Those which bind to the B12-IF complex, preventing uptake, those which bind to IF itself, preventing the binding with B12, and those which bind to parietal cells, preventing the production of IF to begin with
Fat mass reduction, insulin sensitivity, NAD+ tissue levels, AMPK phosphorylation status, and mitochondrial respiration capacity are all measurable
A history of cancer or active malignancy is frequently flagged in the literature as a reason for caution, because pathways involved in angiogenesis and cell migration are also relevant to tumor biology
Oral B12 supplements rely on the digestive system for absorption, which can be significantly impaired by gut conditions, age-related changes, or the absence of intrinsic factor (a protein required for B12 absorption)