B vitamins and berries and age-related neurodegenerative disorders.

Objectives: To assess the effects, associations, mechanisms of action, and safety of B vitamins and, separately, berries and their constituents on age-related neurocognitive disorders-primarily Alzheimer's (AD) and Parkinson's disease (PD).

Data sources: MEDLINE and CAB Abstracts. Additional studies were identified from reference lists and technical experts.

Review methods: Vitamins B1, B2, B6, B12, and folate, and a dozen types of berries and their constituents were evaluated. Human, animal, and in vitro studies were evaluated. Outcomes of interest from human studies were neurocognitive function or diagnosis with AD, cognitive decline, PD, or related conditions. Intervention studies, associations between dietary intake and outcomes, and associations between B vitamin levels and outcomes were evaluated. Specific mechanisms of action were evaluated in animal and in vitro studies. Studies were extracted for study design, demographics, intervention or predictor, and neurocognitive outcomes. Studies were graded for quality and applicability.

Results: In animal studies, deficiencies in vitamins B1 or folate generally cause neurological dysfunction; supplementation with B6, B12, or folate may improve neurocognitive function. In animal experiments folate and B12 protect against genetic deficiencies used to model AD; thiamine and folate also affect neurovascular function and health. Human studies were generally of poor quality. Weak evidence suggests possible benefits of B1 supplementation and injected B12 in AD. The effects of B6 and folate are unclear. Overall, dietary intake studies do not support an association between B vitamin intake and AD. Studies evaluating B vitamin status were mostly inadequate due to poor study design. Overall, studies do not support an association between B vitamin status and age-related neurocognitive disorders. Only one study evaluated human berry consumption, finding no association with PD. Animal studies of berries have almost all been conducted by the same research group. Several berry constituents have been shown to affect brain and nerve tissue function. Blueberry and strawberry extract were protective of markers of disease, although effects on neurocognitive tests were less consistent. Berry extracts may protect against the deleterious effects of compounds associated with AD. Reporting of adverse events was uncommon. When reported, actual adverse events from B vitamins were rare and minor.

Conclusions: The current research on B vitamins is largely inadequate to confidently assess their mechanisms of action on age-related neurocognitive disorders, their associations with disease, or their effectiveness as supplements. B vitamin supplementation may be of value for neurocognitive function, but the evidence is inconclusive.