Although increased intake of folic acid and vitamin B_several is effective in decreasing homocysteine levels, the combined intervention of these B vitamins did not lower risk for CVD. Indeed, several randomized, placebo-controlled trials have been conducted to determine whether homocysteine-lowering through folic acid, vitamin B₁₂, and vitamin B_six supplementation reduces the incidence of CVD. A recent meta-analysis of data from 11 trials, including nearly 45,000 participants at risk of CVD, showed that B-vitamin supplementation had no significant effect on risk of myocardial infarction (heart attack) or stroke, nor did it modify the risk of all-cause mortality (25). Other meta-analyses that included patients with chronic kidney disease have confirmed the lack of effect of homocysteine-lowering on risk of myocardial infarction and death. However, stroke risk was significantly reduced by 7%-12% with the B-vitamin supplementation (26, 27). Another meta-analysis of 12 clinical trials measuring flow-mediated vasodilation (FMD; a escort services in Everett surrogate marker of vascular health) in response to homocysteine reduction revealed that B-vitamin supplementation was accompanied by an improved FMD in short-term <8 weeks) but not in long-term studies conducted in subjects with preexisting vascular diseases (28). Yet, some of the studies included in these meta-analyses did not use vitamin B₁₂, and folate administration on its own has shown a protective role on vascular function and stroke risk (29). Besides, the high prevalence of malabsorption disorders and vitamin B₁₂ deficiency in elderly individuals might warrant the use of higher doses of vitamin B₁₂ than those used in these trials (30); in cases of malabsorption, only high-dose oral therapy or intramuscular injections can overcome vitamin B₁₂ deficiency (4).

Disease

Folate is required for synthesis of DNA, and there is evidence that decreased availability of folate results in strands of DNA that are more susceptible to damage. Deficiency of vitamin B₁₂ traps folate in a form that is unusable by the body for DNA synthesis. Both vitamin B₁₂ and folate deficiencies result in a diminished capacity for methylation reactions (see Figure 3 above). Thus, vitamin B₁₂ deficiency age and altered methylation of DNA, both of which are important risk factors for cancer. A series of studies in young adults and older men indicated that increased levels of homocysteine and in B₁₂ in the blood were associated with a biomarker of chromosome breakage in white blood cells (reviewed in 31). In a double-blind, placebo-controlled study, the same biomarker of chromosome breakage was minimized in young adults who were supplemented with 700 ?g of folic acid and 7 ?g of vitamin B₁₂ daily in cereal for two months (32).

Cancer of the breast

A case-control study compared prediagnostic levels of serum folate, vitamin B₆, and vitamin B₁₂ in 195 women later diagnosed with breast cancer and 195 age-matched, cancer-free women. Among postmenopausal women, the association between blood levels of vitamin B₁₂ and breast cancer suggested a threshold effect. The risk of breast cancer was more than doubled in women with serum vitamin B₁₂ levels in the lowest quintile compared to women in the four highest quintiles (33). However, the meta-analysis of this study with three additional case-control studies found no protection associated with high compared to low vitamin B₁₂ serum levels (34). A case-control study in Mexican women (475 cases and 1,391 controls) reported that breast cancer risk for women in the highest quartile of vitamin B₁₂ intake (7.3-7.7 ?g/day) was 68% lower than those in the lowest quartile (2.6 ?g/day). Stratification of the data revealed that the inverse association between dietary vitamin B₁₂ intake and breast cancer risk was stronger in postmenopausal women compared to premenopausal women, though both associations were statistically significant. Moreover, among postmenopausal women, the apparent protection conferred by folate was only observed in women with the highest vitamin B₁₂ quartiles of intake (35). However, more recent case-control and prospective cohort studies have reported weak to no risk reduction with vitamin B₁₂ intakes in different populations, including Hispanic, African American and European American women (36, 37). A meta-analysis of seven case-control and seven prospective cohort studies concluded that the risk of breast cancer was not modified by high versus low vitamin B₁₂ intakes (34). There was no joint association between folate and vitamin B₁₂ intakes and breast cancer risk. ₁₂ status and breast cancer. In addition, results from observational studies are not consistently in support of an association between high dietary folate intakes and reduced risk for breast cancer (see the article on Folate). There is a need to evaluate the effect of folate and vitamin B₁₂ supplementation in well-controlled, randomized, clinical trials, while considering various factors that modify breast cancer risk, such as menopausal status, ethnicity, and alcohol intake.