For the first time, researchers have found a potential relationship between
vitamin B-12 and the risk of breast cancer.  This case controlled study
compared blood levels of vitamin B-12 in women who later developed breast
cancer with women who remained cancer free.

As it turned out, the women with vitamin B-12 levels in the lower one-fifth
were at increased risk of breast cancer compared to women in the other
four-fifths of vitamin B-12 levels.  This suggests that there might be a
threshold level for vitamin B-12 below which increases in the risk of breast
cancer....researchers suggest that vitamin B-12 deficiency could increase
the risk by allowing DNA strand breaks or altering the structural stability
of DNA.  (Choi S. Vitamin B-12 deficiency:  A new risk factor for breast
cancer?  Nutr. Rev. 1999; 57:250-60.

Also interesting.

Rachel