EatQOD

Even though a number of papers have shown that B12 levels are low in older people and that B12 is poorly absorbed, some scientists in Sweden have linked higher vitamin B12 levels in the blood with a higher risk of advanced prostate cancer.

It is not clear at this time if this is due to supplementation, pattern of food intake, or some genetic pattern that may lead to higher B12 levels.

But if you are taking vitamin B12 supplementation, and especially, higher level supplementation to make sure it is being absorbed, this is a note of caution.

A link to the abstract of the most recent paper is here:
http://www.ncbi.nlm.nih.gov/pubmed/18268110

Their conclusion says: CONCLUSION: This study does not provide strong support for an association between prostate cancer risk and circulating concentrations of folate or vitamin B(12). Elevated concentrations of vitamin B(12) may be associated with an increased risk for advanced stage prostate cancer, but this association requires examination in other large prospective studies.

So it's still a bit iffy.

This time relative to colon cancer. Again the problem seems to be methylation induced by the B vitamins potentially increasing cancer risk. So like anything else, with some of these vitamins, there appears to be a potential downside.

http://www.eurekalert.org/pub_releases/2008-09/wjog-wit_2092408.php

is the link.

"They found that 29.1 % of cases had tumors with at least one methylated gene promoter. In case-case comparison, no significant association was found between methylation in tumors and any single genotype. However, in comparison to controls with the CC genotype, an increased risk of tumor methylation was associated with the CT genotype (OR = 2.5; 95%CI, 1.1 – 5.6). In case-case comparisons, folate/vitamin B12 was positively associated with tumor methylation. Adjusted odds ratios for tumor methylation in cases with high (above median) versus low (below median) serum folate/vitamin B12 levels were 4.9 (95%CI, 1.4 – 17.7), and 3.9 (95%CI, 1.1 – 13.9), respectively. The frequency of methylated tumors was significantly higher in high methyl donor than low methyl donor group, especially in those with MTHFR CT (p = 0.01), and CT/TT (P = 0.002) genotypes but, not in those with the CC genotype (P = 1.0).

The results from this study indicate that high concentrations of serum folate/vitamine B12 are associated with the risk of promoter methylation in tumor-specific genes, and this relationship is modified by MTHFR C677T genotypes. These results support other recent reports that high folate and vitamin B12 status might serve as risk factors for CRC."