My 0.02 USD. I can see someone who limits their intake of B12 and takes PPI and has "just a touch of" atrophic gastritis that they don't know about (pretty common at our age), developing B12 deficiency.
Honestly, I've never seen B12 deficiency develop in someone on a normal American diet (eating anything that don't eat them first) and with a normal gut just because they take PPI, and I've checked for it lots. Ditto magnesium. Ditto calcium and vitamin D.
Checking those levels is easy enough to do. I personally think people should be able to check them on their own, but that's just me.
In my own case, my hemoglobin was lowish until I got treated for low T. Now it's fine. But I'm not a better cyclist for it, or maybe I am. Who knows?
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Momento mori, amor fati.