Recently a friend asked me why it was necessary for vegans to take a vitamin B12 supplement. I had read about this months ago and knew it was important, but I couldn't give her an intelligent response. So I went back into "The China Study," written by Thomas M. Campbell, where he provides a good explanation for this. The following is an excerpt.
"Vitamin B12 is made by microorganisms found in the soil and by microorganisms in the intestines of animals, including our own. The amount made in our intestines is not adequately absorbed, so it is recommend that we consume B12 in food. Research has convincingly shown that plants grown in healthy soil that has a good concentration of vitamin B12 will readily absorb this nutrient. However, plants grown in "lifeless" soil (non-organic soil) may be deficient in vitamin B12. In the United States, most of our agriculture takes place on relatively lifeless soil, decimated from years of unnatural pesticide, herbicide and fertilizer use. So the plants grown in this soil and sold in our supermarkets lack B12. In addition, we live in such a sanitized world that we rarely come into direct contact with the soil-borne microorganisms that produce B12. At one point in our history, we got B12 from vegetables that hadn't been scoured of all soil. Therefore, it is not unreasonable to assume that modern Americans who eat highly cleansed plant products and no animal products are unlikely to get enough vitamin B12.
Though our society's obsession with nutrient supplements seriously detracts from other, far more important nutrition information, this is not to say that supplements should always be avoided. It is estimated that we hold a three-year store of vitamin B12 in our bodies. If you do not eat any animal products for three years or more, or are pregnant or breastfeeding, you should consider taking a small B12 supplement on occasion, or going to the doctor annually to check your blood levels of B vitamins and homocysteine."
Found on page 232