Furthermore, there is ever increasing substantiation that EXCESS VITAMIN B12 is CARCINOGENIC (i.e. CAUSES CANCER). For example, see the following:
Sem Hop. 1970 Jul 10;46(31):2170-4.
[Hazards of administering vitamin B12 to cancer patients].
Cancer incidence and mortality after treatment with folic acid and vitamin B12.
Treatment with folic acid plus vitamin B(12) was associated with increased cancer
Circulating folate, vitamin B12, homocysteine, vitamin B12 transport proteins, and risk of prostate cancer: a case-control study, systematic review, and meta-analysis.
CONCLUSION:
Vitamin B(12) and (in cohort studies) folate were associated with increased prostate cancer risk.
I'm responding to a two-year-old post because I just found this thread and am slowly making my way through it. I learned a lot from your original post that was immediately useful, and I thank you for taking the time to gather all that information and make it available here.
It's hard to know what to conclude from the first three studies, since I couldn't find how much of what form of cobalamin was given, and for how long. To further confuse matters, two of them also included treatment with folic acid.
The last study was more informative. The first thing to note is that it measured folate and B12 levels in a blood sample, not amount consumed via diet or supplementation. In fact fewer than 10% of all the men studied had taken any sort of supplement in the month preceding the study, and the proportion was virtually identical in men with and without cancer.
High circulating levels of a nutrient certainly can come from over-supplementing, but they can also come from an inability to use the nutrient in metabolism, e.g. high levels of phenylalanine in PKU. This second possibility has some support in the study, which states:
(H)igh circulating concentrations of holo-haptocorrin were associated with increased risk, and high circulating concentrations of total-transcobalamin with decreased risk of prostate cancer. [snip] [T]he plausibility of reverse-causation as an explanation for our findings is perhaps reinforced by the absence of an association of holo-transcobalamin, representing the bioavailable fraction of B12, with prostate cancer risk.What the men with prostate cancer had more of is B12 bound to haptocorrin, a transport/storage molecule that cannot be incorporated into cells. What they did not have more of was bio-available B12; in fact levels of that were virtually identical between cases and controls. The authors conclude in the final paragraph that their findings could indicate a reverse connection (the cancer caused the high B12), and they give several possible mechanisms for this.The most they have to say regarding the possibility of high B12 causing prostate cancer is that their study didn't completely eliminate it.
J Clin Pathol. 1993 Jun;46(6):537-9. Correlations between holo-transcobalamin II, holo-haptocorrin, and total B12 in serum samples from healthy subjects and patients.Department of Haematology, St Mary's Hospital Medical School, Imperial College of Science, Technology and Medicine, Norfolk, Lon CONCLUSIONS: The total serum B12 concentration is a relatively poor indicator of holo-TCII concentrations and, therefore, of the ability of serum to deliver B12 to tissues.