Approximately 65,000 participants were recruited into the EPIC-Oxford cohort between 1993 and 1999. Participation in the study required the completion of a diet and lifestyle questionnaire which consists of a food-frequency questionnaire (FFQ) as well as questions on health, family medical history and four dietary categorisation questions: “Do you eat meat?”, “Do you eat fish?”, “Do you eat dairy products?”, “Do you eat eggs?”. On the basis of the response to these four questions, each participant is categorised as either a meat-eater, fish-eater (does not eat meat but does eat fish), vegetarian (does not eat meat or fish) or vegan (does not eat any animal products).
Two methods of recruitment were used: General Practice (GP or nurse) recruitment and postal recruitment.
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Recruitment from the general population was carried out by EPIC nurses working in GP surgeries in Greater Manchester, Oxfordshire and Buckinghamshire. Men and women aged 35 and over on the collaborating GP’s list were invited to participate. Questionnaires were mailed to consenting participants and appointments were made to attend the GP’s surgery for an interview with the nurse. The nurse took anthropometric and blood pressure measurements. The nurse also took a 30ml blood sample and checked the completed questionnaire. In addition, a pilot recruitment phase was conducted by collaborating GPs in Scotland, who recruited 900 women aged 40 to 59 from those attending the surgery. Each consenting woman completed the detailed health and lifestyle questionnaire whilst at the surgery. The GP took anthropometric and blood pressure measurements and a 30ml blood sample.
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The aim of the postal method was to recruit as many vegetarians as possible. Two methods were used to recruit men and women aged 20 and over. First, the main questionnaire was mailed directly to all members of The Vegetarian Society of the UK. Enclosed with each questionnaire was a sheet of labels on which participants were invited to give names and addresses of relatives and friends interested in receiving a questionnaire (“snowballing”). To maintain snowballing momentum, all subsequent questionnaires were mailed with a sheet of labels. Secondly, a short questionnaire (or insert) was distributed to all members of The Vegan Society, enclosed in health/diet-interest magazines, and displayed on health food shop counters. This was contained in a single A4 sheet, which could be folded and sealed for return mailing to the study office. Questions were limited to the four dietary categorisation questions; date and place of birth; sex; height; weight; age left school; smoking history; alcohol consumption; and brief medical history. The main questionnaire was then mailed to all those indicating an interest in receiving one. Participants recruited by these postal methods and who completed the main questionnaire were asked if they would be willing to provide a blood sample. The participant’s GP was then approached to take a blood sample on behalf of EPIC-Oxford.
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The main questionnaire was completed by 57,496 participants. Data from the insert alone were available from an additional 7,993 participants. Participants were recruited from throughout the UK, with 85% from England, 10% from Scotland, 4% from Wales and 1% from Northern Ireland. The cohort is made up of 23% men and 77% women. 53% of the men are meat-eaters, 12% are fish-eaters, 28% are vegetarians and 7% are vegans. 51% of the women are meat-eaters, 17% are fish-eaters, 39% are vegetarians and 3% are vegans.
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Blood samples were provided by 19,500 participants. Blood samples were taken using a specially designed blood kit, either by Research Nurses working in General Practice surgeries, or by General Practitioners or their Practice Nurses. The samples are stored frozen in liquid nitrogen until they are required for laboratory analysis. Analyses will examine specific questions on dietary and other factors in relation to long-term health. The results of laboratory analyses will not be sent to participants.
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Approximately six months after receipt of the completed questionnaire each participant was sent a 7-day food diary in which to record details of everything consumed over a seven day period. 32,000 completed diaries have been received. These are coded for analysis as required.
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Approximately five years after completion of the first questionnaire, a follow-up questionnaire is distributed. On average 70% are returned completed. This questionnaire provides us with information on changes in diet, lifestyle and health.
Additionally, each participant is followed for cancer incidence and cause specific mortality through the NHS Central Registry.
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A subset of participants who had given an initial blood sample and completed a follow-up questionnaire were asked to provide a second blood sample. Participants were selected by age, sex and diet group. 1,000 second blood samples have been collected and stored frozen in liquid nitrogen, until they are required for laboratory analysis. The main purpose of these repeat blood samples is to assess the stability of blood levels of, for example, vitamins, fats and other nutrients.
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