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How much Co is in your water compared to your food. EPA says just 2 ppb in tap water. This means if you drink about 40 fl.oz. per day you only get 2ug of Co from your water. Per the EPA you get about 2-20x more from your food. Pretty much in no cases is your water a source of nutrients.

refs:

https://www.epa.gov/sites/default/files/2016-09/documents/co...

https://www.cdc.gov/nutrition/php/data-research/fast-facts-w...

This is my understanding too. There are microscopic amounts of trace elements here or there but in reality none of them add up to anything meaningful compared to what you get from food, multivitamins, or other less ideal means.

Rather if they did there'd be probably quite a lot of concern.

Not sure where that old wive's tale came from but even my parents had similar concerns against filtered water ages ago.

I've heard doctors repeat it too, though more as an overall mineral deficiency from already poor diets (which is the bigger issue)

It's not used for b12 production in human metabolism. It is, after all, a vitamin. Is this about cobalt deficiency in dairy and meat animals?

We humans cannot synthesize cobalamins from inorganic cobalt.