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The healing cascade involves several converging mechanisms

So people at greatest risk for deficiency include those with low stomach acid (e.g., older adults and people on proton pump inhibitors or antacids) or low IF production (known as pernicious anemia), people on medications that interfere with B12 absorption (such as metformin), people who have had gastric surgery, those with GI conditions that impair nutrient absorption (like celiac or Crohn's disease), and people on vegan and vegetarian diets

It does not invalidate the research, but it does mean human extrapolation should be conservative

Together, they form a powerful duo

Perhaps most notably, the researchers observed no adverse effects even at high doses