Even though many animal source foods contain good amounts of vitamin B12, extracting the vitamin from the food is not so simple. One reason is that vitamin B12 in food is bound to protein, and once the B12-protein complex enters the stomach, the vitamin needs to be separated from the protein to be available for absorption. This is accomplished by a sophisticated mechanism that ensures that the appropriate amount of acid is released into the stomach to begin the process of separating nutrient components from the foods we consume.

Even before we swallow our first bite, the aroma and taste of food triggers a neuronal signal to increase acid production in the stomach. The protein content of the ingested food also neutralizes the acid it encounters in the stomach and the rise in pH is detected by specialized 'sensing' cells in the gastric antrum. This releases the hormone gastrin, which then stimulates the parietal cells of the stomach to produce more acid, lower the pH and be able to dislodge B12 from its protein carrier. Once vitamin B12 is freed, it is immediately snatched up by haptocorrin (a B12 binding protein that is produced in the salivary glands, swallowed with the meal and standing by to bind free B12 in the stomach) and escort it on the first part of its journey to the absorption site at the end of the small intestine.

B12 is so important to survival and health in the mammal, that this elaborate hormone mediated 'sense and secrete' pathway also stimulates the gastric parietal cells to produce Intrinsic Factor (IF) to capture B12 from degraded haptocorrin and deliver it to the Cubilin receptors for absorption in the distal ileum. In other words, the protein-pH-IF axis is critical for being able to extract B12 from foods for absorption.