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This post distills Chapter 19 of the Handbook of the Biology of Aging:

Crimmins, E. M., Zhang, Y. S., Kim, J. K., & Levine, M. E. (2021). Trends in morbidity, healthy life expectancy, and the compression of morbidity. In Handbook of the Biology of Aging (pp. 405-414). Academic Press.

Trends in morbidity, healthy life expectancy, and the compression of morbidity

Before we begin, we’re going to get icy rational for a minute and talk about two new concepts.

  1. Compression of morbidity.

Morbidity means “suffering from disease or disability.”

Imagine that in the past, people tended to get disabled at age 65, and die at age 80. What if now, they get disabled at 75, and die at age 82? That means they live two years longer, but suffer eight fewer years of disability. Their healthspan, or the number of years they live in good health, increased faster than their lifespan. If trends continue, maybe they’ll routinely live in perfect health until they suddenly collapse on their 100th birthday! This is what we mean by “compression of morbidity.”

  1. Population statistics vs. lifecycle statistics.

In America, 16.8% of people are over age 65. That’s a population statistic. It tells you what fraction of people have a certain trait. By contrast, a 2005 study found that the average age of diagnosis of adult-onset diabetes has decreased from 52 to 46. That’s a lifecycle statistic. It tells you facts about when, or how much, of a person’s life cycle has a certain trait.

These don’t seem intuitively that different, and in a static population, they are identical. But if you improve the actuarial tables, making it more likely that we live to older ages, there’s no effect on population statistics, but lifecycle statistics change. Decreasing fertility changes population statistics (resulting in an older population), but not lifecycle statistics (it doesn’t change how long we live).

We have to keep the distinction crisp in our minds in order to check if morbidity is compressing or not.

Point #1: Compression of morbidity is when healthspan is catching up to lifespan. Population statistics and lifecycle statistics seem similar but are importantly different.

It’s hard to define “healthy” (no diagnosed disease? feeling good? free of disability? no underlying disorders?), so studies get different results when they ask whether we’re getting healthier over time. Getting better at preventing death often means keeping more sick and disabled people alive, so better medicine can make the stats for “number of sick people” go up. And some public health scientists unfortunately seem not to understand population vs. lifecycle statistics as well as you do.

Point #2: You have to define “health” carefully to study health trends. Not all researchers do, so there’s some confusion in the public health literature.

A few population health statistics are improving, like the fraction of the population with (or having experienced) minor disability, heart attack, and high cholesterol or blood pressure. Yet a higher fraction of the population was getting heart disease, cancer, and stroke until 2000, when trends stabilized.

Point #4: Population health statistics vary quite a bit.

Now let’s consider lifecycle statistics.

Life expectancy at birth is stalled, or getting worse, and we’re not succeeding in delaying the onset of heart disease, cancer, or stroke. However, life expectancy at 65 is still slowly going up. People are living longer free of disability, due to delayed onset and improved recovery. We’re also succeeding at keeping disabled people alive, which makes the “number of people with disability” stat look worse.

Let’s consider hypertension, diabetes, cancer, heart disease, stroke and arthritis, comparing the USA in 1998 vs. 2014. Individuals live more years free of these diseases, but also (with the exception of cancer) live even more years with disease. The average person spends a smaller fraction of their life with cancer, the same post-stroke, and a larger fraction with the other diseases.

Considering 300 diseases in the USA from 1990 vs. 2017, healthspan (health-adjusted life expectancy) grew by 2 years, but life expectancy grew by 3 years. That’s expansion of morbidity, not compression, partly caused by better life-saving interventions that keep seriously ill or disabled people alive.

Point #5: Even though we’re living longer lives, and more healthy years of life, morbidity has expanded.

Delaying the onset of disease and disability means fighting aging itself, by slowing damaging underlying disorders while enhancing our ability to repair the body. Most near-term medical research is focused on slowing the progression of an existing disease, and this is where we’re currently making most progress in healthspan and lifespan.

Point #6: We're working harder to slow disease than to delay its onset, and changing that would mean refocusing medical research to fight aging itself.

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