Our meta-analysis, including 13 RCTs with over 39,000 participants, found a trend toward mild benefit that did not reach statistical significance of folic acid supplementation on the risk of stroke among high cardiovascular risk persons. This result contrasts with the prior meta-analysis conducted in 200713
which showed a positive result for stroke reduction. The main difference is the inclusion in our analysis of four large trials, encompassing 21,881 participants, that have been completed since 2007 which generally found neutral effects.
Several subgroup analyses suggested potential benefit in particular settings, but must be regarded as hypothesis-generating given the multiplicity of analyses performed. Multiple analyses suggested that trials using treatment regimens of greater intensity or duration were of modest benefit, including trials in which the active treatment consisted of vitamins B6 and B12 in addition to folic acid, trials with substantial reductions in homocysteine levels in the active arm, and trials with longer treatment duration. However, meta-regression analyses were discordant, failing to show a linear relationship of homocysteine lowering or treatment duration to stroke prevention.
We also found that active treatment showed clinical benefit in trials in which men predominated among enrolled patients. This result is concordant with observational studies which showed higher folate intake is associated with reduced ischemic stroke risk in male US health professionals and male Finnish smokers,15, 16
but not female US nurses.14
Potential explanations for this gender interaction are that men have higher stroke event rates, increasing study power to detect treatment effects, and gender differences in the severity and treatment-responsiveness of hyperhomocysteinemia, both greater in men than women.
A mild benefit of folic acid supplementation was shown in trials in which stroke was not a qualifying event but not in secondary prevention trials. This observation raises the possibility that homocysteine lowering is beneficial at early stages of vascular disease elaboration, but less effective in the face of established, advanced disease. In a study with participants without diabetes or cardiovascular disease, B vitamin supplementation significantly reduced progression of early-stage subclinical atherosclerosis (carotid artery intima media thickness) but had no effect on progression of aortic or coronary artery calcification, markers of late-stage atherosclerosis.11
One explanation that has been advanced for neutral results in individual trials of folate therapy is that spread of fortification of the food supply with folic acid increased the background dietary intake of folate among all enrolled patients, diminishing the potential for the active treatment arm to exert a treatment effect. A physiologic study found that low-dose (0.4mg/d) folic acid treatment, comparable to daily intake and dietary fortification, improved vascular endothelial function and high-dose (5mg/d) folic acid treatment provided no additional benefit.44
Whereas plasma 5-methyltetrahydrofolate levels increased proportionately with treatment dose of folic acid, vascular tissue 5-methyltetrahydrofolate showed no further increment with high-dose compared with low-dose folic acid.44
These findings may partly explain the improvement in stroke mortality after folic acid fortification in Canada and the United States.45
Our meta-analysis found a trend toward treatment benefit in trials conducted only or partly in countries without background fortification of the food supply, but no evidence of benefit of pharmacologic folate therapy in trials performed wholly in countries with fortification.
Some studies reported that homocysteine reduces the concentration of high density lipoprotein (HDL) cholesterol in plasma by inhibiting the hepatic synthesis of apolipoprotein A1 (apoA-I), the main HDL apolipoprotein.46, 47
These studies not only explain the documented inverse correlation between the plasma concentrations of HDL cholesterol and homocysteine,48
but also raise the real possibility that a homocysteine-induced inhibition of apoA-I synthesis is the mechanism linking homocysteine to the development of atherosclerosis. A low concentration of HDL cholesterol has been shown in epidemiological studies to be predictive of stroke49
and treatment that increase the level of HDL cholesterol in plasma may related to regression of atherosclerosis.50
Since most participants in our meta-analysis were high cardiovascular risk persons and intensive lipid profile intervention was likely to be applied, the additional benefit for homocysteine lowering might be difficult to demonstrate. On the other hand, a meta-analysis of prospective observational studies did not adjust for HDL cholesterol levels and it is likely to overestimate the association of homocysteine and CVD.7
Some limitations need to be mentioned. Meta-analysis is retrospective research that can be constrained by comprehensiveness of searches, methodological rigor of the included studies, and publication bias. We tried to maximize study identification and minimize bias by developing the study protocol a priori, performing a thorough search of several databases, and using explicit criteria for study selection, data collection, and data analysis. An additional limitation of meta-analysis is that vary with respect to the characteristics of participants, duration and intensity of treatment, the type of cerebrovascular event identified and the expertise of stroke adjudicators, and other design features. However, formal testing did not identify any substantial resulting heterogeneity among trial findings.
In conclusion, meta-analysis of completed trials does not demonstrate a major benefit of folic acid supplementation in averting stroke. However, potential mild benefits were observed in primary stroke prevention, especially when folate is combined with B vitamins and in male patients. As folic acid supplementation is an inexpensive, safe, and widely applicable intervention, further investigation in these settings is warranted.