In my book on, in his book on B12 deficiency, Chris says a high serum B12 does not necessarily rule out functional active B12 deficiency. In fact, I’ve come to view high serum B12 in the absence of supplementation as a potential red flag for active B12 deficiency. Can you explain a little bit more about what the mechanisms are here?

Chris Kresser:  I don’t know that that’s fully understood, but I have seen it mentioned in the scientific literature. One possibility is that the inactive B12 is not being, which is the less active forms of cobalamins, B12 are not being converted into more active forms. And so you can see the cobalamins in the blood, but they’re not actually entering the cell, perhaps, where they’re used. That’s one possible mechanism that I can think of.

Generally, when you see high B12 on the blood test, it’s going to be because they’re supplementing, may also be if it’s just very mildly above the range, it may be that they are eating a really nutrient-dense diet, particularly with organ meats or shellfish. And in a minority of cases, you will see some people who are not supplementing or not eating a particularly large amount of B12 that have high serum B12, and when you test urine methylmalonic acid or homocysteine or serum methylmalonic acid or whole OTC. If you have access to it, you’ll often find that those patients have low levels of active B12.