The only papers I could find in Google Scholar about this connection all come from India, and does not seem strongly connected. The study at
https://journals.lww.com/jfmpc/fulltext/2025/04000/prevalenc... , for example, says:
> While some studies have hypothesized that the use of RO water could contribute to vitamin B12 deficiency, no significant differences were observed in this study.[20] Symptoms of deficiency were not significantly associated with serum vitamin deficiency status. Only VDD was significantly associated with fatigue as a symptom. This discrepancy raises questions about the current normative values for vitamin B12 and vitamin D3 in the Indian population and suggests the need for further research.
A whole lot of people drink RO water. If it were a simple correlation, I would expect to see cases and papers from all across the world.
I also know there's a long history of false claims along the lines "distilled water sucks the minerals from your body", also called "hungry water". I first heard in the 1980s as a supposed reason for not using distilled water in a radiator. Or even commentary of it in the Carnivorous Plant FAQ at https://www.sarracenia.com/faq/faq3385.html .
Because of that long history, and the lack of a good mechanism for how it should work, I need a much higher level of evidence for a direct, causal connection.