If you’re really interested in nutrition, you should be reading Bix regularly:
“Vitamin B12 follows a rather circuitous path to absorption. First, of course, you have to eat it. Then you have to free it from the proteins to which it’s bound, so you need a healthy amount of stomach acid. Stomach acid levels decline as we age. They’re also affected by drugs like antacids which are taken for gastric conditions that occur more frequently in the elderly. Once the vitamin is freed, it must bind to a specific protein called Intrinsic Factor (IF) which is secreted by cells that line the stomach. That’s a major stumbling block right there. Anything that damages those parietal cells will cause outright B12 deficiency. (Some people don’t secrete much IF to begin with.) Like? … Ulcers, tumors, scar tissue from ulcers & tumors, any kind of inflammation or gastritis, alcohol intake, and, naturally, the removal of these cells through weight loss or cancer surgery. Intrinsic Factor also doesn’t bind well in an acidic environment. Remember we needed an acidic environment, a low pH, to release B12 from its protein source? Well, now we need a higher pH, about 7, for binding. The pancreas releases buffers to raise the pH of gastric juice. Anything that interferes with pancreatic secretion (diabetes, cancer, cystic fibrosis, etc.) will interfere with B12 absorption. Right, now the IF-B12 complex travels to the small intestine where it’s absorbed. B12 needs to be freed from IF. The pancreas would have secreted enzymes to do that. No enzymes? Poor absorption. Finally it gets absorbed. Any damage to the lower part of the small intestine (celiac disease or other gluten-induced damage, cancer surgery, scar tissue, overrun by bacteria from the colon) will result in poor B12 absorption.”
BTW, Bix seems to be a vegetarian or vegan, but don’t hold that against him.