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.