The evidence about the prevalence of vitamin B12 deficiency in vegans comes mainly from this review of studies in EJCN, 2015:
- The results differed widely (2.4-78%) among studies, but 45% of vegans in average were B12 deficient, based on low blood B12 levels.
- More than 85% of vegans who were not taking B12 supplements were B12 deficient, based on low blood B12 levels. (Why not 100%? Because, at the time of a study, some participants were on a vegan diet for only 1 or 2 years, which may be too short to develop deficiency).
1) In various studies described in The prevalence of cobalamin deficiency among vegetarians assessed by serum vitamin B12: a review of literature (EJCN, 2015) (summarized in Table 1), the percent of vegans with B12 deficiency (by the criteria of low B12 (cobalamin) levels, which ranged widely from <95 to <250 pmol/l) were 2.4 to 78%.
2) In the study mentioned in the question: Vitamin B-12 status, particularly holotranscobalamin II and methylmalonic acid concentrations, and hyperhomocysteinemia in vegetarians (AJCN, 2003), which included only 29 vegans, 27 of them or 92% were B12 deficient on the basis of low holotranscobalamin II levels (<35 pmol/l), but only 52% on the basis of low cobalamin levels (<156 pmol/l).
Lab tests for vitamin B12 deficiency
According to Institute of Medicine (US), blood levels of vitamin B12 (cobalamin) that reflect deficiency are 120 to 180 pmol/L (170 to 250 pg/mL) for adults. But, as deficiency develops, blood levels may be maintained at the expense of B12 in the tissues. Thus, normal blood levels do not necessarily indicate adequate B12 status.
Methylmalonic acid (MMA) levels may more accurately reflect the B12 status, because they indicate a metabolic change that is highly specific to B12 deficiency (Office of Dietary Supplements). Blood levels <270 nmol/l are normal (The Vegan Society).
Time in which vitamin B12 deficiency can develop
According to Vegan Society:
In the absence of any apparent dietary supply, deficiency symptoms
usually take five years or more to develop in adults, though some
people experience problems within a year. A very small number of
individuals with no obvious reliable source appear to avoid clinical
deficiency symptoms for twenty years or more.
The recommended dietary allowance (RDA) for vitamin B12 for adults is 2.4 µg/day. It is not clear at which lower daily intakes deficiency could develop, but vegetarians, who consume milk with 1.1 µg B12/cup (237 mL) and eggs with 0.6 µg B12 also often develop deficiency (EJCN, 2015).
Prevention of vitamin B12 deficiency in vegans
It seems that for most vegans, the only option to prevent B12 deficiency is to consume B12 fortified foods or supplements. Some plant foods contain vitamin B12, but this is either inactive (in spirulina, chlorella, tempeh, miso, kombu) or not present in sufficient amounts (in white button mushrooms, Korean purple laver or nori or nutritional yeast); human intestinal bacteria may also produce some vitamin B12 but not in sufficient amounts (Vegan Health, Vegetarian Nutrition Dietetic Practice Group).
Vitamin B12 deficiency in vegans who do not take supplements
- AJCN, 2003, Table 1: 29% (5 from 17) of vegans who were taking B12 supplements and 83% (10 from 12) of those who were not, were B12 deficient.
- Annals of Nutrition and Metabolism, 1983: From 83 participants (vegetarians/vegans), 92% of vegans who were not taking supplements, 64% of the lactovegetarians, 47% of the lacto-ovovegetarians and 20% of the semivegetarians had low serum vitamin B12 levels (<200pg/ml or <150 pmol/L). However, their complete blood count values did not deviate greatly from those found for nonvegetarians (no anemia), even though some had been vegans or lactovegetarians for over 10 years.