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Compared with both a traditional Mediterranean diet (TMD) enriched with nuts and a low-fat control diet, adherence to a TMD enriched with olive oil decreased cholesteryl ester transfer protein activity (relative to baseline, P=0.028), reported Montserrat Fitó, MD, PhD, coordinator of the Cardiovascular Risk and Nutrition Research Group at the Hospital del Mar Medical Research Institute in Barcelona, and colleagues.
It also increased HDL's ability to esterify cholesterol, paraoxonase-1 arylesterase activity, and HDL vasodilatory capacity (relative to control, P=0.039, P=0.012, and P=0.026, respectively), they wrote in Circulation.
"Following a Mediterranean diet rich in virgin olive oil could protect our cardiovascular health in several ways, including making our 'good cholesterol' work in a more complete way," Fitó stated in a press release.
Writing in an accompanying editorial, Daniel J. Radar, MD, of the University of Pennsylvania in Philadelphia, noted that the findings increase understanding of how dietary intervention can affect HDL functionality.
"Hernáez et al demonstrate a dissociation between the effects of a TMD on HDL-C levels (trending lower) versus on HDL CEC [cholesterol efflux capacity] (significantly higher) and greater positive effects of the TMD on CEC compared with those of a low-fat diet.
"Although efforts to promote HDL CEC through pharmacological means are still in clinical development, these results indicate that a Mediterranean diet is a practical lifestyle-focused approach to improving HDL function and has the proven benefit of reducing cardiovascular risk and the potential to reduce the progression of age-related macular degeneration. Whether promotion of HDL CEC causally contributes to the benefits of the Mediterranean diet remains to be established," he wrote online.
Fitó and colleagues randomly selected a subsample of 296 people at high risk of cardiovascular disease among those participating in the PREDIMED (PREvención con DIeta MEDiterránea) study. The participants (mean age 66) provided blood samples at both the beginning and the end of the 1-year study period. They were randomly assigned to one of three diets:
- A traditional Mediterranean diet enriched with virgin olive oil (about 4 tablespoons) each day (n=100)
- A traditional Mediterranean diet enriched with extra nuts (about a fistful) each day (n=100)
- A healthy "control" diet that reduced consumption of red meat, processed food, high-fat dairy products and sweets (n=96)
Using statistical analyses, Fitó's team assessed the effects of both TMDs on the role of HDL particles on reverse cholesterol transport (cholesterol efflux capacity, HDL ability to esterify cholesterol, and cholesteryl ester transfer protein activity), HDL antioxidant properties (paraoxonase-1 arylesterase activity and total HDL antioxidant capacity on low-density lipoproteins), and HDL vasodilatory capacity (HDL ability to induce the release of nitric oxide in endothelial cells).
The researchers also studied the effects of a TMD on several HDL quality-related characteristics (HDL particle oxidation, resistance against oxidative modification, main lipid and protein composition, and size distribution).
Fitó and colleagues found a decline in total cholesterol levels among those on the low-fat control diet (P=0.039 and P=0.007 relative to baseline and the TMD olive oil, respectively) -- mainly due to a decrease in LDL cholesterol levels (P=0.019 and P=0.004 relative to baseline and the TMD olive oil, respectively).
The TMD with olive oil and with nuts both increased cholesterol efflux capacity relative to baseline (P=0.018 and P=0.013, respectively), but the low-fat diet did not yield significant results.
Additionally, the content of triglycerides in HDL core decreased significantly after both the TMD olive oil and TMD nuts interventions compared with the low-fat diet (P=0.027 and P=0.035, respectively).
All three diets increased the percentage of large HDL particles (relative to baseline, P<0 .001="" p=""> Fitó and colleagues wrote they were surprised that the low-fat diet, which was rich in fruits and vegetables just like the Mediterranean diets, had a negative impact on HDL's anti-inflammatory properties.
They also discussed several possible explanations for the TMD-mediated increase in HDL functionality, including an improvement in the oxidative status of the lipoprotein and a better HDL composition profile.
Radar, too, had several theories for the differences observed between the TMD and the low-fat diet: "One possible difference is alcohol intake. The TMD participants were instructed that 'when the volunteer consumed alcohol, to moderately drink red wine (1 small glass/meal),' whereas the low-fat diet participants were given no similar instructions. Alcohol consumption is known to influence HDL-C levels and has been shown in a small experimental study to promote CEC."
The researchers concluded that "further studies are warranted to investigate the mechanism by which the TMD improves HDL function and whether these properties convey cardioprotective effects."
"Our study could be useful to remind that the Mediterranean Diet is beneficial even through unexpected mechanisms, highlighting again the importance of taking care of our cardiovascular health through a simple and cheap way such as a healthy diet," Fitó explained to MedPage Today.
Studying the cardioprotective mechanisms of an antioxidant-rich diet may contribute to the development of future therapeutic targets oriented to improve HDL functions, he added.
Study limitations were the inclusion of elderly people at high cardiovascular risk which may prevent extrapolation of the results to the general population.
Fitó noted a final limitation: "Some of the techniques used to measure HDL functions, although they were non-invasive and highly-sensitive, were in vitro simulations of processes that happen in our arteries (such as the removal of cholesterol or the ability of HDLs to induce the relaxation of blood vessels). These tests may not fully reflect the real complexity of the previous cardiovascular phenomena," he wrote to MedPage Today.
The researchers reported no financial disclosures of interest.
- Reviewed by Henry A. Solomon, MD, FACP, FACC Clinical Associate Professor, Weill Cornell Medical College and Dorothy Caputo, MA, BSN, RN, Nurse Planner
CirculationSource Reference: Rader D "Mediterranean approach to improving high density lipoprotein function" Circulation 2017; DOI: 10.1161/CIRCULATIONAHA.117.026278.
CirculationSource Reference: Hernáez A, et al "Mediterranean diet improves high-density lipoprotein function in high-cardiovascular-risk individuals" Circulation 2017; DOI: 10.1161/CIRCULATIONAHA.116.023712.