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Nutrition and Dementia Revisited
This blog has previously looked at managing nutritional issues in dementia, and at the potential protective impact of a diet rich in oily fish or supplements of omega-3 polyunsaturated fatty acids. However, the recent publication of Alzheimer’s Disease International’s Nutrition and dementia includes a review of potentially preventative nutritional factors makes it timely to revisit this significant topic.
It is estimated that there are 7.7 million new cases of dementia each year worldwide. Reducing the incidence and slowing the progression of dementia is a health priority, and there is an understandable interest in identifying modifiable risk factors, such as nutrition.
Sufficient nutritional intake is important to prevent certain micronutrient deficiencies potentially harmful to the brain. Furthermore animal and biological models have suggested that nutrients can be important in modifying damage in the brain whether through antioxidant activity, reducing damaging oxidative stress in the brain, or through being able to effect production and activity of chemicals needed for brain function. In addition, epidemiological or observational studies have indicated certain differences between the diets of healthy older people, compared with those with dementia, such as increased fruit and vegetable intake. However, interpretation of such studies, particularly those with cross sectional designs, is difficult as it may be that people with dementia alter their dietary habits, so the observations are a consequence rather than a cause of disease. Or it could be that fruit and vegetable consumption is merely a marker for other healthy eating patterns. As followers of this blog will have heard repeated often, experimental evidence from randomised controlled trials (RCTs) provides the best evidence base. But even such trials have limitations in nutrition and dementia. Dementia is a complex disease with a long time lag between the beginning of the complex mechanisms that may lead to dementia and the actual appearance of clinical symptoms. RCTs may be difficult to conduct if they need to be implemented in midlife to delay or prevent dementia onset in later life.
What dietary patterns have been associated with lower risk?
A Mediterranean diet (with high intake of cereals, fruit and vegetables, fish and legumes) has been associated in some studies with decreased risk of heart disease and type 2 diabetes. It is thought a Mediterranean diet can also perhaps reduce risk of dementia through its effect on the vascular system in the brain, just as it does in the cardiovascular system, by protecting against oxidative stress or by limiting inflammatory processes that can damage the blood vessels. Studies from some (but not all) observational studies suggest an association between adherence to a Mediterranean diet and reduced dementia risk. One RCT in elderly patients (average age 74.1 years) conducted over 6 years showed a limited but positive effect of a Mediterranean diet supplemented with extra virgin olive oil on lower risk of mild cognitive impairment, but further intervention studies are needed to more fully understand the potential role for Mediterranean diets in dementia risk and the protective factors within it before clear recommendations can be made.
There are indeed many dietary factors that might plausibly decrease risk for the onset of dementia. However there is no clear and consistent evidence to date to support a causal protective role for the most cited nutrients in this area, namely vitamins B6, B12, C or E, folate or omega-3 polyunsaturated fatty acids.
The B vitamins, B6, folate and B12, have all been proposed to have protective effect on cognitive ageing, but despite the relatively large number of observational studies that have been conducted, there is insufficient evidence for an association. However, there is some consistent evidence that high levels of homocysteine (that can be caused by folate or B12 deficiency) are associated with cognitive decline. Although RCTS have shown that supplementation with B vitamins can reduce homocysteine levels, they have not demonstrated a significant impact on cognitive function.
Oxidative damage of cell components is thought to play a part in the development of dementia. Antioxidants may reduce the damage of oxidative species like free radicals. Hence, nutrients with antioxidant properties (such as vitamin C, E and flavonoids) are of interest in dementia prevention, with most research having focused on vitamin E. A beneficial association has been reported in observational studies where status has been assessed through vitamin E intake, measured using food frequency questionnaires rather than by more stringent biochemical methods. RCTs of vitamin E supplementation have failed to show clear and consistent benefit on cognitive decline or dementia incidence in those with mild cognitive impairment.
To supplement or not to supplement?
Overall there is currently insufficient evidence to confirm a relationship between the nutrients often reported as being beneficial to cognitive function in popular science. Although some poorer quality studies have shown positive results, preventive interventions (RCTs) have largely failed to provide any strong evidence to recommend the use of micronutrient supplements. The negative results may in part be due to methodological issues; studies are often small and of short duration. In addition, nutrients may be required synergistically (as they would be in a balanced diet) for healthy brain function. Moreover the majority of studies do not focus specifically on those with deficiencies. Thus, considerably more research into micronutrients and brain function is needed before we can make more meaningful conclusions.
Let’s not forget the impact of undernutrition
Whilst nutrient deficiencies and supplementation are more widely discussed, the issue of undernutrition in dementia is not well recognised, yet the evidence on the association between dementia and weight loss is compelling. Weight loss often accompanies dementia and may worsen the clinical course of the condition, lead to greater functional impairment and dependence, and increase the risk of morbidity, hospitalisation, institutionalism and ultimately mortality. Mechanisms that underlie this weight loss are likely to be multifactorial; dementia-related brain atrophy may impact on appetite control and energy balance, and altered feeding behaviours can severely disrupt dietary intake. While studies have shown that oral nutritional supplementations (ONS) can be used to maintain or improve weight among people with dementia, evidence for the benefit of ONS on cognitive function have not been described.
Eating is part of our everyday life and important to all, yet it is almost a totally neglected area of focus in relation to dementia. Ensuring nutritional status of people with dementia should be a priority, and, for those in residential care, one that should be assisted by clearly complying with CQC nutritional standards.
Ayela Spiro, British Nutrition Foundation – QCS Expert Nutrition Contributor