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Observational studies suggest a link between high serum concentrations of homocysteine and venous thromboembolism (VTE)(VTE).. If the link is causal, then reducing homocysteine concentrations should help prevent deep vein thromboses and pulmonary emboli. But trials of vitamin supplements designed to do just that have been disappointing.
The latest trial randomised 5522 middle aged and older men and women to take a supplement of folic acid, vitamin B6, and vitamin B12 or placebo for five years. In each group, 44 participants had a symptomatic deep vein thrombosis or pulmonary embolus (or both) during follow-up (incidence rate 0.35 per 100 person-years in each group; hazard ratio 1.01; 95% CI 0.66 to 1.53). The supplement did not prevent venous thromboembolism overall, despite bringing down homocysteine concentrations compared with placebo. Neither did it work for any subgroup, including patients with the highest serum concentrations of homocysteine at baseline.
All the men and women in this trial had cardiovascular disease, or at least two major risk factors including diabetes. Both groups started the trial with mean serum homocysteine concentrations of 11.5 µmol/l, and nearly three quarters lived in Canada and North America, where folic acid is added to bread flour.