In this large, randomized, double-blind, placebo-controlled trial with 7.3 years of treatment among 4,252 women at high risk for CVD, we found no significant effect of homocysteine-lowering treatment by a combination pill of folic acid and vitamins B6 and B12 on risk of type 2 diabetes. There remained no evidence for a treatment effect in a sensitivity analysis restricted to women compliant with their study pills over the follow-up period. Our study provides the first randomized trial data regarding the long-term effect of folic acid/B vitamin supplementation on the risk of type 2 diabetes, although our findings remain to be corroborated by future research.
It has been hypothesized that B vitamins may help reduce the risk of type 2 diabetes by ameliorating metabolic abnormalities, such as oxidative damage, inflammation, and endothelial dysfunction, which characterize all phases of insulin resistance and pancreatic β-cell function and are implicated in the development and progression of type 2 diabetes. Due to their relative safety and low cost, B vitamin supplements have been targeted as potential therapeutic agents in previous randomized controlled trials for prevention of vascular diseases in high-risk populations. In contrast, direct evidence from randomized trials of B vitamin supplementation for type 2 diabetes has been very limited. Some (20
) but not all (30
) secondary prevention trials have suggested that folic acid supplementation may be effective in the improvement of oxidative stress (23
) and endothelial dysfunction (20
) in patients with type 2 diabetes. To our knowledge, no large clinical trials have specifically examined homocysteine-lowering interventions on the primary prevention of type 2 diabetes. In the present study, we provide evidence that folic acid and B vitamins have a neutral effect on the risk of type 2 diabetes among nondiabetic women; this is consistent with the absence of benefit from this intervention in lowering risk of cardiovascular events (31
). Given the efficacy of the intervention in reducing homocysteine levels, our trial casts doubt on the etiologic role of hyperhomocysteinemia in the development of type 2 diabetes.
A major concern has been raised about the limited effect of folic acid treatment in a folate-fortified population. To assess the changes in homocysteine levels in response to background folate fortification in the U.S. population and to the randomized treatment with folic acid and B vitamins in our trial, Albert et al. (25
) have conducted an analysis of baseline and follow-up folate and homocysteine levels in a randomly chosen subpopulation (150 in the active group and 150 in the placebo group) of WAFACS participants. The prefortification prevalence of hyperhomocysteinemia (≥15.0 μmol/l) in our population (27.7%) was larger than the average prevalence estimates of increased homocysteine levels (≥13.0 μmol/l) for men and women (25%) in the National Health and Nutrition Examination Survey (32
) and in the Framingham population (19%) (33
). Despite significant elevation in plasma folate levels due to mandatory folate fortification, homocysteine levels changed relatively little over a 7-year period. In contrast, our folic acid/B vitamin intervention lowered homocysteine levels by ~18.5% (2.27 μmol/l) (25
). This reduction in homocysteine levels, however, was not associated with protection against the development of type 2 diabetes in this randomized trial. It is unknown whether a greater magnitude of homocysteine-lowering would have conferred protection against diabetes.
Genetic variations in enzymes involved in homocysteine metabolism may modulate the effect of treatment on risk of type 2 diabetes via their effects on circulating homocysteine levels (34
). Interindividual genetic variability in our study population is an unlikely explanation for our null findings, however, because genetic factors should have been comparable in the active treatment and placebo groups by the randomization procedure. There may be subgroups of individuals with B vitamin deficiencies or genetic variants in the pathway of homocysteine metabolism that could particularly benefit from homocysteine-lowering therapy. Future well-designed and large-scale genetic studies within randomized trial settings are warranted to test the hypothesis. Of interest, our prespecified subgroup analyses showed a trend toward decreased risk associated with folic acid/vitamins B6 and B12 among women with a family history of diabetes, but these findings may have been due to chance and need to be confirmed in future investigations.
Some limitations of our trial also deserve consideration. First, declining compliance over time in the trial may have diluted the findings. In sensitivity analyses restricted to women compliant with their study pills, however, the overall null effects and trends were unchanged. Second, the use of a combination pill did not allow us to investigate the effects of individual components or potential interactions among them in relation to type 2 diabetes. Third, misclassification in the self-reported diagnosis of type 2 diabetes is another concern. Since the proportions of undiagnosed cases are likely to be similar in the treatment and placebo groups due to effective randomization and double-blinding strategies, such a misclassification is more likely to be nondifferential but could have attenuated the results. Fourth, we did not measure homocysteine and folate levels in all participants to assess a modifying effect of baseline levels; however, we found no evidence of benefits of B vitamin therapy among individuals with different levels of dietary folate, vitamin B6, and vitamin B12 intakes at baseline. Fifth, confounding by extraneous risk factors cannot be completely excluded, although the baseline characteristics were well balanced in the treatment and placebo group, as expected in a large-scale trial with effective randomization. Finally, our results based on women at high risk for CVD may not be generalizable to men or to the general population.
In conclusion, in this study within a large, randomized, placebo-controlled trial among over 5,400 women at high risk for CVD, we observed no apparent benefit or harm of folic acid and vitamins B6 and B12 supplementation on the risk of type 2 diabetes over 7 years of treatment. Our findings do not support recommending B vitamin supplements for diabetes prevention, although additional evidence from future large trials in populations with moderate-to-severe hyperhomocysteinemia or in regions without grain fortification will be needed.