Cognitive decline is a significant problem of ageing with major health and socio-economic impacts; therefore, preventing or delaying the onset of cognitive disorders should be a public health priority. In particular, there is a need to identify modifiable factors that could be targeted to promote better brain health in ageing.
Undoubtedly, nutrition has an important role in preserving cognitive health and thus improving the quality of life in older age. The impact of specific nutritional factors on the brain in ageing is an area of active research.
Can B vitamins have a role in dementia prevention?
One of the most discussed nutrients in terms of brain health is the group of B vitamins. This largely stems from the hypothesis that high blood homocysteine may be a potential risk factor for dementia, and B vitamins can influence the level of homocysteine.
B vitamins have important functions in the body, but do we know whether they have a role in the prevention of mild cognitive impairment (MCI) and dementia?
The Scientific Advisory Committee on Nutrition (SACN), the independent body that advises the government on nutrition and related health matters are currently looking at this in a new position statement due to be published later in the year, and they have recently reported on their preliminary findings.
Research on vitamin B and cognitive function has been reviewed
SACN looked at the body of scientific literature on B vitamins like folic acid or folate (its natural form), vitamin B6 and vitamin B12, and cognitive function in adults. The committee found the evidence to be inconsistent. This may in part because of the complexity of nutrition trials, but also because the research studies typically use different doses and combinations of B vitamins, and the supplements are given for different lengths of time.
SACN specifically looked at systematic reviews and meta-analyses to try and get a stronger idea of what the effects may be. For example, a meta-analysis of randomised controlled trials (RCTs) exploring cognitive ageing in healthy older adults comparing B vitamins with placebo had no significant effect on a standard cognitive test called the Mini–Mental State Examination (MMSE). Another systematic review looking at observational cohort studies (a less robust type of study), in adults aged 45 years and above, found a significant association between greater B vitamin intake and a reduced risk of dementia. This review though has a great many limitations, making it difficult to draw any firm conclusions from this.
Vitamin B supplementation has also been looked at in people with MCI and dementia to see if they could improve cognitive function. In patients with MCI, a meta-analysis showed a small but beneficial effect of B vitamin supplementation on memory. However, the effect became non-significant when the data on a group with high plasma homocysteine were excluded. No significant differences in general cognitive function, executive function and attention were found in this analysis. In patients with Alzheimer’s disease (AD), B vitamins supplementation had no significant cognitive benefits either when measured with the Alzheimer’s Disease Assessment Scale or the MMSE.
Folic acid may be of particular interest
Folic acid is a B-vitamin of particular interest at the moment. There is some discussion as to mandatory fortification of flour with folic acid in order to reduce the risk of neural tube defect pregnancies. So any other implication this may have on the health of the population is important to consider.
In relation to cognitive health, the evidence for folic acid in adults comes from intervention (RCTs) and cohort studies typically undertaken in adults aged over 65 years. Intervention studies with folic acid alone or in combination with other B vitamins report mixed results but overall no consistent significant effects on any general or specific domains of cognition. Cohort studies consistently report either no significant relationships or that relatively higher folate status is associated with lower risks of cognitive decline and dementia.
13 studies looked at B-vitamins in the prevention and treatment of Alzheimer’s disease and dementia
One of the most quoted systematic reviews in this area, undertaken by Professor Alan Dangour and his team, looked at dietary intake (including supplementation) or blood levels of folate/folic acid alone or with other B vitamins and change in cognitive performance, dementia or AD. In total, thirteen studies – three prospective cohorts and ten RCTs, were identified.
The cohort studies considered dietary intakes of folate, vitamin B6 and vitamin B12 and risk of AD. Two of the three studies reported a significantly decreased risk of AD with higher folate consumption. One study also observed a significant association between higher intake of vitamin B6 and decreased risk of AD, but no association was found between vitamin B12 intake and AD in any of the studies. Although all participants were free of dementia at baseline, some studies did include people with poor cognitive performance and cognitive impairment.
With regards to the RCTs identified in this review, in three trials with folic acid supplementation alone in subjects without diagnosed dementia, statistically, significant improvements were reported in some of the memory and cognitive function tests. However, two of these trials also reported declines in some cognitive function tests. In the one folic acid trial on subjects with diagnosed dementia, no significant difference was found in the change in test scores between the folic acid and placebo groups. In the six trials that supplemented with folic acid in combination with other B vitamins, none of the trials reported improvements in cognitive performance.
Insufficient evidence for an association between B vitamins intake and cognitive impairment, cognitive decline or dementia
The authors noted that there is insufficient evidence to draw conclusions on the association between intakes of B vitamins and cognitive impairment, cognitive decline or dementias in either healthy adults or people with cognitive impairment or dementia. And really that currently is the conclusion to date – that reviewing all the scientific literature in this area there is no convincing evidence that supplementation or higher intakes in adult life is protective. But we do know that healthy dietary patterns are.