The "homocysteine-hypothesis" of vascular disease has attracted considerable interest, as total plasma homocysteine levels can be easily lowered by folic acid and vitamin B12, raising the prospect that cardiovascular disease could be lowered by such B-vitamin supplementation.
Two large B-vitamin intervention trials have been performed in Norway during the period 1998 to 2005, NORVIT and WENBIT, both registered at ClinicalTrials.gov, identifiers NCT00266487 and NCT00354081, respectively. The main objective in these trials was to study the effects of homocysteine-lowering therapy with folic acid and vitamin B12 to reduce the risk of cardiovascular events in patients with established coronary artery disease. The B-vitamin intervention, which included vitamin B6 in a 2x2 factorial design, was identical in the two trials. Follow-up was terminated for NORVIT on March 31st 2004 and for WENBIT October 5th 2005. Results from the NORVIT trial was published April 2006 {Bonaa, 2006} and preliminary results from the WENBIT trial were presented at the annual congress of the European Society of Cardiology September 4th 2007 {Zegers, 2007}. The WENBIT trial is completed and submitted for publication early in year 2008.
So far, none of the B-vitamin intervention trials have shown any statistically significant favourable effect of homocysteine-lowering therapy with folic acid with or without concomitant vitamin B12 on cardiovascular events {Bazzano, 2006}. In NORVIT there was even a trend towards an increased risk of cardiovascular events (myocardial infarctions) in patients receiving the combination of folic acid, vitamin B12 and vitamin B6. This trend was not observed in WENBIT. However, the treatment with folic acid / B12 was associated With a more rapid progression of angiographic coronary atenoses {Løland, 2010}. Thus, the "homocysteine-hypothesis" of vascular disease has been attenuated through the emergence of these negative trial results, whereas a potential harmful effect of the B-vitamin intervention has been revealed.
There is so far no data on possible long-term effects following years of B-vitamin supplementation. By combining analyses and follow-up in the NORVIT and WENBIT cohorts, we will probably have some more answers both considering possible subgroup and long-term effects of the B-vitamin intervention.
Current data indicate that folate prevents cancer, especially breast and colorectal cancer. However, during the last few years several reports have challenged this assumption. Swedish observational studies found increased risk of colorectal cancer at high blood folate levels {Van Guelpen, 2006} and increased risk of prostate cancer at high levels of folate and vitamin B12 {Hultdin, 2005}. In a randomised trial with folic acid versus placebo to prevent colorectal adenomas, one found increased risk of cancer in the group receiving folic acid, especially of prostate cancer {Cole, 2007}. In a long-term follow-up of women taking high doses of folic acid throughout pregnancy one found a doubled risk of deaths attributable to breast cancer {Charles, 2004}. Recently it has been hypothesized that the implementation of folic acid fortification of foods may have been wholly or partly responsible for the observed increase in colorectal cancer rates in the USA and Canada in the mid to late 1990s {Mason, 2007}. This has led to new hypotheses that folate may prevent carcinogenesis but may enhance the growth of established cancer cells {Ulrich, 2007}. The question of possible adverse effects of folic acid supplementation will be of major importance when public health administrations decide whether to implement or enhance programs folic acid fortification of foods.
The effect of the B-vitamin intervention will also be studied in relation to other life-style diseases like diabetes and osteoprosis.
Additionally, the combined NORVIT-WENBIT cohort will used for observational studies evaluating new risk phenotypes or genotypes and their potential effect modification by the B-vitamin interventions.
