Long Distance Runners, Ancient Greece, image from a Greek amphora

The Mythical Race Between Diet and Exercise

You can’t outrun a bad diet. It’s a clever turn of phrase that resonates. But like many things that resonate about diet, exercise, and obesity, it might be a little too clever. In a very gentle way, David Allison, Dennis Bier, and Julie Locher point this out in a brief commentary appearing this week in the International Journal of Obesity. The race between diet and exercise is more mythical than literal.

A Gentle Caution

Allison, Bier, and Locher raise their note of caution based on an article in Science about the work of Herman Pontzer. While agreeing that Pontzer’s work is “highly important, interesting, and exemplary,” they suggest that Pontzer may go a little too far when he says “exercise generally fails to aid weight loss.”

Their point is important for two reasons. First, it’s worth remembering that carefully controlled studies have shown that exercise indeed can cause weight loss. The effect might not be a potent as many people believe, but it’s not nothing.

Correlation and Causality

The second reason takes us back to a very basic truth. Correlation does not prove causality – nor does the lack of correlation disprove the potential for causality. Allison et al point out that Pontzer makes his comments about exercise and weight loss based on very large, high quality sets of observational data. But they write that we must remember this:

“No amount of measurement quality, new technology, or large sample size can substitute for randomization of experimental units to different levels of the independent variable (i.e., the postulated causal factor) in allowing causal inferences to be made. This has been demonstrated theoretically and illustrated empirically. This is not merely a pedantic point, but an increasing concern in science where trust in ‘communication of science’ is at stake. It is also of concern in some fields of human investigation where observational studies prevail and where causal inferences are often defended on the basis of large sample size.”

In other words, no matter how big and fancy your data and methods are, correlations that arise from them still do not establish a causal effect.

Maybe it’s time to retire the well-worn metaphor of a mythical race between diet and exercise.

Click here for the article from Science and here for the commentary from Allison et al. For more on the complicated relationship between exercise and body weight, click here.

Long Distance Runners, Ancient Greece, image from a Greek amphora / Wikimedia Commons

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December 3, 2022

4 Responses to “The Mythical Race Between Diet and Exercise”

  1. December 04, 2022 at 11:33 am, John DiTraglia said:

    Ummmm. “exercise generally fails to aid weight loss.” may not be 100% accurate but it’s 99.992316% accurate. I think David Allison, (one of my heroes by the way) et al’s commentary is cavilling. Their biggest worry is that it might cause people to not exercise which would be admittedly deadly but talk about baseless conjecture. It would be too bad if the truth caused bad behavior but still.

    • December 04, 2022 at 12:20 pm, Ted said:

      Splitting hairs is frustrating, but sometimes a hairy question requires it. In my view, there’s a world of difference between “exercise generally fails” and “exercise is not very potent” for weight loss. The problematic word is “generally.” It flags a generalization and generalizations are hazardous.

  2. December 06, 2022 at 9:56 am, Anna said:

    I’d be curious to hear your thoughts on the young topic of muscle-centric medicine. If it’s new to you, it’s based on the idea that we are under-muscled, not just overly fat. Dr. Gabrielle Lyon and colleagues consider muscle to be a 6th vital sign that plays a role in longevity, preventing dementia and other aging diseases. I find the mindset shift of adding (muscle) over subtracting (fat) especially productive in a society infiltrated with stigma. Curious to hear your thoughts. Thanks for all you do.

    • December 06, 2022 at 10:13 am, Ted said:

      Thanks, Anna for suggesting we think about this. I’m eager to read more and also cautious about broad concepts that might be helpful for some and not for others. Certainly, sarcopenia is an idea that is getting a lot of attention.