Does Food Retail Access Influence Dietary Intake?
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Does food retail access influence dietary intake? Martin White A thesis submitted in partial fulfilment of the requirements for the degree of Doctor of Medicine (MD) School of Health & Population Sciences College of Medical & Dental Sciences University of Birmingham February 2010 University of Birmingham Research Archive e-theses repository This unpublished thesis/dissertation is copyright of the author and/or third parties. The intellectual property rights of the author or third parties in respect of this work are as defined by The Copyright Designs and Patents Act 1988 or as modified by any successor legislation. Any use made of information contained in this thesis/dissertation must be in accordance with that legislation and must be properly acknowledged. Further distribution or reproduction in any format is prohibited without the permission of the copyright holder. Abstract The extent to which the food retail environment, including the availability, price and quality of foodstuffs, has an impact on what people eat remains unclear. This study aimed to determine whether the retail environment, of a household‟s usual main food store or of the area surrounding the home, is independently associated with the dietary intake of individual householders. The study employed a cross-sectional design and comprised simultaneous surveys of all retail outlets selling foodstuffs, and of households and the individuals living in them in the city of Newcastle upon Tyne, UK in 2000-2002. 5044 adults aged 16-97 years living in 3153 households provided data, including a 134-item food frequency questionnaire (FFQ) and detailed socio-demographic information. Detailed data on 33 commonly consumed foods was obtained from 560 food stores. Indices of relative intakes of fruits and vegetables, non-starch polysaccharide and total fat were derived from the FFQ. Availability of foodstuffs was generally good across the city, but poorer people lived closer to stores selling a wider range of foodstuffs, and fresh fruits and vegetables were more costly in more affluent areas. Diet, and related behaviours, attitudes and knowledge were strongly socio-economically patterned, with higher fat intake and lower fruit and vegetable intake, poorer dietary knowledge, more frequent food shopping at discount and convenience stores, and travel on foot or using public transport to food stores were all more common among less affluent and less educated households. In multilevel regression analyses, no area level variables were associated with variation in any of the dietary indices. Overall, variation in dietary intakes was most strongly associated with social, demographic and behavioural variables, and dietary knowledge. Dietary quality, in Newcastle upon Tyne at least, is not strongly associated with food retailing. Access to healthy foods in the retail environment may be an important pre-requisite of a healthy diet. However, where such access is uniformly good dietary quality is most importantly associated with individual lifestyle choices, which may be driven socio-economic factors. Public health interventions to improve diet need to focus on the knowledge and behaviours needed to acquire, prepare and consume a healthy diet, as well as the economic means to do so. iii iv Dedication To my father, RHRW. v vi Contents Abstract ..................................................................................................................................................... iii Dedication .................................................................................................................................................. v Contents ................................................................................................................................................... vii List of Tables .......................................................................................................................................... xiii List of figures ......................................................................................................................................... xvii Acknowledgements and Declaration .................................................................................................. xix Abbreviations ......................................................................................................................................... xxi Glossary of terms .................................................................................................................................. xxii Section 1 – Rationale & Methods .......................................................................................................... 1 1. Introduction...................................................................................................................................... 3 2. Background: current knowledge, local and policy context ....................................................... 5 2.1 Introduction ................................................................................................................................. 5 2.2 Current evidence on food access, diet and obesity ............................................................... 7 2.2.1 The social and spatial patterning of dietary intake ....................................................... 7 2.2.2 The social and spatial patterning of food retail access ................................................ 8 2.2.2.1 Food retail access .................................................................................................... 11 2.2.2.2 The cost of a healthy diet ...................................................................................... 12 2.2.3 The relationship between retail access and dietary intake ......................................... 13 2.2.3.1. Ecological studies ................................................................................................... 13 2.2.3.2 Individual level studies ........................................................................................... 14 2.2.3.3 Quasi-experimental studies ................................................................................... 15 2.2.4 The relationship between retail access and obesity .................................................... 16 2.3 A hypothesised causal framework for the relationship between dietary intake and food retail access ............................................................................................................... 17 2.4 Methodological limitations of existing research and key requirements of future studies ......................................................................................................................................... 20 2.5 Review of key methodological research to support study design ..................................... 22 2.5.1 Choice of setting, study population and sample size considerations ...................... 22 2.5.1.1 Why Newcastle upon Tyne was chosen as the study area ............................... 22 2.5.1.2. Sample size ............................................................................................................... 25 2.5.2 Exposure assessment: measuring the retail environment ......................................... 26 2.5.2.1. Sampling food stores.............................................................................................. 26 2.5.2.2. Assessment of the food environment within stores ......................................... 26 2.5.3 Outcome assessment: measuring dietary quality in a large population sample ................................................................................................................................ 27 2.5.3.1 Food frequency questionnaires ............................................................................ 28 2.5.3.2 The use of composite indices to summarise dietary data ................................ 28 2.5.4 Calibration of exposure assessment and outcome measurement using local data ............................................................................................................................ 29 vii 2.5.5 Area level data and spatial considerations ................................................................... 30 3. Scientific rationale, aims and objectives .................................................................................... 33 3.1 Brief statement of the scientific rationale ............................................................................. 33 3.2 Study aims and research questions ........................................................................................ 33 4. Methods .......................................................................................................................................... 35 4.1 Study design .............................................................................................................................. 35 4.2 Research Governance .............................................................................................................. 36 4.3 Setting ......................................................................................................................................... 36 4.4 Retail Survey .............................................................................................................................. 36 4.4.1 Identification and recruitment of food stores ............................................................ 36 4.4.2 Development of