Recruitment and methods

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.

General Practice recruitment

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 invited participants and appointments were made to attend the GP’s surgery for an interview with the nurse. Each participant consented to provide a blood sample and for further information to be provided from his or her medical records. The nurse then took anthropometric and blood pressure measurements and a 30ml blood sample, and also 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 woman completed the detailed health and lifestyle questionnaire whilst at the surgery, and consented to provide a blood sample and for further information to be provided from her medical records. The GP took anthropometric and blood pressure measurements and a 30ml blood sample.

Postal recruitment

The aim of the postal method was to recruit as many vegetarians as possible. Information about the study and questionnaires were sent to members of the Vegetarian Society of the UK, the Vegan Society, and previous participants in the Oxford Vegetarian Study, and also distributed through other health related magazines and health food shops. Potential participants were invited to complete the questionnaire after reading the information provided about the study. Participants recruited by this postal method were asked on the questionnaire if they would consent to the study team obtaining information from their NHS medical records. They were also asked if they would be willing to provide a blood sample, and if they agreed to this then their GP was then approached to take a blood sample on behalf of EPIC-Oxford.

Participants

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.

Blood samples

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 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.

Follow-up questionnaires

Approximately five years after completion of the first questionnaire, a follow-up questionnaire was distributed. About 70% were returned completed. This questionnaire provides us with information on changes in diet, lifestyle and health.

A second follow-up questionnaire was sent in 2007, approximately 10 years after the participants joined the study. About 61% were returned completed.

The third follow-up questionnaire was sent in 2010, approximately 15 years after the participants joined the study. About 65% were returned completed.

Food diaries

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. A second 7-day food diary was sent to participants in 2007/2008. The food diaries are coded for analysis as required.

Newsletters and correspondence

The following letters and newsletters have been sent to participants:

Second blood samples

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.

Medical follow-up, ethics and regulation

As well as the follow-up through questionnaires, each participant in EPIC-Oxford is followed for cancer incidence, the diagnosis in hospital of a range of other diseases potentially related to diet, and for causes of death through NHS and other government registers. In England and Wales this linkage is done through NHS Digital. To do this, identifiable data including your name, NHS Number and date of birth plus a unique study identifier is supplied by the study investigators to NHS Digital. NHS Digital has a record of all hospital admissions and outcomes data from the Hospital Episode Statistics (HES) dataset and links this information to individual participants in the study. This linkage is carried out under Section 251 of the NHS Act 2006. In addition NHS Digital are able to provide us with information about people who may have passed away (mortality data). This information includes date and cause of death information supplied by NHS Digital on behalf of the Office for National Statistics and is sourced from civil registration data. NHS Digital then supply the linked mortality data information together with the unique study identifier back to the study. For participants in Scotland equivalent data linkage is performed Public Benefit and Privacy Panel for Health and Social Care and for Northern Ireland this linkage is done through the Central Services Agency.

When we receive linked data, we remove all identifiers except for data of birth and date of death. All subsequent analyses use only subsets of the de-identified data. The data will be held only at the Cancer Epidemiology Unit at the University of Oxford. No data is released or shared in any form that would enable individual participants to be identified.

Participants can request more information about how we use their data and also find out about their options to withdraw consent for us to use their data in our FAQ section.

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