Participants
The Coronary Diet Intervention with Olive Oil and Cardiovascular Prevention (CORDIOPREV) study is an ongoing prospective, randomised, open, controlled trial of 1,002 patients receiving conventional treatment for coronary heart disease (CHD) who had their last coronary event more than 6 months before enrolment in one of two different dietary models (a Mediterranean diet and a low-fat diet) over a period of 5 years. The patients were recruited between November 2009 and February 2012, mostly at the Reina Sofia University Hospital, Cordoba, Spain. The eligibility criteria, design and methods of the CORDIOPREV clinical trial have been reported elsewhere [11, 12], and the protocol is registered at ClinicalTrials.gov (registration no. NCT00924937). The outcome investigated in this study relates to the one of the secondary outcomes of the CORDIOPREV study: metabolic control of carbohydrates (CORDIOPREV-DIAB).
Patients who had not had a diabetes diagnosis and/or were not receiving glucose-lowering treatment before the beginning of the study underwent an OGTT as part of the protocol of the CORDIOPREV-DIAB study. Patients were asked about their medical diagnosis and glucose-lowering treatment in an interview with an internal medicine physician and their response was confirmed by electronic medical records, which are available for all patients in Andalucía, Spain. According to the baseline OGTT, 242 patients had normal glucose tolerance (fasting glucose <5.5 mmol/l, 2 h glucose <7.7 mmol/l, HbA1c <47.5 mmol/mol [6.5%]); 77 had impaired fasting glucose (IFG; fasting glucose 5.6–6.9 mmol/l), 80 had impaired glucose tolerance (IGT; 2 h glucose 7.8–11.0 mmol/l); 53 had both IFG and IGT; and 190 had type 2 diabetes (fasting glucose ≥7 mmol/l, 2 h glucose ≥11.1 mmol/l or HbA1c ≥47.5 mmol/mol [6.5%]). Once in the study, patients who received a biochemical diagnosis of diabetes, but who were not put on pharmacological treatment by their physicians, continued in the study. Patients who initiated pharmacological treatment for diabetes were excluded from the study.
A total of 642 patients were randomised to two dietary treatment groups: 327 to a Mediterranean diet group and 315 to a low-fat diet group. There were no significant differences in the demographic and metabolic characteristics between the groups (see electronic supplementary material [ESM] Table 1). Patients lost before follow-up at 2 years are detailed in ESM Fig. 1.
The local ethics committees approved the trial protocol and its amendments, which follow the Declaration of Helsinki and good clinical practice. The experimental protocol conforms to international ethical standards [13]. Written informed consent was obtained from all study participants.
Study diets
The low-fat diet comprised <28% of energy from fat (12% monounsaturated, 8% polyunsaturated and 8% saturated fatty acids), 57% from carbohydrates and 15% from proteins. The Mediterranean diet comprised 35% of energy from fat (22% monounsaturated, 6% polyunsaturated and 7% saturated fatty acids), 50% from carbohydrates and 15% from proteins. To ensure that the main fat source of the Mediterranean diet (extra-virgin olive oil) was identical for all patients in this group, the olive oil was given to the participants by the research team. Food packs, including low-fat foods (cereals, legumes, pasta, etc.) were provided for the patients randomised to the low-fat diet group.
Dietary assessment and anthropometric measurements
The CORDIOPREV study was supervised by a team of registered dietitians. At the beginning of the study and every year, the dietitian had a face-to-face interview with each patient, during which was carried out a 137-item semi-quantitative food frequency questionnaire, validated in Spain [14], and a validated 14-item questionnaire of adherence to the Mediterranean diet to determine a Mediterranean diet score [15]. Participants in each intervention group received the same intensive dietary counselling. The dietitian adapted the participants’ customary diet to the Mediterranean diet or to the low-fat diet, focusing on the overall daily diet rather than on isolated nutrients. Weight, height and waist circumference were also measured at baseline and again each year according to standardised protocols.
Biochemical measurements
Venous blood for analysis of the participants’ biochemical variables was collected in tubes containing EDTA after a 12 h overnight fast at baseline and after 2 years of follow-up. Lipid variables, serum insulin, plasma glucose and plasma concentration of C-reactive protein (CRP) were determined as previously reported [11, 12].
Estimation of IR, insulin secretion and beta cell function indices
Patients underwent a standard OGTT at baseline and after 2 years of follow-up. After an overnight fast, blood was sampled from a vein before oral glucose intake (0 min) and again after a 75 g flavoured glucose load (Trutol 75; Custom Laboratories, Baltimore, MD, USA). Blood samples were taken at 30, 60, 90 and 120 min to determine glucose and insulin concentrations [16].
The following indices were then estimated. The indices used to determine tissue-specific IR were: the hepatic insulin resistance index (HIRI) and the muscle insulin sensitivity index (MISI). HIRI was estimated by the validated method of Matsuda and DeFronzo [16], expressed as fasting insulin (pmol/l) × fasting glucose (mmol/l). MISI was measured according to the method of Abdul-Ghani et al [17]: MISI = (dG/dt) / mean plasma insulin concentration, where dG/dt is the rate of decay of plasma glucose concentration from its peak value to its nadir during the OGTT. Other IR indices determined were: insulin sensitivity index (ISI) = 10,000/√ [(fasting insulin [pmol/l] × fasting glucose [mmol/l]) × (mean OGTT insulin [pmol/l]) × (mean OGTT glucose [mmol/l])] [16]; and HOMA-IR [18]. Insulin secretion was measured by the insulinogenic index (IGI): IGI = [30 min insulin − fasting insulin (pmol/l)] / [30 min glucose − fasting glucose (mmol/l)] [19]. Finally, beta cell function was estimated by calculating the disposition index as follows: disposition index = ISI × [AUC30 min
insulin / AUC30 min
glucose], where AUC30 min is the area under the curve between baseline and 30 min of the OGTT for insulin (pmol/l) and glucose (mmol/l) measurements, respectively, calculated by the trapezoidal method [20].
Determination of muscle and liver IR groups
At baseline, the patients were distributed into four groups according to the presence or absence of muscle and/or liver IR. For this purpose, we used a method based on that described by Abdul-Ghani et al [21]. The patients were divided into tertiles according to HIRI and MISI. The highest tertile of HIRI and the lowest tertile of MISI were considered to indicate IR in each organ. A second operational definition based on the median value for IR in skeletal muscle and liver gave similar results.
Statistical analyses
Normal distribution was tested for all measured variables, and skewed variables were normalised by log10. Statistical analysis was carried out using SPSS software version 18.0 for Windows (SPSS, Chicago, IL, USA). The data were presented as mean ± SD for continuous variables and as frequencies for categorical variables. In the statistical tests, age, sex, BMI and change in weight were included as covariates. The statistical significance of the differences in mean values at baseline according to IR groups was assessed by a univariate general linear model. A repeated measures ANOVA test was used to determine the statistical differences between variables at baseline and after 2 years of follow-up. The mean percentage change in each variable was calculated as follows: ([V
2 − V
0] / V
0) × 100, where V
0 is the value of each variable at baseline and V
2 is the value after 2 years of follow-up. A negative value of mean percentage change indicates that the variable has decreased after 2 years of follow-up, and a positive value indicates that the variable has increased. Bonferroni’s test was used in all cases where post hoc analyses were required, in which p < 0.05 was considered to be significant.