Hi Xjhuez, Our whole b12 absorbtion system is built around extracting an exceedingly rare item available in parts per trillion in a few foods. It is optimized to get the first 5-10 mcg. Our bodies have evolved to concentrate and retain this exceedingly rare item. It is absorbed very efficiently via a complicated active system that in a natural circumstance, "if anything goes wrong, your fault , my fault, nobody's fault and you will die" (Big Jake). This is based on 50 years of absorbtion and serum studies on cobalamins and appears rock solid. Back in the 50s dozens of studies were done comparing oral with injectable for serum levels to try establish what needs to be done to keep people with a malfunctioning absorbtion system alive without consuming 2 liters of raw liver puree a day.