Can the Mediterranean diet lower HbA1c in type 2 diabetes? Results from a randomized cross-over study

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Abstract

Background and aims

To investigate the impact of a diet modeled on the traditional Cretan Mediterranean diet on metabolic control and vascular risk in type 2 diabetes.

Methods and results

Twenty-seven subjects (47–77 yrs) with type 2 diabetes were randomly assigned to consume either the intervention diet ad libitum or their usual diet for 12 weeks and then cross over to the alternate diet. Most of the meals and staple foods for the intervention diet were provided. Lipids, glycemic variables, blood pressure, homocysteine, C-reactive protein, plasma carotenoids and body composition (anthropometry and dual energy X-ray absorptiometry) were assessed at baseline, and at the end of both diet periods. Dietary adherence was monitored using plasma carotenoid and fatty acid (FA) analysis, complemented by diet diaries. Compared with usual diet, on the ad libitum Mediterranean intervention diet glycosylated haemoglobin fell from 7.1% (95% CI: 6.5–7.7) to 6.8% (95% CI: 6.3–7.3) (p = 0.012) and diet quality improved significantly [plant:animal (g/day) food ratio increased from 1.3 (95% CI: 1.1–1.5) to 5.4 (95% CI: 4.3–6.6) (p < 0.001)], plasma lycopene and lutein/zeaxanthin increased (36% and 25%, respectively), plasma saturated and trans FAs decreased, and monounsaturated FAs increased.

Conclusion

A traditional moderate-fat Mediterranean diet improves glycemic control and diet quality in men and women with well-controlled type 2 diabetes, without adverse effects on weight.

Introduction

Diabetes causes significant morbidity and premature mortality. Major determinants of type 2 diabetes are modifiable [1]. Intensive diet and lifestyle interventions are effective in preventing and managing diabetes [2].

The well-known cardioprotective benefits of the traditional Cretan Mediterranean diet were first described in the Seven Countries Study [3]. Since then numerous benefits have been attributed to Mediterranean-style diets, the Lyon Diet Heart Study demonstrated a remarkable 70% reduction in recurrent myocardial infarction and sudden death in high risk patients [4]. Until recently, there was a paucity of published data on the impact of a traditional Cretan Mediterranean cuisine, characterized by an abundance of plant foods (particularly edible wild leafy greens, tomatoes, onions, garlic, herbs, wholegrain cereals, legumes and nuts) and olive oil as the principal source of fat, on metabolic control in type 2 diabetes, as most studies focused on varying the type and amount of fat [5], [6], [7]. A number of recent prospective studies have shown that stronger adherence to a traditional Mediterranean-type diet may prevent the onset of type 2 diabetes [8], [9]. Furthermore, a recent meta-analysis demonstrated that greater adherence to a Mediterranean diet was associated with significant improvement in health status and reduction in all-cause, cardiovascular and cancer mortality [10].

A growing number of dietary approaches to improving long term metabolic control in diabetes have recently been trialed [11], [12], [13], [14], [15], [16], [17]. A number of trials of shorter duration, have demonstrated improvements in insulin resistance (as measured by the HOMA method) with the Mediterranean diet [18], [19]. Of the few longer-term studies examining HbA1c none have investigated the impact of the traditional Mediterranean diet in its entirety (where majority of foods provided); one utilized a restricted calorie Mediterranean-style diet [13], another a high-monounsaturated fat diet modeled on the Mediterranean diet [15], another provided detailed guidelines on a Mediterranean diet [16], and another provided general dietary guidelines: low fat vs low carbohydrate diet [11] – The few dietary interventions assessing impact on HbA1c have been as effective as drug intervention trials, but without the side effects observed in drug trials [20], [21], [22]. Therefore, there is a pressing need for longer duration dietary intervention studies, utilizing the traditional Mediterranean diet in its entirety, with HbA1c as an outcome.

The aim of the present study was to examine the impact of a traditional Mediterranean-type cuisine on HbA1c and vascular risk in type 2 diabetes.

Section snippets

Subject recruitment, study design and population

Volunteers with well-controlled type 2 diabetes were invited to participate via local newspaper advertisements (Fitzroy, Melbourne). Exclusion criteria were: presence of disabling stroke, cancer not in remission, renal failure, or liver disease. Thirty-one Australian-born people, not previously exposed to a Mediterranean diet, were recruited into the study. Eligible subjects were provided with a plain language statement including study aims and investigations, consent form, diet diary, physical

Results

Twenty-seven adults (sixteen men and eleven women) completed the study, mean age 59 years. Gender specific data are pooled as there were no differences in baseline characteristics by gender. The sequence of the diet did not influence the outcome, therefore, data for each diet were pooled. Sixteen subjects (59%) were obese, with a mean BMI of 30.7 (kg/m2) (95% CI: 28.9–32.6). The subjects had a mean HbA1c of 7.1% (95% CI: 6.5–7.8) at recruitment and their mean duration of diabetes was six years

Discussion

This dietary intervention study confirms previous studies demonstrating the benefits of a traditional Mediterranean-type diet in the prevention and management of diabetes [8], [9], [15], [16], [17], [28]. However, this current study is one of the few studies to examine the impact of a traditional Mediterranean-type diet (in a randomized cross-over trial), in its entirety, where the majority of foods (>70%) are provided to subjects, on weight and metabolic control of type 2 diabetes. The

Acknowledgments

This work would not have been possible without the Melbourne Collaborative Cohort Study and the infrastructure support provided by the Cancer Council Victoria. The National Health and Medical Research Council of Australia part-funded this study (Project # 124317), which represents the collaboration of and contribution by many.

The authors would like to acknowledge Connie Karschimkus for assisting with the laboratory analyses, Dr Su Qing for analysis of plasma carotenoids, and Diana So for

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  • Cited by (0)

    Supported in part by grants from the NHMRC in Australia, Diabetes Australia, and Diabetes CCRE (Centre for Clinical Research Excellence in Clinical Science in Diabetes).

    Australian Clinical Trials Registration Number (ACTRN): ACTRN012607000394448.

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