Blood B12 Level: Not Reliable
A serum B12 (sB12) level below the normal range indicates that B12 levels are becoming depleted. However, as described below, a sB12 level in the normal range does not ensure that B12 levels are healthy. Unfortunately, medical practitioners still use serum B12 to evaluate function, even of vegans.
- Seaweeds Can Falsely Inflate sB12 Levels
- Transcobalamin II
- B12-Deficient Nerve Damage With Normal sB12 and No Macrocytosis
Methods for determining sB12 levels rarely, if ever, distinguish between B12 and all inactive B12 analogues. Seaweeds contain a variety of inactive B12 analogues. Someone who is eating large amounts of seaweed may have serum B12 levels well above normal, but much of it could be inactive B12 analogues that may actually be interfering with B12 function (see the section Inactive Analogues: Worse than Useless in Vitamin B12 Analogues).
Transcobalamin II (TC2) transports B12 to tissues. In non-vegetarians TC2 normally contains about 20% of B12 in the blood (1). TC1 and TC3 are the proteins that normally store the other 80% of the B12 in the blood (1). If TC2 lacks B12, the vitamin will not be delivered to tissues, regardless of whether the total sB12 is low, normal, or high (2). When absorption of B12 via the intestines slows, B12-TC2 levels fall rapidly (1). There is evidence that TC2 is depleted of B12 within days after absorption stops (1).
|Table 1. Transcobalamin II Levels in Herrman et al. 2003 Study.|
|B Vit||No B Vit||B Vit||No B Vit|
|Serum B12 (pg/ml||387||409||242||259||170|
B12-TC2 - B12 on Transcobalamin II
MMA - Methyl malonic acid
What this means is that a malabsorption disease can result in tissues not receiving B12 even though serum B12 appears normal. And, even at a normal serum B12 level, when intake of B12 is low or none, the tissues might not be receiving nearly as much B12 as they would otherwise be receiving when intake is normal.
In 2003, Herrmann et al. published a study looking at vegan and vegetarians' TC2 levels (3). Some vegans and vegetarians were taking B vitamins, though amounts of B12 were not reported. Results are shown in Table 1.
From the table, you can see that vegans' B12-TC2 levels were quite low, while homocysteine levels, especially for the vegans not taking any B vitamins, were much higher than for the non-vegetarians. Statistical analysis by the authors showed that B12-TC2 levels were the best predictors of homocysteine levels.
Some people with normal B12 levels and without macrocytic anemia suffer from B12-deficient nerve damage, elevated homocysteine, and elevated MMA acid levels. See Lack of Anemia Does Not Mean B12 Status is Healthy under "Coenzyme Functions of Vitamin B12" for more details.
3. Herrmann W, Schorr H, Obeid R, Geisel J. Vitamin B-12 status, particularly holotranscobalamin II and methylmalonic acid concentrations, and hyperhomocysteinemia in vegetarians. Am J Clin Nutr. 2003 Jul;78(1):131-6.