow aging changes. On the other hand, smoking accelerates aging of the skin, heart, lungs, blood vessels, and bone. Excessive noise affects the ears; ultraviolet light ages the eyes and skin; excessive dietary protein seems to increase the aging of the kidney. 

Wait a minute, you may be thinking. What about preventive health care, disease risk factor modification, or my genetic endowment? After all, Aunt Mary lived to 103. To inform us we have a vast body of literature on proximate-cause epidemiology. Proximate cause epidemiology is the study of causes of death and the risk factors for those causes. Cardiovascular disease is an example that has been extensively studied, with published risk factors such hypertension, diabetes mellitus, elevated serum lipids, smoking, family history, and others. Modifying these risk factors may reduce our likelihood of dying from heart disease (probably not by very much), but has little or no effect on mortal hazard. In other words, we may be able to change the likely cause of our death without meaningfully lengthening our lives. It is not a major worry of mine what my death certificate will ultimately read as my primary cause of death. One’s concern is to have a meaningful life as long as it lasts. As stated by British humanist E. M. Forster, “We must be willing to let go of the life we’ve planned to have the life that is waiting for us.”

A lot of risk factor modification is much ado about the trivial. If we carefully examine the mountains of evidence, it becomes clear that the impacts are on the order of absolute reductions in deaths of one half to two percent. In other words, 50 to 200 people need to be treated over extended periods of time (a decade or so) to prevent one premature death (that otherwise would not have occurred). Realistically, an intervention, such as aggressively treating high blood pressure, might optimistically reduce an otherwise normal individual’s risk of a bad outcome, such as a stroke or heart attack, from five percent to three percent: a two-percent reduction over five to ten years.

It is easy to be confused about risk factor modification by what we read or hear in social media. Absolute risk reduction — the difference between our baseline risk and the reduced risk with the intervention — is what really matters. But clinical studies and the media frequently trumpet relative risk reduction, which is the percentage your risk has been reduced. In the example above the relative risk reduction would be a 40-percent drop in the risk from 5 to 3 percent. Which sounds more convincing, “We can reduce your risk of a stroke or heart attack by 40 percent,” or “We can lower your absolute risk of a stroke or heart attack by 2 percent (or 1 in 50)?” Both statements are mathematically equivalent.

The target of modern preventive health care is to extend longevity by reducing premature death, which certainly seems reasonable in very young populations with many decades of remaining life. However, defining premature death becomes increasingly problematic the older we become and ultimately misses the point, because the death rate is still one per person. To me, at some phase of life, the target of prevention needs to shift from maximizing longevity to maintaining function and minimizing dependency. As we live longer and better with the compression of our disability to the later stages of life, we should focus on those factors that threaten our independence, such as problems of vision, hearing, mobility, and memory loss. I tell my very elderly patients that my goal is to keep each of them smiling and happy for as long as possible. So far, no one has voiced a different objective.