Strongest evidence: The most robust finding is for reduced homocysteine levels, with high evidence strength from 7 studies (6 beneficial, 1 neutral). Effects were small to moderate, observed at doses around 1 mg/day over 8–12 weeks. For increased folate levels, evidence is moderate (3 studies, 2 beneficial with large effect size, 1 neutral), also at 1 mg/day.
Mixed or weaker evidence: No outcomes with low or very low evidence strength were identified in this synthesis. The folate increase finding is based on only 3 studies, so conclusions are preliminary.
Effective dose patterns: Both outcomes converge on a dose of 1 mg/day. The median study duration for both outcomes was 84 days, suggesting that at least 12 weeks of supplementation is typical for observing effects.
Population insights: Beneficial effects on homocysteine were seen in healthy adults, patients with coronary heart disease, type 2 diabetes, and older adults (50–65 years). For folate levels, studies included healthy adults and individuals with phenylketonuria. One study in children with sickle cell disease showed no benefit, indicating that results may not generalize to pediatric populations or those with certain comorbidities.
Notable caveats: The homocysteine literature is subject to publication bias (null results are less likely to be published). Most trials were short (4–12 weeks), so longer-term effects are less documented. For folate increase, one study used rice fortification with multiple micronutrients, not direct supplementation, and another used l-5-methyltetrahydrofolate, which may not represent all forms of vitamin B9.