The SUN project was designed in collaboration with the Harvard School of Public Health and its methodology is similar to that used in large American cohorts such as the Nurses' Health Study and the Health Professionals Follow-up Study. A detailed description of its methods has already been published.
The SUN Study is an open-enrollment cohort including 13,500 university graduates in December 2003 (SUN meaning Seguimiento Universidad de Navarra), recruited and followed-up through biennial mailed questionnaires. Diet was evaluated at baseline with a semiquantitative food-frequency questionnaire, previously validated in Spain. This questionnaire has shown interesting results in a preliminary case-control study of first non-fatal myocardial infarction (171 matched case-control pairs). Some of these results are presented because the aim of the case-control study was to address the same hypotheses as the cohort using the same dietary assessment method.
In the initial questionnaire of the SUN cohort, information about other risk factors for hypertension and cardiovascular disease was collected, as well as the presence of these conditions. In the first follow-up questionnaire, information about a new physician-made diagnosis of hypertension was included. To assess the relationship between olive oil consumption and risk of hypertension, a Cox model was used. To build a diet score, 8 energy-adjusted items were categorized in quintiles and the value of the quintile (+1 for the 1st quintile, +2 for the 2nd quintile, etc.) was directly imputed for olive oil, fruits, vegetables, fiber, alcohol, fish/shellfish, legumes, and low-fat dairy products. Other 2 items (meat/meat products and glycemic load) were categorized in energy-adjusted quintiles and inversely weighed (+1 for the 5th quintile, +2 for the 4th quintile, etc). The 10 items were summed up, yielding a composite score ranging from 10 to 50 points. Hazard ratios (HR) and their 95% confidence interval (CI) for quintiles of energy-adjusted olive oil consumption and for the overall diet score were calculated.
In the preliminary case-control study protection against myocardial infarction was evidenced among those participants with high intake of olive oil, fish and n-3 PUFA; similarly a high intake of dietary fiber, fruits, folate and wine comsumption were found to be associated with a lower risk of myocardial infarction; whereas other items (meats and products with a high glycemic load) were associated with a higher risk, thus highlighting the importance of considering the overall dietary pattern.
The follow-up rate in the cohort for a median of 28,5 months was 88.4% after 5 mailings in 4,825 participants included in this analysis, accounting for a total study base of 11,003 person-years. The HR (95% CI) of incident hypertension for the 2nd, 3rd, 4th and 5th quintiles of olive oil consumption were 0.60 (0.34 - 1.04), 0.70 (0.41 - 1.18), 0.48 (0.27 - 0.86) and 0.79 (0.48 - 1.32 respectively after adjusting for gender, age, physical activity, body mass index, total energy intake, sodium intake, alcohol intake, and other dietary exposures. There was no linear trend among categories of olive oil consumption.
Compared with those with <25 points in the diet score, the hazard ratio (HR) - 95% confidence interval (CI) of incident hypertension for those scoring 25-29 points was 0.73 (0.44 - 1.22), for those scoring 30-34 was 0.62 (0.37 - 1.03), and for those scoring >34 points was 0.74 (0.44 - 1.25). When considering the Mediterranean score as a quantitative variable the HR for each additional point was 0.98 (0.95 - 1.01; p=0.18), the inclusion of a quadratic term significantly improved the model. The HR (95% CI) for the linear term was 0.76 (0.63 - 0.91; p=0.003) and 1.004 (1.001 - 1.007; p=0.005) for the quadratic term, representing a flattening (i.e., a plateau) of the dose-response curve for the highest scores.
A moderate olive oil consumption was inversely associated with the risk of myocardial infarction and hypertension. A composite score reflecting a typical Mediterranean dietary pattern was also associated with a lower risk of myocardial infarction in the preliminary case-control study. Our data suggest a threshold effect in the dose-response relationship between olive oil intake and the risk of hypertension. A Mediterranean dietary score was associated with a lower risk of hypertension after adjustment for main risk factors for hypertension with an apparent plateau of high scores.
Harvard University - University of Athens, Greece