The Shepherd’s Dream

The Difference Between Measured and Imagined Life Expectancy

As an article of faith, we like to believe that healthy habits will lead us to a longer life. So of course, it makes sense to develop healthy habits for eating, enjoy an active life, and get enough good sleep every night. But putting a number on the benefit of those habits is not so easy. Because it’s no small task to actually measure the effects of healthy habits on life expectancy and the imagined number for those effects can be wildly wrong.

This is a case where thinking does not make it so. Life has many surprises for us.

Bold Promises from Observational Data

Nevertheless, researchers publishing in Nature Food make some bold and remarkably specific claims about gains in life expectancy a person can get from changing their diet – undaunted by the limitations of their observational data. Lars Fadnes and colleagues write:

“Sustained dietary change from unhealthy dietary patterns to the Eatwell Guide dietary recommendations is associated with 8.9 and 8.6 years gain in life expectancy for 40-year-old males and females, respectively. In the same population, sustained dietary change from unhealthy to longevity-associated dietary patterns is associated with 10.8 and 10.4 years gain in life expectancy in males and females, respectively.”

The resulting headlines don’t even bother with the qualifier that this is a mere association:

“You Can Add 10 Years to Your Life Simply by Changing Your Diet, Massive Study Finds”

But the limitations of this study take up almost as many words as this entire post. Those limitations start with a simple principle that has profound implications. Based on associations, competent scientists cannot promise an effect. And, as the authors note, sustained changes in diet to a pattern deemed healthy are exceedingly unlikely. Because “dietary patterns fluctuate over time.”

The Challenge of Showing a Mortality Benefit

The lesson here is simple. It is not easy to actually show that behavioral changes will bring a mortality benefit. With the epic Look AHEAD study, researchers looked hard for evidence that changing diet and exercise patterns could extend a person’s life. In the end, they found that such a benefit does not happen, concluding:

“Clinicians should be reassured about recommending that adults with type 2 diabetes with overweight or obesity participate in intensive lifestyle programs focused on weight loss. However, such recommendations should be based on the beneficial effects of intensive lifestyle intervention on a variety of physical and psychological outcomes and on quality of life, not on expectations of lower mortality risk.”

These are words to the wise about expecting a longer life as a payoff for eating healthy. Don’t kid yourself. Do it because it helps you feel better and enjoy life. Not because of an imagined extension of life expectancy.

Click here for the study from Nature Food and here for perspective from the Look AHEAD study.

The Shepherd’s Dream, from ‘Paradise Lost,’ painting by Henry Fuseli / WikiArt

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November 26, 2023