Rationale for the Use of a Mediterranean Diet in Diabetes Management

Nutrition Therapy Recommendations

It is helpful to begin with a discussion of nutrition therapy for A1C, blood pressure, and cholesterol. The ADA's diabetes nutrition recommendations focus mainly on carbohydrate monitoring to manage blood glucose.² No limits or specific ranges are given for carbohydrates; rather, the method of monitoring carbohydrates can be tailored to patients' individual preference or level of understanding (e.g., carbohydrate counting, exchanges, or experience-based estimation). In addition, the ADA recommendations emphasize reducing saturated fat to < 7% of overall calories and minimizing trans fats to prevent or treat heart disease.²

Lifestyle modifications, with a particular emphasis on food and nutrition, are recommended for anyone who is not at goal for blood pressure (i.e., who has a blood pressure ≥ 130/80 mmHg for an individual with diabetes). The Joint National Committee on the Prevention, Detection, Evaluation, and Treatment of High Blood Pressure recommends adopting the Dietary Approaches to Stop Hypertension (DASH) eating plan,⁶ which is rich in fruits and vegetables, includes some low-fat dairy products, and calls for sodium intake of ≤ 2.4 g/day. This diet also calls for moderation of alcohol (no more than two drinks per day for men and one drink per day for women, with one drink defined as 12 oz of beer, 5 oz of wine, or 1.5 oz of distilled spirits [e.g., whiskey]). Following the DASH style of eating has demonstrated reductions in blood pressure similar to those of treatment with one anti-hypertensive medication.⁷

The Third Report of the Expert Panel on Detection, Evaluation, and Treatment of High Blood Cholesterol in Adults (Adult Treatment Panel III [ATP III]) presents guidelines for cholesterol management to reduce the risk of CHD or future events.⁸ ATP III recognizes diabetes as a CHD risk-equivalent, and as discussed previously, cholesterol management is a high priority in the treatment of diabetes. The nutrition recommendations within the guidelines, known as the Therapeutic Lifestyle Changes (TLC) diet, provide percentile recommendations for macronutrients to reduce risk.⁸ Although the guidelines are succinct, this approach makes it difficult to interpret the guidelines for both clinicians and patients (i.e., specific recommendations depend on personalized calorie goals and require nutrition label-reading skills).

Table 1 provides the specific details of the TLC diet recommendations. Of note, total fat is provided in a range of 25–35% of total calories to reflect consumption of different types of fats. People are allowed to eat more fat as long as most of the fat comes from healthier sources (e.g., monounsaturated and polyunsaturated fats), limiting the amount of saturated and trans fats. Unique recommendations within the TLC diet include options to add stanols/sterols (2 g/day) and increased soluble fiber (10–25 g/day) to maximize LDL cholesterol lowering.

Although some duplication does exist among the nutrition guidelines (e.g., recommendations for fruit, vegetable, alcohol, and saturated and trans fat consumption), there are some inconsistencies. What if there were one nutrition strategy that could target all three parameters (blood glucose, blood pressure, and cholesterol), keeping it simple for both health professionals and patients? More importantly, what if this single set of nutrition recommendations could be explained in simple terms (e.g., foods that people eat) and would not require advanced skills (e.g., label reading, calculations)?

The Mediterranean diet may be such a solution. It captures elements of both the TLC and the DASH diets and provides more tangible recommendations (e.g., “Eat more fruits, and limit red meat” vs. “Limit intake of saturated fat to < 7% of overall calories”). Essentially, the Mediterranean diet, by design, affects blood glucose, blood pressure, and cholesterol management, making it an intriguing choice for diabetes clinicians and their patients.

The Mediterranean Diet

Researchers have used the term “Mediterranean diet” to describe a centuries-old eating pattern that includes mostly plant-based, nutrient-dense foods such as fruits, vegetables, legumes, nuts, and whole grains, with olive oil as the main fat source. Additionally, the Mediterranean-type diet includes foods rich in omega-3 fatty acids (usually eaten as fatty fish and walnuts), with limited amounts of red meat and other animal products (e.g., butter and other dairy foods).⁹ Moderate consumption of wine (i.e., one glass of wine per day for women, and two per day for men) with meals is generally indicated as a component of the eating pattern because there is a potential decrease in lipid peroxidation when dietary polyphenols (from wine) and oxidized fats are consumed in the same meal.¹⁰

Because the Mediterranean diet is meant to represent the eating patterns of all the countries bordering the Mediterranean Sea, use of this term in research is a misnomer; there is no one single diet that can represent all the economic, cultural, and religious differences that influence the eating patterns of all of these countries. Variations in Mediterranean food patterns make this issue particularly difficult to study, and contradictory results are often found in studies that focus solely on the component foods or food groups.¹¹ Yet researchers continue to classify this eating pattern as the “Mediterranean diet,” which is why the authors have used the term in this article.

To remedy this classification issue (i.e., variability in actual eating patterns within the Mediterranean region), researchers have developed Mediterranean diet scores or indexes.^12,13 These indexes attempt to assess adherence to a traditional Mediterranean-type diet in research to facilitate analyzing the eating pattern and any related health benefits as a whole. The common belief is that there are unknown, but potentially powerful, synergistic qualities of an overall Mediterranean-style eating pattern that may transcend the qualities of the individual foods within the eating pattern that are known to improve health.

Health Benefits and Evidence to Support the Use of a Mediterranean Diet in People With Type 2 Diabetes

The health benefits of choosing an eating pattern similar to the Mediterranean diet are numerous. Benefits specifically related to diabetes include reduction in overall mortality and mortality of CVD,¹⁴ prevention of diabetes, and improvement in glycemic control and cardiovascular risk in people with diabetes.¹⁵

In a meta-analysis of 17 studies,¹⁵ the Mediterranean-type diet was found to improve fasting glucose and A1C levels for those with type 2 diabetes. In several studies,^16–18 the Mediterranean diet lowered fasting glucose levels in those with diabetes more than did low-fat diets.

The reason for lower fasting glucose levels may be related to the positive effect on insulin sensitivity that results from replacing saturated and trans fats with unsaturated fats.¹⁶ Specifically, a low-carbohydrate Mediterranean diet (35% carbohydrate, 45% fat [50% of which is monounsaturated fat], and 20% protein) demonstrated glycemic benefit compared to both a control diet and a traditional Mediterranean diet (both of which contained 40–55% carbohydrate, 30% fat, and 15–20% protein).¹⁹

One study by Esposito et al.²⁰ found that adherence to a Mediterranean-type diet decreased A1C levels and postprandial glucose levels measured independently by subjects in their natural environment. In addition, participants with the highest adherence to the Mediterranean diet had lower BMIs, waist-to-hip ratios, and prevalence of metabolic syndrome.

It is important to note that some studies may not separate the effects of the Mediterranean diet from other factors influencing outcomes, such as other risk factor management resources.¹⁵ Therefore, further studies are needed to clarify these issues.

In terms of diabetes prevention, an 83% lower risk of diabetes has been found among those who closely adhered to a Mediterranean diet.²¹ Furthermore, adherence to a Mediterranean diet, even without calorie restriction, seems to be effective in preventing diabetes among those at high cardiovascular risk.²²

The abundant use of olive oil, fruits, and vegetables in this pattern of eating leads to additional health benefits. Using monounsaturated fatty acids (MUFAs) such as those found in olive oil or polyunsaturated fats such as those in seeds instead of saturated or trans fatty acids reduces the risk of diabetes.¹⁶ The Mediterranean diet is also inversely associated with arterial blood pressure, and olive oil use specifically may be as important as fruit and vegetable intake for both systolic and diastolic blood pressure control.²³

The Mediterranean diet may even have positive effects on inflammatory markers²⁴ and may reduce the risk for peripheral artery disease.²⁵ Two outcome-based trials that included patients with diabetes demonstrated that a Mediterranean diet reduced cardiovascular and total mortality.^26,27

Practical Applications for Diabetes Clinicians

The Mediterranean diet, which is based on a traditional eating pattern, may be a simpler nutrition strategy that can help patients with diabetes achieve more optimal glycemic control and reduce the risk of complications. Before starting patients on this type of eating pattern, it is important to assess what particular meal-planning approach they currently use to account for carbohydrates (e.g., carbohydrate counting, exchanges, or experience-based estimation) in their daily eating pattern and to provide further education if needed. This is essential because patients will likely need to replace existing carbohydrates with healthier versions (e.g., fruits, vegetables, whole grains, and legumes) to adopt a style of eating similar to the Mediterranean diet and achieve their glycemic goals.

Explaining the Mediterranean-style eating pattern as a whole and then allowing patients to choose areas to focus on first is recommended to improve adherence. Specific areas that may be suggested for initial nutrition therapy include reducing the number of meals per week that feature red meat by substituting legumes or fish; increasing the amount of fruits and vegetables consumed; and replacing saturated fats with monounsaturated fats.

One study²⁸ conducted to assess the impact of a Mediterranean diet using exchanges for breast cancer prevention found that women who received fairly extensive counseling were able to increase the amount of monounsaturated fat in their diet with very little impact on their overall fat intake. They were also able to double their intake of fruits and vegetables. To increase adoption of Mediterranean diet principles, diabetes clinicians should provide specific examples of foods to include rather than focus on macronutrients or micronutrients as a group.²⁹

Conclusion

Ultimately, patients with diabetes need practical recommendations to help them choose healthier foods without an overly complicated process. Practical information can help patients increase their self-efficacy in eating healthier and better managing their diabetes. An eating pattern similar to the traditional Mediterranean diet can be integrated with existing national guidelines for the management of diabetes, blood pressure, and cholesterol. Existing data suggest that the Mediterranean diet has health benefits, including improved glycemic control and reduced cardiovascular risk,¹⁵ and may offer benefits to diabetes patients and clinicians alike in terms of palatability, ease of explanation and use, and promotion of improved health.