Introduction to Vitamin B12: Are You Getting It?.
Does Vitamin B12 Matter?
There are two types of B12 deficiency: mild and overt.
Overt B12 Deficiency:
B12 protects the nervous system. Without it, permanent damage can result (e.g., blindness, deafness, dementia). Fatigue, and tingling in the hands or feet, can be early signs of deficiency. B12 also keeps the digestive system healthy.
Mild B12 Deficiency:
By lowering homocysteine levels, B12 also reduces the risk of heart disease, stroke, and other diseases. Vegans and near-vegans who do not supplement with vitamin B12 have consistently shown elevated homocysteine levels. See the section Homocysteine, B12, Vegetarians, and Disease.
B12 is generally found in all animal foods (except honey). The overwhelming consensus in the mainstream nutrition community, as well as among vegan health professionals, is that plant foods do not provide vitamin B12. (Luckily, vitamin B12 is made by bacterial fermentation such that it does not need to be obtained from animal products.) Despite this, some vegan advocates still believe that “plant foods provide all the nutrients necessary for optimal health” and, therefore, do not address vitamin B12 when promoting the vegan diet. Other vegan advocates acknowledge the need for B12, but only as an afterthought.
The result is that many vegans do not eat B12 fortified foods or supplements. Many have developed overt B12 deficiency. In some cases, the symptoms have cleared up after taking B12 supplements, but not everyone has been so lucky.
While many current vegans report feeling better on a vegan diet, the most common complaint I hear from ex-vegans is that they didn’t feel healthy. This seems reasonable: The people who feel good on the diet stick with it. The people who feel bad, don’t. Could it be that some of the people who go back to eating animal products are feeling the effects of a reduced B12 status? Many vegans would not consider this a possibility, because humans need very little B12 and new vegans usually have a healthy store which can last months or years.
The fact that vegans tend to have lower B12 levels than lacto-ovo vegetarians or non-vegetarians is often countered with, “Few vegans have ever shown signs of B12 deficiency.” However, most vegans appear to supplement their diet with B12 (often unknowingly through fortified foods), which could explain why most vegans never show overt B12 deficiency.
As for vegans whose diets are not supplemented, I disagree that they rarely show signs of B12 deficiency. As the reader will soon see, there have certainly been plenty of vegans who have suffered from B12 deficiency in the scientific literature. I meet vegans on a regular basis who report having been diagnosed with B12 deficiency or who came down with symptoms of B12 deficiency. It is time that there were no more. Vegans can ensure optimal B12 status, reducing their risk for many diseases, by following the recommendations.
This article is a thorough review of the scientific literature about vitamin B12 and the vegan diet, including every relevant study on vegans and vitamin B12 published since 1980. Vegan advocates who may otherwise not be interested in the details of vitamin B12 are encouraged to read the Recommendations and Can a Natural Diet Require Supplements?
B12-Related Laboratory Values
This article often refers to various laboratory values related to B12. A reference can be found in Appendix: B12-Related Laboratory Values.
Vitamin B12: A Pesky Molecule
B12 is a complicated vitamin with a unique absorption mechanism, wide array of deficiency symptoms, and a number of inactive analogues (molecules that appear to be active B12, but actually are not) that possibly interfere with its function.
via Introduction to Vitamin B12: Are You Getting It?.