Vitamin B12 deficiency is being found much more often than before. Docs are still often not aware. There are 3 major reasons for increasing prevalence:

1. aging population demographics leading to increasing prevalence of decreased stomach acidity;

2. Medications (mostly the antacids like Nexium or Prilosec, but also Metformin and some others);

3. vegan, vegetarian, and plant-based diets.

This New England Journal article covers several key facts about what it calls a "common clinical presentation". It is a 57 yo patient with B12 deficiency with a normal serum B12 level. The patient had homocysteine and methylmalonic acid levels that were very high.