Mandatory fortification of certain food with folic acid was introduced in the US and Canada in 1998 with the primary outcome designed to be a reduction of birth defects. The programme has been successful in cutting the number of children born with neural tube defects by over 25 per cent.
Europe, on the other hand, has been dragging its feet on the issue of fortification, although a recent meeting between the Food Standards Association (FSA) and industrial lobby groups indicated that fortification could be soon be made compulsory.
Researchers from the US Centers for Disease Control and Prevention (CDC) compared stroke mortality rates in the United States and Canada for 1990 to 1997, and 1998 to 2002, with stroke mortality rates in England and Wales.
The population-based cohort study design calculated stroke mortality rates by using statistics from the national centres for health statistics in the respective countries.
Between 1990 and 1997, the rate of death from strokes was already in decline in the US (0.3 per cent). After introduction of the mandatory fortification program, the rate of death from strokes decreased significantly to 2.9 per cent.
In Canada, the fall in stroke mortality was even more pronounced, with an average decrease of one per cent for the period 1990 to 1997, and 5.4 per cent for 1998 to 2002.
In England and Wales, the researchers calculated that the mortality rate was actually greater for 1999 to 2002 than for 1990 to 1997.
"The epidemiological analysis presented here certainly does not establish causality, but the trends we observed are consistent with the hypothesis that folic acid fortification is contributing to a reduction in stroke deaths," wrote lead author Quanhe Yang in the journal Circulation (Vol. 113, pp. 1335-1343).
Medical records obtained during the various stages of the National Health and Nutrition Examination Survey (NHANES) allowed the researchers to evaluate blood levels of folate and total homocysteine concentrations, with elevated homocysteine levels being seen as an independent risk factor for stroke.
For the United States population, the levels of homocysteine decreased across all age groups and races studied after the introduction of fortification.
As could be expected, the folate levels in the blood increased significantly, giving more support for the hypothesis that folic acid can reduce stroke mortality.
The data do not establish causality, and this study is based on a review of old records - albeit relatively complete records.
"If folic acid fortification is responsible for even a fraction of the accelerated improvement we observed, this public health benefit is an important bonus to the reduction in neural tube defect rates previously demonstrated," said Yang.
The results are presented only days after the publication of two large intervention trials, the Heart Outcomes Prevention Evaluation (HOPE) 2 trial and the Norwegian Vitamin (NORVIT) trial, which both reported no benefits of B vitamin (including folic acid) supplementation on patients with heart disease or recovering from a heart attack.
The CDC researchers took a moment to consider the results of the NORVIT study and countered by saying the study "has already engendered considerable controversy because of its complex design and limited statistical power."
US fortification levels are set at 140 micrograms of folic acid per 100 grams of flour, while Canada has set a limit of 150 micrograms of folic acid per 100 grams.
Studies have shown that folic acid is more easily absorbed from fortified foods (85 per cent) and supplements (100 per cent) than the folate found naturally in foods (50 per cent).