Homocysteine and B vitamins

High homocysteine levels may be caused by low folate and vitamin B6 levels (1).  B12 levels are also a contributing factor (2). Homocysteine accumulates when it is not converted to methionine in methylation reactions that requires vitamin B12 and folate (2). Additionally, via the Transsulfuration pathway, homocystein will convert to cystathione via vitamin B6 dependant enzymes. (2)

Here is a diagram of this conversion of homocysteine to cysteine (2):

So we see that deficiency in folate, b12 or b6 can lead to elevated Homocysteine levels. Studies have shown that supplementing with these vitamins reduces homocysteine levels. (1,2). 

Sources:

(1) Schnyder G et al. Effect of homocysteine-Lowering Therapy with Folic Acid, Vitamin B12, and Vitamin B6 on Clinical Outcome after PTCA. JAMA 2002; l288:973.

(2) Vitamin B6. Oregon State, Linus Pauling Institute, Miconutrient Center. 

https://lpi.oregonstate.edu/mic/vitamins/vitamin-B6#cardiovascular-disease-prevention (Links to an external site.)Links to an external site.