IF it was a case of taking just one, I'd suggest testing for B12 & if low making it that one .
I take B12 in sublingual form
As mentioned Metformin AND being 60+ can lower it.
Too low can mean injections, & treatment for life ...so I took it seriously, due diligence & lots of research.
Having watched my mum's decline into dementia, the possible impact of the role B12 might play fully captured my attention.
Up shot was I asked for my B12 to be tested at my HBA1c blood check.
( Not an issue asking & getting it tested, but beware it's not tested unless requested.)
Found mine was at very low end of recommended range 190-900.
Ordered B12, in sublingual form, as it seems in tablet form it can't pass the stomach in any meaningful way.
Next HBA1c it went from 225 - 335...not ideal but an improvement & away from the edge of danger, I felt.
For B12 context & info, B movie below in my signature highlights the various & detrimental ways low B12 impacts people.
The
Wikipedia page is a good starting point
As for other vitamins, D3+K2 here as well .
And 2 Brazil nuts for 'brain power' can't prove it's helped, but they taste good and are rather filling, surprisingly.
Google selenium &
Brazil nuts
Good luck finding the info you're looking for .