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Dietary data: Objective and outline of this guide

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Why study diet in cohort and longitudinal studies?

Diet is a major modifiable health behaviour that can impact a wide variety of health outcomes [2]. The studies in the CLOSER consortium provide a resource in which both drivers and consequences of dietary intake can be explored. Together they also provide opportunities to examine longitudinal and secular trends in dietary intake. However, measuring diet is not straightforward. An individual’s diet is the result of complex social, economic and cultural circumstances [3-6] and dietary intake will vary according to age, season, day of week, and working patterns, and it has considerable random variation [7]. The nutrient composition of foods themselves can also vary depending on soil composition, fortification practices and changes in manufacturing [6]. Further, measurement error and potential reporting bias can lead to under- or over-estimation of associations between diet and health [6].

Nutritional epidemiology has been criticised [8]. Findings of associations from particular diet-related variables and health from observational studies are often not replicated in randomised controlled trials (RCTs). It is suggested that this is due to residual confounding, measurement error, and the fact that capturing dietary intake is too complex for questionnaire methods [8]. However recently there have been significant improvements in the validity of dietary assessment methods. Further, despite RCTs being the gold standard study design in many instances they are not always suitable for nutrition; it is unethical to assign individuals to an inadequate nutrient level and difficult to capture the effects of long-term overall diet [9]. So, while no current dietary assessment method is able to measure diet exactly, understanding how diet is assessed in each of the original eight CLOSER studies, what data were collected, and undertaking cautious interpretation of results can support the use of these data.

 

Overview of this guide

This guide aims to provide guidance on the definition, measurement and interpretation of the dietary data collected in the CLOSER studies. It is intended to support researchers in the use of the dietary data both within single studies and across the studies. The guide is limited to describing dietary intake data and does not include information about eating behaviours, eating difficulties, food expenditure, biomarkers of nutrition or analytical methods. Details about the overall concept of dietary patterns and how they are constructed can be found elsewhere [10].

Before detailing how dietary assessment was conducted and how it has been used for research in each of the studies, a brief overview of contextual and policy considerations and a summary of dietary assessment tools is presented.

 

Explore additional background detail:

Learn more about the individual studies covered by this guide and their dietary measurements:

Learn about harmonisation in the context of dietary data:

Further information:

This page is part of the CLOSER resource: ‘A guide to the dietary data in eight CLOSER studies’.