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Dietary Surveys and Nutritional Epidemiology
Abbreviations: ESFA, exhaustive single food analysis; FDR, false discovery rate; PCA, principal components analysis
Over the last two decades, there has been an explosion in the use of principal components analysis (PCA) to identify dietary patterns in nutritional epidemiological studies(1-10). The main reason for the explosion of PCA is that recent results have raised questions about the role of diet in the aetiology of certain chronic diseases. A lack of empirical evidence from randomised controlled trials based on observational findings has been noted in chronic diseases such as asthma(11), cancer(12,13)and CVD(14), and it has been argued that single foods or nutrients may be less important than dietary patterns in causing disease. Since the use of a single variable to explore associations between diet and disease has led to unreliable results, an alternative is to look at a small number of dietary dimensions each made up of a combination of foods with the use of PCA.
PCA of data from FFQ allocates food items according to the degree with which their reported intakes are correlated. The principal components identified are referred to as 'dietary patterns' and can be investigated in relation to health and disease. This approach proved successful, for example, in two large US cohorts, in which the same two patterns of diet - a 'prudent' dietary pattern characterised by intake of vegetables, fruit, legumes, whole grains, fish and poultry and a 'Western' dietary pattern characterised by intake of red meat, processed meat, refined grains, sweets and desserts, French fries and high-fat dairy products - were identified and subsequently linked to differences in the occurrence of CHD(15,16), colon cancer(17,18)and chronic obstructive pulmonary disease(19,20).
PCA is often claimed to resolve the issue of confounding between different dietary exposures(21,22), though, even where there is a causal effect from diet, it is not clear whether the foods singled out in a PCA are those that are directly associated with the risk...