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Looking across the sea – what does the US say about diet, cognition and mood?
The Dietary Guidelines for Americans are reviewed, updated and published every 5 years. The 2015 Guidelines – due to be published later this year – are advised by a report of the scientific evidence from an expert group, the Dietary Guidelines Advisory Committee (DGAC). This committee has now finished its review and this will be used to inform the recommendations in the influential guidelines.
In their review, the 2015 Dietary Guidelines Advisory Committee has reported on the relationship between nutrition and cognitive diseases and this month’s nutrition article takes a closer look at this.
As within the UK, neuro- and psychological function and disease is increasingly recognised as a high national priority for health promotion and chronic disease prevention. Two major components of neuropsychological function are cognition, the ability to reason, and mood, balanced and appropriate to enable optimal cognition.
Nutrition and cognitive impairments
The rising numbers of older adults and the potential human and financial cost of age-related cognitive impairments, such as Alzheimer’s disease and other dementias have helped drive national interest, both here and in the US. The theory that nutrition can reduce and/or play a role in the treatment of these diseases and their related burdens has been studied in relation to several nutrients and foods, including the B vitamins, vitamin E and selenium.
The long chain omega-3 fatty acids found in oily fish, eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) are among the most studied nutrients for neural health, in part because DHA is a major component of the brain, specifically grey matter and its synapses, and the specialised light detecting cells of the retina. DHA, in particular, supports the amplitude and signalling speed of neural response. EPA has emerged as a nutrient with suggested anti-depressive properties and studies to define its role in prevention and therapy are underway. Recently, data from a meta-analysis suggested them effective in patients with diagnosis of major depressive disorder (MDD) and in depressive patients without diagnosis of MDD. Indeed, the American Psychiatric Association as well as the UK Royal Society of Psychiatrists suggests that EPA and DHA supplements can be considered as a complementary therapy alongside conventional pharmacological treatment for major depressive disorder, although this has not reached national recommendations.
The increasing amount of science in this area allowed the DGAC to conduct two systematic reviews in answer to ‘What is the relationship between dietary patterns and risk of neurological and psychological illnesses?’ in order to inform the guidelines for healthy dietary patterns in the 2015 Dietary Guidelines.
Review 1: Cognitive Impairment, Dementia, and Alzheimer’s Disease
The goal of the first systematic review was to determine whether dietary patterns are associated with risk of age-related cognitive impairment, dementia, and Alzheimer’s disease.
This systematic review included 30 studies (28 published since 2008). Despite some inconsistencies in this body of evidence, common elements of dietary patterns were reported to be associated with measures of cognitive impairment, dementia, and/or Alzheimer’s disease:
- Patterns higher in vegetables, fruits, nuts, legumes, and seafood were generally associated with reduced risk of age-related cognitive impairment, dementia, and/or Alzheimer’s disease
- Patterns higher in red and/or processed meats were generally associated with greater age-related cognitive impairment. Relatively few studies reported on refined sugar and added salt, and patterns including these nutrients tended to report greater cognitive impairment with high intakes
Although the number of studies in this area is expanding, there are a number of issues that limit the ability to draw strong conclusions. The studies reviewed used a wide range of methodology in study design, definition and measurement of cognitive outcomes and assessments of dietary intakes.
A number of potential sources of error have been identified. For example, dietary patterns were derived using dietary intake measured at baseline only and, therefore, may not reflect patterns consumed before the study or changes in intake that may have occurred over the duration of the study. Similarly, several studies measured cognitive function only at a single time point and therefore could not assess change in cognitive function over time. Finally, not all potential but important confounders were always adjusted for; such as existing or family history of cognitive decline, dementia or Alzheimer’s disease and baseline health status, and this hampers the usefulness of the study conclusions.
DGAC concluded that there was limited evidence to suggest that a dietary pattern containing an array of vegetables, fruits, nuts, legumes and seafood consumed during adulthood is associated with lower risk of age-related cognitive impairment, dementia, and/or Alzheimer’s disease.
Review 2: Depression
The goal of the second review was to determine whether dietary patterns are associated with risk of depression.
The systematic review included nineteen articles all of which were published since 2008 that assessed the relationship between dietary patterns and depression. Despite the use of different methodologies and outcomes in this body of evidence, some protective dietary patterns emerged in adults:
- Patterns emphasising seafood, vegetables, fruits, and nuts, were generally associated with reduced risk of depression
- Patterns emphasising red and processed meats and refined sugar were generally associated with increased risk of depression
The ability to draw strong conclusions was limited by a number of significant issues, again including the use of a range of methodology in study design. For example there was considerable variability in how the outcome of depression was assessed, with some studies using various depression scales, some using physician diagnosis/hospital discharge records, and others using proxies such as use of depression medication. Diet was only measured at a single time point and may therefore not be reflective of the diet in the period of time the disease was developing.
DGAC concluded that limited evidence suggests that patterns emphasising seafood, vegetable, nuts and legumes are associated with lower risk of depression in men and women (excluding those with post-natal depression). More research is needed to determine whether dietary patterns are associated with risk of depression in vulnerable populations though, such as women in the post-partum period, children and adolescents, as well as those in various ethnic and cultural groups. Studies have not reliably assessed these subgroups and no conclusions can be drawn for them.
What are the Implications of this?
Dietary patterns emphasizing vegetables, fruits, seafood, legumes and nuts, similar to those that achieve chronic disease risk reduction, are consistent with maintaining neurocognitive health, including cognitive ability in healthy aging, and balanced mood. So make sure they are on your menus!
Ayela Spiro, British Nutrition Foundation – QCS Expert Nutrition Contributor