There is increasing interest into the role of dietary patterns on many health and cognitive outcomes in nutrition research. Dietary patterns, also sometimes referred to as whole-of-diet analysis can include the assessment of an overall diet style such as the Mediterranean diet (185) and other healthy diets informed by national dietary guidelines (186), or multivariate statistical approaches to characterise diets using reported dietary intake (187). By assessing overall diet, this approach acknowledges differences in combinations of food consumption and the importance of examining the balance of the various components of dietary intake, rather than just focusing on single foods and nutrients in isolation (188). While there are a number of
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comprehensive reviews on the role of various dietary patterns in maintaining health and preventing various health conditions, the following section will provide a brief overview of the current literature on the Mediterranean diet, Western-style diet, diets, vegan/ vegetarian diets and diets rich in fruits and vegetables on cognitive function. 2.1.1 Mediterranean Diet
The Mediterranean diet, which is characterised by a high intake of fruits, vegetables, legumes, nuts and fish, a high intake of monounsaturated fatty acids, omega-3 fatty acids (and low saturated fatty acids), a low-to-moderate intake of dairy products, red meat, poultry and eggs, and a moderate intake of alcohol, particularly wine, has been associated with a reduced risk of cognitive decline (39-41). Indeed, one review of cross-sectional and prospective studies concluded that the Mediterranean diet is protective against cognitive decline attributed to vascular or degenerative origin (189). Further, another review of prospective studies found that higher adherence to the Mediterranean diet in older adults was beneficial for reducing the risk of developing MCI, and was also associated with better global cognitive performance, particularly for episodic memory (190). These findings are supported by the results from a large primary prevention trial for CVD in participants at high vascular risk (PREDIMED) which found that in a sub study of 522 participants using data from PREDIMED-NAVARRA centre, those allocated to a Mediterranean type diet supplemented with either extra virgin olive oil or nuts for 6.5 years had significantly higher global cognitive function scores after the intervention when compared to participants in the low-fat control diet group (mean differences in MMSE scores ranging from 0.51-0.62 between Mediterranean diet plus olive oil vs control diet and 0.33-0.57 for Mediterranean diet plus nuts vs control diet for tests of cognitive function) (191). More recently, another sub study of PREDIMED using data from cognitively normal adults from the Barcelona-North study centre found less decline in the global cognitive composite score after a median follow-up of 4.1 years for those assigned to both Mediterranean diet groups (mean change in MMSE scores for Mediterranean diet plus olive oil 0.05 and -0.05 for addition of nuts) compared to the control diet (mean change in MMSE scores of -0.38) (192). In contrast, a 2015 systematic review on the role of the Mediterranean diet on cognitive function found that overall, findings to date have been mixed, with only four of six cross-sectional studies and only half of the longitudinal studies along with one RCT and three meta- analyses reporting that better adherence to the Mediterranean diet was associated
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with less cognitive decline and dementia (193). These mixed findings were attributed to potential differences in the classification of the dietary patterns in different countries, which may be related to the use of various different assessment tools for dietary intake (193). On this basis, it is evident that further studies are still needed to evaluate whether the Mediterranean diet can improve cognitive function.
It has been suggested that the mechanism by which the Mediterranean diet might reduce the risk for cognitive decline (or improve cognitive function) in older adults may relate to the high intake of anti-oxidants associated with this diet, which may reduce oxidative stress that has been linked to impaired cognitive function (194). It may also relate to the reduction or limiting of foods which are high in sugar, saturated fats and salt which have been linked to a decreased risk for cardiovascular risk factors (195) that are also associated with cognitive decline. However antioxidants and saturated fats can also be found in a variety of other foods in other dietary patterns (such as the Western-style diet) and the role of these nutrients in regards to cognitive function will be discussed in section 2.2.
2.1.2 Western-style Diet
In contrast to the Mediterranean diet, Western-style diets, which are typically characterised by a higher intake of animal and saturated fats, refined sugars and lower intakes of fruits and vegetables, have been associated with poorer cognitive function (42-44). For instance, a 3-year prospective study found that higher adherence to a Western-style dietary pattern was associated with a decline in visuospatial abilities in older adults (196) compared to a Mediterranean diet pattern. Principal component analysis of data from the AusDiab study, a population-based national survey of non-institutionalised Australian adults, also found that foods associated with a Western-style diet was negatively associated with cognitive function (197). The potential mechanism by which a Western-style diet may adversely influence cognitive function may be relate to an increase in pro- inflammatory markers and oxidative stress, both of which have been strongly implicated in cognitive decline (44, 198). However, one review stated that the majority of the current research investigating the relationship between the Western- style diet and cognitive function has primarily focused on the role of saturated fats and simple sugars despite the many other macro and micro-nutrients which could also play a role (e.g. protein, salt) (44). This narrow view may be contributing to a
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negative bias and an inaccurate overview of the effects of the Western-style diet on cognitive function overall.
2.1.3 Vegan/ Vegetarian Diets
Vegan and vegetarian diets, in a similar way to the Mediterranean diet, are rich in antioxidants and fibre and low in saturated fats, but do not include seafood, meat or meat products (products resulting from the processing of meat) (199). While these diets have been shown to be associated with a lower risk of diabetes, CVD and obesity (199), there is other evidence which suggests that these diets may lead to deficiencies in vitamin B12, creatinine and omega-3 fatty acids (200-202), which have been found to be protective of cognitive decline. Further, another five day study conducted in 128 young women which supplemented vegetarians with 20g of creatinine, found improvements in memory compared to non-vegetarians who were also supplemented (202). While this suggests that deficiencies in particular nutrients related to vegan and vegetarian diets may cause these diets to be associated with an increased risk for cognitive decline, there appears to be very limited studies which have specifically investigating the effect of vegetarian or vegan diets on cognitive function. However, one cross-sectional study in 404 adults aged 60 years and older with and without MCI found that being vegetarian increased the risk for cognitive impairment four-fold compared to non-vegetarians (203).
2.1.4 Fruits and Vegetables
Diets rich in fruits and vegetables have been found to be associated with lower rates of heart disease and stroke, which are known risk factors associated with cognitive decline (204, 205). However, as identified in a 2012 review there is currently limited evidence for a beneficial effect of increased fruit and vegetable intake on cognitive function (206). Most of the evidence supporting a positive effect appears to be derived from cross-sectional studies (206). When looking at the findings from longitudinal cohort studies, there are mixed findings with some studies only reporting benefits for either vegetables or fruits but not both (206). In a prospective study using data provided over a 10-16 year period from healthy women aged over 70 years (with a median vegetable intake of 3.1 serves and 2.4 serves for fruits), it was found that those who had higher intakes of green leafy vegetables had slower rates of cognitive decline (0.05 units which is the equivalent of being 1-2 years younger in age) than those who had lower intakes (205). It was also found that the
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difference in cognitive function between the lowest and the highest quintiles for total vegetable intake was equivalent to being 1.5 years younger in age (205). However there were no significant findings in regards to fruit intake (205). Consistent with these findings, principal component analysis using data from the AusDiab study described above, found that for every one unit increases in foods contributing to a fruit and vegetable pattern, there was a 1.06-fold decrease in the odds of having cognitive impairment (197).
In a similar fashion to the Mediterranean diet, the cognitive benefits associated with fruits and vegetables may be attributed to antioxidants. There are also a number of vitamins in fruits and vegetables such as B vitamins, vitamin C and vitamin E which may also play a role (207, 208) and this is discussed further in the next section. 2.1.5 Summary of dietary patterns and effects on cognitive function
Dietary patterns can provide a holistic overview of dietary intake and are therefore likely to portray realistic effects of foods. Overall, current evidence suggests that there may be some benefits of the Mediterranean diet on cognitive function but the evidence is not conclusive. In contrast, the Western-style diet has been associated with increased oxidative stress and inflammation which may contribute to an increased risk for cognitive decline, but again the current evidence is mixed. The focus of studies on saturated fats and simple sugars may be contributing to an overall negative view of the Western-style diet on cognition. While diets rich in fruits and vegetables have been consistently associated with a reduced risk for factors associated with cognitive decline such as heart disease, studies have found mixed findings for the effects of fruits and vegetables on cognitive decline itself. Further, there have been no specific studies looking at vegan/vegetarian diets and their effect on cognitive function.