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Or perhaps even more likely, that both are caused by things like good nutrition.

I don’t follow. Good nutrition doesn’t lead to poor fitness afaik.

The parent is possibly commenting on how nutrition standards in western countries have (arguably) declined recently.

Or even that there is a 3rd confounding factor.. say amount of sleep...

That's along the lines of hypotheses like lung cancer being the common cause behind smoking and yellowed fingertips.

No, not really.