Health Benefits of Mediterranean Diet Potentially Overstated

The Mediterranean diet may protect against cardiovascular disease (CVD), but both the quantity and quality of the available evidence appears to be limited and may not be strong enough to support widespread recommendation in guidelines, according to a systematic review and meta-analysis presented at the American College of Cardiology (ACC) Scientific Sessions 2015.

Researchers at the meeting reported that the Mediterranean dietary pattern is broadly recommended for the prevention of chronic disease. However, it appears the evidence base is more limited than is widely understood.

Therefore, the research team sought to define the most likely effects of the Mediterranean diet on vascular disease and mortality. The researchers systematically searched MEDLINE, EMBASE and the Cochrane Central Register for randomized controlled trials comparing Mediterranean with control diets. The analysis included six studies with a total of 10,950 participants.

The researchers found that smaller randomized clinical trials, which examined the effects of Mediterranean diet on intermediate metabolic outcomes, suggested small to moderate beneficial effects for multiple determinants of vascular risk such as blood pressure (BP), triglycerides and HDL cholesterol. 

“Despite these encouraging findings, neither the cohort studies nor the [randomized controlled trials] of intermediate outcomes show benefits of a magnitude congruent with the very large point estimates of effect on cardiovascular outcomes proposed by some of the individual trials included in our review,” said lead study investigator Thaminda Liyanage, who is a postgraduate fellow at The George Institute for Global Health at the University of Sydney in Australia.

Further, Dr. Liyanage said there were serious concerns about the integrity of the data for one large study (n=1,000). There was evidence of protection with the Mediterranean diet against major vascular events, coronary events, stroke and heart failure, but not for all-cause mortality or CV mortality, according to the analysis. 

These findings are clinically relevant because CVD is a leading cause of mortality worldwide, reportedly accounting for 25% of the estimated 52 million deaths in 2010, Dr. Liyanage said.

Dr. Liyanage noted that several potential mechanisms of benefit, such as antioxidant or anti-inflammatory effects, have been postulated for the Mediterranean diet. However, large trials of intervention strategies specially targeting these pathways have been universally negative.

“This raises an important question about the plausibility of large beneficial effects of the Mediterranean diet and raises significant concern about the possible effects of random or systematic errors,” Dr. Liyanage told Endocrinology Advisor. “In regard to the former, the total volume of data available is small and much less than might typically be available for comparable investigations of the effects of drugs on the same outcomes.”

He said a well-conducted and adequately powered trial is warranted to precisely define the overall balance of benefits and risks associated with the Mediterranean diet.

The American Heart Association/ACC, the European Society of Cardiology and the National Heart Foundation of Australia all recommend the Mediterranean-style diet to reduce CV risk.

“It seems unlikely that clinicians and patients fully appreciate the limitations of the data and the extent of the uncertainty,” said Dr. Liyanage.

Reference

1. Liyanage T et al. Abstract 1178-116. Presented at: American College of Cardiology (ACC) 64th Annual Scientific Session & Expo; March 14-16, 2015; San Diego.