Study design
The EPIC-InterAct study is a large prospective case-cohort study nested within the European Prospective Investigation into Cancer and Nutrition (EPIC) [15]. Its aim is to investigate the influence of lifestyle and genetic factors (and their interaction) on the risk of developing type 2 diabetes in a large cohort of adults from eight European countries [12]. The EPIC study was initiated in the late 1980s as a collaboration between 23 research institutions in ten European countries (Denmark, France, Germany, Italy, the Netherlands, Spain, Sweden, UK, Greece and Norway). Greece and Norway did not participate in the EPIC-InterAct study. The EPIC study has been previously described by Riboli et al [15].
During a mean follow-up of 11.7 years a total of 12,403 verified newly diagnosed cases of type 2 diabetes were identified among 340,234 EPIC participants (men and women aged 20-80 years at baseline with a stored blood sample and reported diabetes status). Using a case-cohort design, a centre-stratified random subcohort of 16,835 participants (5%) was selected from the 340,234 EPIC participants. After exclusions of participants with prevalent diabetes (n = 548) or those with missing data on diet (n = 117), smoking status (n = 241), BMI (n = 169), physical activity (n = 289) or education (n = 479), as well as those who fell in the top or bottom 1% of the ‘energy intake/energy requirement ratio’ (n = 619), a total of 26,088 individuals remained for the analysis (14,529 subcohort non-cases, 729 subcohort type 2 diabetes cases and 10,830 non-subcohort type 2 diabetes cases).
All participants in the EPIC cohorts involved in the InterAct project gave their informed consent. The EPIC-InterAct study has been approved by the responsible ethics committees.
Dietary exposure assessment
At baseline, in the EPIC study, total daily consumption of food items was assessed for each participant by country-specific validated dietary questionnaires [16]. The total daily intake of each nutrient and the total daily energy intake were calculated [17]. The mean daily intake of meat iron was calculated as the mean of the total daily intake of iron (both heme and non-heme iron) from all types of meat. Glycaemic index and glycaemic load values were calculated based on glucose as a reference [18].
In the present study, the food group ‘red meat’ included the daily consumption (g) of unprocessed beef, pork, veal, mutton, lamb, goat and horse, hamburgers, meatballs and minced meat. The food group ‘poultry’ included chicken, hen, turkey, duck and goose; also domestic rabbit was added to this group. The food group ‘processed meat’ included bacon-, ham- and liver-containing items and all other processed meats (black pudding, chorizo, sausages, corned beef). The category ‘offals’ included liver, kidney, tripe, tongue, heart and sweetbread. A ‘red and processed meat’ group was created by combining the ‘red meat’ and the ‘processed meat’ groups. A ‘total meat’ group was also created by combining all types of meat.
Non-dietary exposure assessment
At baseline, in the EPIC study, information on lifestyle habits (including tobacco smoking, alcohol drinking and physical activity), education, occupation, previous diseases and reproductive history was collected by means of a specific lifestyle questionnaire. Height, weight, waist and hip circumference of participants were also collected.
Ascertainment and verification of type 2 diabetes incident cases
In each InterAct centre a list of all ascertained type 2 diabetes incident cases was created including individuals with evidence of type 2 diabetes from self-reported history of diabetes during follow-up contacts, linkage to primary and secondary care registers, type 2 diabetes-specific medication use and type 2 diabetes diagnosis reported in hospital admissions records or in mortality data. A detailed description of the ascertainment and verification procedure in all InterAct centres is provided elsewhere [12]. Follow-up was censored at the date of diagnosis, 31 December 2007 or the date of death, whichever came first.
Statistical analysis
Sex-specific quintiles of daily consumption of total meat, red meat, processed meat, red and processed meat, poultry and meat iron were obtained based on the subcohort sample. Sex-specific tertiles were obtained for offals due to their small daily consumption.
Within the subcohort sample the distribution of the main baseline non-dietary characteristics according to sex-specific quintiles of total meat consumption were explored. Generalised linear models, adjusted for age and daily energy intake at enrolment, were used to explore the baseline dietary variables.
Cox regression, with an estimation procedure for case-cohort designs according to the Prentice weighting method [19], was used to estimate HR and 95% CI of incident diabetes according to sex-specific quantiles of consumption of total meat, red meat, processed meat, red and processed meat, poultry, offals and meat iron. The time at entry was age at recruitment; exit time was the age at which participants were diagnosed with diabetes, died, were lost-to-follow-up or were censored at the end of the follow-up period, whichever came first. HR and 95% CI were computed with the lowest quantile of consumption as reference. Tests for linear trend in HR were based on medians across quantiles. In the basic model the baseline hazard function was stratified by centre and adjusted for sex and energy intake (natural log kJ). In multivariate model 1, analyses were performed stratified by centre and adjusted for sex, energy intake, smoking status (dummy variables for former and current smokers; never smokers as reference), alcohol (quintiles of daily intake), physical activity (four levels) and educational level (five levels) based on a priori knowledge of the main risk factors for type 2 diabetes [3, 20–22]. In multivariate model 2, a term for BMI (continuous) was added. The same models were performed with each type of meat and meat iron as continuous variables (50 g increments for total meat, red meat, processed meat, red and processed meat, and poultry; 10 g increments for offals; 1 mg increments for meat iron). Further analyses were performed with adjustment for intake of other foods and nutrients such as potatoes, total vegetables, bread and pasta, fish, cakes, soft drinks, coffee (50 g increments), total fats, magnesium, fibre and glycaemic load (continuous). Further analyses were also performed with meat consumption values scaled by additive calibration [23].
Possible effect modification by sex was tested by adding to multivariate model 2 the corresponding interaction terms obtained from quantiles of intake of the several meat items. The likelihood ratio test was used to assess the significance of the interaction terms. Multivariate model 2 was then performed separately by sex. Possible effect modification by BMI was also tested by adding to the model the corresponding interaction terms. Models were performed separately by BMI categories (<25, 25–30, ≥30 kg/m2).
Sex-specific models, simultaneously adjusted for red meat, processed meat, poultry and offals (sex-specific quantiles of consumption) were also performed in order to explore the effect of the different meat groups adjusting for each other.
Random-effect meta-analysis was used to assess heterogeneity (I
2 statistic) among countries in the association between meat consumption (overall and by specific types) or meat iron intake and occurrence of type 2 diabetes.
Sensitivity analyses were performed by excluding participants with myocardial infarction, angina, stroke, hypertension or hyperlipidaemia at baseline and by excluding the first 2 years of follow-up.
Analyses were performed with Stata 9.2 (StataCorp, TX, USA).