In one study, 50 healthy vegetarian men and women, average age 46, took 500 mcg of vitamin B12 per day and then a placebo, or the reverse, in two 12-week phases. Participants had been on a vegetarian diet for at least six years, and 70 percent started the study with low levels of vitamin B12.
After the vitamin B12 phase, but not the placebo phase, blood levels of vitamin B12 rose significantly and homocysteine levels—a sign of inflammation linked to heart disease—were significantly lower. The ability of blood vessels to expand also increased by about 10 percent, and thickness of the carotid artery walls decreased by 0.02 mm, making them more flexible.
The carotid arteries of the neck are similar to coronary arteries of the heart, suggesting vitamin B12 benefits the heart as well.