By David Kiefer, MD, Editor

Clinical Assistant Professor, Department of Family Medicine, University of Wisconsin; Clinical Assistant Professor of Medicine, Arizona Center for Integrative Medicine, University of Arizona, Tucson

Dr. Kiefer reports no financial relationships relevant to this field of study.

SUMMARY POINT

  • Some health benefits, not harms, are seen with a Mediterranean diet that does not have a restriction in fat intake.

SYNOPSIS: Studies examining the Mediterranean diet with unrestricted fat intake find benefits on cardiovascular disease, diabetes, and breast cancer risk.

SOURCE: Bloomfield HE, Koeller E, Greer N, et al. Effects on health outcomes of a Mediterranean diet with no restriction on fat intake: A systematic review and meta-analysis. Ann Intern Med 2016; Jul 19. doi: 10.7326/M16-0361 [Epub ahead of print].

Which is better, a low-fat diet or a low-carbohydrate diet? This question still is posed to clinicians frequently, and working out the physiological details and individualizing a response are challenges. The August issue of Integrative Medicine Alert included a review of a sub-analysis of the PREDIMED, or PREvención con DIeta MEDiterránea, dataset,1 showing that long-term vegetable fat enrichment of a Mediterranean diet leads to less weight gain and less of an increase of central adiposity than a control diet. As if to punctuate this work, Bloomfield and colleagues undertook a systematic review and meta-analysis on studies examining a Mediterranean diet with no fat restriction. This effort was slightly different from the Estruch et al controlled trial, a supplementation of the Mediterranean diet with either olive oil or mixed nuts. Bloomfield et al searched several databases for Mediterranean diet studies compared to control diets published between 1990 and 2016. A Mediterranean diet in their analysis was defined by having at least two of the criteria as listed in Table 1.

Table 1: Criteria (verbatim) for Mediterranean Diet in the Bloomfield et al Meta-analysis

  • A high monounsaturated-to-saturated fat ratio
  • High fruit and vegetable intake
  • High consumption of legumes
  • High grain and cereal intake
  • Moderate red wine consumption
  • Low consumption of meat and meat products with increased intake of fish

Studies were included in the meta-analysis if they had at least 100 study participants and were followed for at least one year, although these restrictions were not mandated for studies on rheumatoid arthritis nor cognitive impairment. The authors also commented on adherence to the Mediterranean diet in North America by looking at this variable in studies conducted in the United States or Canada. A total of 90 articles published on 56 trials were included in this analysis.

Table 2 summarizes the results found in the statistical analyses. Embedded in this work was the PREDIMED trial, contributing results to both the cardiovascular disease and all-cause mortality results, as well as cancer outcomes (the only trial to do so). In addition, two trials on cognitive impairment offered mixed results (one favorable, the other no difference) on the effect of the Mediterranean diet. One cohort study on rheumatoid arthritis found no statistically significant effect of the Mediterranean diet. No studies adequately addressed adherence. The authors also commented on the risk of bias of the individual studies reviewed, finding a variety in quality; this is relevant to the interpretation of results in terms of believability, clinical applicability, and generalizability. When only the most rigorous studies were included, as these authors tried to do, it softened the results slightly compared to other work on this topic. When tied into past studies, including Cochrane analyses, Bloomfield et al singled out the cardiovascular connection as being a unique positive outcome because of the emphasis on unrestricted fat intake and mentioned physiological mechanisms as well.

Table 2: Results of Mediterranean Diet Groups Compared to Control for Different Variables

Variable

Mediterranean Diet

Control Diet

Statistical Difference*

Primary prevention of all-cause mortality

   

No difference

Cognitive impairment

   

Mixed results

Rheumatoid arthritis

   

No difference

Major cardiovascular events

Lower incidence

 

HR, 0.71
(CI, 0.56-0.90)

Breast cancer

Lower incidence

 

HR, 0.43
(CI, 0.21-0.88)

Diabetes

Lower incidence

 

HR, 0.70
(CI, 0.54-0.92)

*HR = hazard ratio; CI = 95% confidence interval

The take-home points here are as follows: 1) the benefits of the Mediterranean diet for some significant health outcomes cannot be denied, 2) fat intake may have a protective effect on cardiovascular outcomes as per a few clinical trials (the most interesting result here, and one that continues to debunk the “dangers” of fat in the diet), and 3) more, high-quality controlled trials are necessary to comment on most other health conditions, including the ability of people to adhere to this type of diet.

REFERENCE

  1. Estruch R, et al. Effect of a high-fat Mediterranean diet on body weight and waist circumference: A prespecified secondary outcomes analysis of the PREDIMED randomised controlled trial. Lancet Diabetes Endocrinol 2016; Aug 4. doi: 10.1016/S2213-8587(16)30085-7.