We examined associations between two distinct dietary patterns, i.e. ‘whole food’ (rich in fruit, vegetables, dried legume and fish) and ‘processed food’ (rich in processed meat, chocolates, sweet desserts, fried food, refined cereals and high-fat dairy products), and cognitive deficit in a middle-aged population. In the fully adjusted models, but without taking into account the influence of education, our results suggested that the ‘whole food’ pattern was associated with lower and the ‘processed food’ pattern with increased odds of cognitive deficit. However, adjustment for education considerably attenuates these associations, suggesting that education is an important confounder in the association between nutrition and cognition.
While dietary patterns have been investigated inrelation to several chronic diseases such as cardiovascular diseases [23
], or diabetes [24
], studies on the relation between dietary patterns and cognitive functioning are less frequent. One exception is a recent study [4
] that examined the association between dietary pattern, using dietary indices, and the risk of Alzheimer's disease and cognitive decline in an elderly population. They showed that high adherence to a Mediterranean diet [25
] decreased the risk of cognitive decline and Alzheimer's disease in a nondemented, multiethnic elderly cohort (n = 2,258, mean age 77.2 ± 6.6 years). This association remained significant after adjustment for education. The use of an ‘a priori’ definition like the Mediterranean score presents the inconvenience of weighting equally the underlying individual food component categories, which, in turn, are composed of a number of food constituents. Using an ‘a posteriori’ factor analysis, our results, unadjusted for education, support those reported using the Mediterranean diet [4
] by suggesting that a diet rich in fruits, vegetable and fish is associated with lower odds of cognitive deficit while a diet rich in processed meat, chocolates and sweeteners, desserts, fried food, refined grains and high-fat dairy products is associated with greater odds of cognitive deficit.
In our analysis, the diet and cognition relationship remained unchanged after adjustment for sex, age, energy intake, marital status, physical activity, smoking habits, chronic diseases (diabetes, dyslipidemia, CHD, hypertension), BMI and mental health. Our finding, before adjustment for education, of a relationship between the ‘whole food’ dietary pattern and cognitive deficit is supported, partly, by results of two prospective studies that found high intake of vegetables to be associated with a slower rate of cognitive decline at older ages [26
]. The beneficial effect of fruits and vegetables on cognition could be a result of high amounts of antioxidants in these foods. However, the literature on the association between antioxidant levels in the blood or estimated from food intake and cognitive performances or dementia is inconsistent and dependent on the specific nutrient examined [28
]. Our ‘whole food’ dietary pattern also included a high intake of fish and there is consistent evidence to support this finding. Many studies have shown high fish consumption to be associated with low incidence of dementia [29
] including Alzheimer's diseases [29
], slower cognitive decline in elderly [32
] and lower cognitive impairment in a middle-aged population [34
]. The protective effects of fish consumption has been traditionally attributed to its high content in long-chain omega-3 polyunsaturated fatty acids which are a major component of neuron membranes and have vascular and anti-inflammatory properties [35
]. Then, the association between the ‘whole food’ diet and cognition observed in our study could be explained by the cumulative and synergic effect of nutrients from different sources of foods rather than by the effect of one isolated nutrient.
The ‘processed food’ factor described in our study was highly loaded by sweets, desserts, fried food, processed food, refined grain products and high-fat dairy products and was very close to the ‘Western’ pattern defined in the American population [36
] which has been shown to be correlated with markers of systemic inflammation [37
]. Several lines of investigation have suggested that inflammation is involved in the pathogenesis of dementia [38
]. However, the association between inflammation and cognition is still under debate [43
] and more studies are needed to better understand the associationbetween the ‘processed food’ intake, inflammation process and cognition.
In this middle-aged British population, we showed education to influence the relationship between dietary pattern and cognition. The test for interaction suggests that it does not moderate the association between dietary factors and cognition, in that the diet-cognition association is similar in high- and low-education groups. The attenuation of the diet-cognition association after adjustment for education is a statistical result and could suggest two things. One, that education mediates the association between dietary factors and cognition in that dietary factors influence education which then influences cognition. However, the first part of this causal chain is unlikely as education was assessed prior to the dietary measures, and it appears unlikely that dietary factors influence education in this way. The second explanation for the substantially attenuated association between dietary patterns and cognition is that education acts as a confounder. Previous research shows that education is linked to dietary behavior [6
], the exposure being considered here and cognition [8
], the outcome. Thus, we argue that education plays an important confounding role in the association between dietary patterns and cognition. The fact that this effect for education is evident after adjustment for multiple covariates is remarkable, particularly as Whitehall II is a white-collar middle-aged cohort.
The confounder role of education could work in several ways. One, education is associated with dietary habits and nutrient intake. Low education is associated with poor health behaviors, smoking, and less regular physical activity. Thus, participants with lower education have less healthy eating patterns compared to those with higher education. Furthermore, there is some evidence to show that lower socioeconomic position, of which education is a measure, is associated with purchase of foods that are cheaper per unit of energy rather than foods rich in protective nutrients [44
]. Finally, low education is also related to poorer health-related nutrition knowledge [6
] which determines food choice. Second, education as a risk factor of cognitive impairment could confound the diet-cognition relationship. Low education has been shown to be associated with increased risk of dementia [8
]. These observations are supported by the cognitive reserve hypothesis [48
], which stipulates that cognitive reserve delays the onset of clinical manifestations of dementia.
Our study has several potential limitations. First, the use of a semi-quantitative food questionnaire, only on specific foods, is recognized to be less precise than dietary assessment using a diary questionnaire. However, in this study population, at a previous wave of data collection, we have shown that nutrient intakes estimated by the FFQ method were well correlated with biomarker levels and with intake estimates from the generally more accurate 7-day diary [13
]. Second, the cross-sectional framework of the analyses makes it impossible to draw causal inferences on the association between nutrition and cognition. Third, Whitehall II study participants are office-based civil servants, who are not fully representative of the British population [12
]. Finally, the factor analysis approach used to identify these patterns involves several arbitrary decisions such as the consolidation of food items into food groups, the number of factors extracted, the methods of rotation or labeling of the factors [51
Despite these limitations, by considering an overall diet approach rather than a ‘single’ nutrient or food approach, our study is the first to show, in a middle-aged general population, that education, through its role as a powerful confounder, shapes the relationship between the two dietary patterns – ‘whole food’ and ‘processed food’ pattern – and cognitive function.