Mediterranean diet: diabetes prevention

recent spanish study, sponsored mostly by the spanish governnment but with material support from the california walnut commission and morella nuts (though without their input on design, data analysis or reporting of results) assessed the effect of a Mediterranean diet in the prevention of diabetes (see dm prevention medit diet annals 2013 in dropbox, or Ann Intern Med. 2014;160:1-10). weight loss has been repeatedly shown to decrease diabetes incidence, and there are several observational trials suggesting that rice and high glycemic index diets lead to more diabetes and that green vege diets decrease incidence. this spanish trial was a subgroup analysis of a 4-year randomized controlled trial done in several primary care centers, involving 3500 men and women aged 55-80 at high cardiovasc risk (either diabetes or at least 3 of: smoking, hypertension, high chol, low HDL, overwt/obesity, fam hx of premature cad; study randomization was not stratified by diabetes status, but diabetics were excluded from the analysis) randomly assigned to one of three diets:
        --Mediterranean diet supplemented with extra-virgin olive oil (EVOO) -- pts given 50 ml to use per day
        --Mediterranean diet supplemented with nuts -- patients given 30g/d: 15g of walnuts, 7.5g of almonds, an 7.5g of hazel nuts
        --control diet, with advice on a low-fat diet 

baseline mean BMI of 30, 16% smokers, 90% with htn, 54% with hyperlipidemia (mean LDL 140, HDL 56). none of the dietary interventions focused on weight loss or increasing physical activity.  results: 

        --reasonable adherence to diet by food questionnaires as well as by objective biomarkers in small random sample (urinary hydroxytyrosol level in EVOO group and a-linolenic acid in nut group). only minor changes in body weight, waist circumference and physical activity, and no diff between the groups 
        --EVOO diet, nuts diet, and control diet had, respectively 80, 92, 101 cases of new-onset diabetes, with rates of 16.0, 18.7 and 23.6 cases per 1000 person-years and multivariate-adjusted hazard ratios of 0.60 (0.43-0.85) and 0.82 (0.61-1.10) comparing EVOO and nuts diets, respectively, to controls (ie 40% and 18% reductions, with the 18% reduction not quite being statistically significant). diabetes incidence curves separate after 1.5 years and maintain this separation for up to 5.5 years.

Mediterranean diet does have significant fats (34-40% of energy, mostly derived from vege sources such as olive oil and nuts) and low dairy. includes moderate consumption of alcohol (mostly wine) and frequent use of sauces with tomato, onions, garlic, other spices. in this study dietitians gave personalized input to patients about using EVOO for cooking and dressing, weekly intake of nuts, increased fruit/vege/legume/fish, eating white meat instead of red or processed meat, and avoiding butter, fast foods, sweets, and sugar-sweetened beverages. also reduction in alcohol other than wine (moderate amount, with meals). diet assessed q3months in individual and group visits, focusing on shopping lists, meal plans, recipes. pts in control group also with personalized advice and group sessions of same frequency and intensity as those in each of the Mediterranean diet groups. new-onset diabetes defined as fasting bs>126 or 2-hr >200 after 75g glucose oral load (ie, not A1C).

so, impressive intervention trial showing that there seems to be significant benefit from a diet based on lots of fruits/veges/low animal fats with additional EVOO and with wine thrown in, as compared to a low-fat diet. we cannot conclude from this study, however, that adding the olive oil to the Mediterranean diet is better than the Mediterranean diet alone, just that it is better than a low-fat diet. it is likely that the EVOO, which adds more monounsaturates (olive oil) and anti-oxidants (the extra-virgin preparation) are additionally beneficial.