ScienceDaily article linkThe above link describes how they studied people's stools and discovered that certain kinds of bacteria helped people produce active vitamin D. they supposed that supplementation and sunlight had little to no impact on active vitamin D levels, and that active vitamin D levels are the ones that matter wuth COVID-19 and other stuff.
What kinds of bacteria helped? They never said. They did say a chemical that those bacteria tended to produce, though.
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