Mediterranean Diet for the Prevention and Treatment of Metabolic Syndrome

Abstract

Metabolic syndrome (MetS) is characterized as a cluster of risk factors which includes abdominal obesity, hypertension, impaired fasting glucose, and dyslipidemia (elevated triglycerides [TGs] and decreased high-density lipoprotein cholesterol [HDL-C] levels).¹ It is associated with increased risk of developing type 2 diabetes mellitus (T2DM) and cardiovascular disease (CVD). Over the past years various definition criteria have been proposed by several organizations. However, these criteria may not present the same predictive ability.² Recently, the importance of a commonly used set of criteria for worldwide use for different ethnic groups and sexes has been pointed out, with emphasis given on population- and country-specific cutoff points for waist circumference.³ According to the World Health Organization, the Adult Treatment Panel III of the National Cholesterol Education Program and the International Diabetes Federation the prevalence of MetS is rapidly increasing, and is now considered as a global health problem.⁴

The term “Mediterranean diet” represents a food pattern typical in Crete and Southern Italy in the early 1960s.⁵ Generally, the term “Mediterranean diet” has been associated with dietary patterns found in olive growing areas of the Mediterranean. Its main characteristics are olive oil as the main source of fat, moderate amounts of dairy products, low to moderate amounts of fish and poultry, daily consumption of fresh fruits and vegetables, daily consumption of cereals (preferably whole grain), low amounts of red meat, and moderate amounts of alcohol (mainly red wine).^6,7

Several cross-sectional studies reported inconsistent associations between the Mediterranean-style diet and the incidence of the MetS. The Canarian Nutrition Survey (ENCA) showed that some components of the syndrome, such as blood pressure (BP), were negatively correlated with higher adherence to the Mediterranean diet, but the overall effect was negligible.⁸ In the ATTICA study, HDL-C and TG concentrations were not associated with adherence to Mediterranean diet in overweight and obese people, although the authors found negative relation with BP.⁹ Nevertheless, data from the same study revealed that men and women without any evidence of CVD or DM had 19% lower risk of MetS based on the adoption of a Mediterranean diet and light-to-moderate physical activity.¹⁰ Babio et al have observed an inverse relationship between adherence to the Mediterranean diet and prevalence of MetS in 808 elderly participants at high risk of CVD from the PREvención con DIeta MEDiterránea (PREDIMED) study. Participants with the lowest adherence to Mediterranean diet had almost twice the risk of having MetS compared to the participants with the highest adherence.¹¹

Considering prospective cohort studies, the Supplementation en Vitamines et Mineraux AntioXydants study reported a reduced incidence of MetS in those participants following a traditional Mediterranean diet, over a 6-year follow-up period mainly by affecting waist circumference, BP, TG, and HDL-C concentrations.¹² These results were confirmed in the Framingham Offspring Study.¹³ In this prospective study, consumption of a diet in accordance with the principles of the Mediterranean diet was associated with reduced abdominal obesity, insulin resistance, and atherogenic dyslipidemia. Analysis of the data from an open-enrolment cohort in Spain, consisting of 2563 university graduates (Seguimiento University of Navarra study), showed that the cumulative incidence of MetS decreases as adherence to Mediterranean diet increases.¹⁴Multivariate-adjusted results for age, sex, physical activity, smoking, and total energy intake revealed a strong inverse association. Specifically, participants with the highest values of adherence to the Mediterranean diet showed an odds ratio of 0.2 compared to those with lowest adherence.

The limitations of observational research require critical evaluation. In order to confirm the effects of a traditional Mediterranean diet on the MetS, intervention studies are required. Until now, only few intervention trials have been performed. Esposito et al showed that a Mediterranean-style diet is effective enough to reduce the prevalence of MetS, by independently affecting all of the MetS components. After 2 years, only 47% of the patients who followed the Mediterranean diet were classified as patients with MetS compared to 87% of the control group (who followed a prudent diet that consisted of 50%-60% carbohydrates; 15%-20% proteins; and <30% fat).¹⁵ The PREDIMED was a randomized trial that was conducted in Spain. Participants were patients (men, aged 55-80 years and women, aged 60-80 years) at high risk of CVD and were assigned to 3 different diets, a high-fat, nonenergy-restricted traditional Mediterranean diet enriched with nuts (30 g/d mixed nuts), a high-fat, nonenergy-restricted traditional Mediterranean diet enriched with olive oil (1 L/week virgin olive oil), and a low-fat diet (control diet). All diets were given ad libitum.¹⁶ After 1 year, the prevalence of MetS was significantly reduced only in the group which consumed a Mediterranean diet supplemented with nuts (13.7% vs 6.7% in the olive oil group and 2.0% in the low-fat diet group). No weight change was observed in any group. Richard et al compared the effects of a Mediterranean diet with or without weight loss in men with MetS.¹⁷ In all, 26 men with the MetS consumed a North American control diet for 5 weeks followed by a 5-week Mediterranean diet, both under weight maintaining conditions. Participants then underwent a 20-week weight loss period, after which they consumed the Mediterranean diet for 5 weeks under weight stable conditions. The Mediterranean diet in the absence of weight loss was efficient in reducing total cholesterol and low-density lipoprotein cholesterol concentration but with no significant changes in any of the components of the MetS. Weight loss led to a significant alleviation of the majority of the MetS components. Jones et al studied the effects of a Mediterranean-style low-glycemic load diet in women with MetS.¹⁸ In this randomized trial, 44 women consumed a Mediterranean-style low-glycemic load diet and 45 consumed a Mediterranean-style low-glycemic load diet enriched with phytochemicals. Both intervention diets produced similar results. After 12 weeks a significant reduction was observed in the waist circumference, TG concentrations, and BP. The HDL-C concentrations were significantly decreased in week 8 but restored at week 12 while plasma glucose levels did not change over the course of intervention. Although no caloric restriction was applied a significant weight loss was observed in both the groups.

Several authors have evaluated the effects of the Mediterranean diet on individual components that characterize the MetS. A series of cross-sectional studies in the general population^19

–23 and diabetic patients^24,25 have reported various findings. The overall assessment drawn from the majority of these studies is that the Mediterranean diet exerts a modest beneficial effect on most of MetS factors. On the other hand, intervention trials produced controversial results in healthy participants,^26

–29 but the relatively short implementation period (1-6 months) of a Mediterranean-style diet together with the small number of participants (16-72) should be taken into consideration. Overweight and obese patients with^30,31 or without DM^32,33 seems to benefit by adopting a Mediterranean-style diet, with the most prominent results produced by the 2 studies^30,32 with the highest number of participants and the longest intervention period. Adherence to the Mediterranean diet mainly affects BP and to a lesser degree waist circumference, TG concentrations, and HDL-C levels. Glucose concentrations are also decreased but this effect is observed mainly in diabetic patients. The inverse relationship between the adherence to the Mediterranean diet and lower MetS prevalence could be explained by the typical composition of the Mediterranean diet. The main component which characterizes the Mediterranean diet, olive oil, has been shown to improve insulin sensitivity.³⁴ Vegetables, fruits, and olive oil are rich in minerals such as potassium and calcium and this has been associated with a reduced BP.³⁵ It also supports low red meat consumption; meat consumption has been linked with increased arterial BP.³⁶ Omega-3 fatty acids, contained in fish, lower plasma TG.³⁷ Furthermore, moderate intake of alcohol has been shown to increase HDL, mainly due to an increase of HDL₂ and HDL₃.³⁸

The discrepancy of the results could be attributed to the quality, methodology, and the overall design of the studies. Analysis of key elements across studies should include: (1) the type of population selected (healthy or not); participants with a specific disease could be more aware or even have partly adopted a healthier lifestyle. (2) The length of the intervention or follow-up period; short-term trials may fail to produce clear results. (3) The number of the participants; adequate number of participants strengthen the results. (4) The ethnicity of the sample; cultural and demographic variables need to be considered. (5) Adjustment for confounding factors (age, sex, smoking, physical activity, body mass index, supplements, medication; etc). (6) The methods applied to evaluate adherence to the Mediterranean diet. (7) The definition criteria used to diagnose MetS.

As was pointed out, the number of studies on Mediterranean diet and its effects on the MetS are, until now, quite limited. Moreover, several issues need to be addressed. The long-term effects of a low-calorie Mediterranean diet designed for weight loss should be compared to the ad libitum consumption of a Mediterranean diet. Future intervention studies should analyze the impact of the glycemic load/glycemic index of the Mediterranean diet on MetS components. Finally, studies should be designed to evaluate the net effect of the Mediterranean diet on the MetS rather than the individual action on each of the MetS components.

In conclusion, implementation of Mediterranean diet might be a promising approach for patients at high risk of MetS or patients with established MetS. In addition to weight loss, lifestyle modifications including regular physical activity should be considered. Even small changes toward a healthy lifestyle may be important in the prevention and treatment of the MetS.

Footnotes

Declaration of Conflicting Interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

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