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What does the evidence say about the Mediterranean diet?

Watts, Sharon A. DNP, FNP-BC, CDE; Stevenson, Carl BSN, RN; Patterson, Julianne MSN, RN, CDE

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doi: 10.1097/01.NURSE.0000530407.38450.a3
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OVER 84 MILLION adults in the United States with prediabetes need lifestyle education to prevent the onset of diabetes.1,2 (See Startling statistics.) Nurses can take steps to help mitigate the harm of both prediabetes and diabetes that are fueled by fast, inexpensive, and overly processed food. This article reviews what generally constitutes the Mediterranean diet (MD), presents the evidence about the role the MD plays in type 2 diabetes mellitus (T2DM) prevention, and suggests how nurses can present the concept of this diet to patients and their families.

Defining terms

Primary prevention aims to prevent disease or injury before it occurs in the population at large. Examples of primary prevention include preventing exposures to hazards that cause disease or injury, altering unhealthy or unsafe behaviors that can lead to disease or injury, and increasing resistance to disease or injury should exposure occur.3 Nurses could teach patients about the prevention of diabetes by diet and exercise, an important primary prevention.

Secondary prevention aims to reduce the impact of a disease or injury that's already occurred through evidence-based interventions. Examples of secondary prevention include treating disease or injury as soon as possible to halt or slow its progress, encouraging personal strategies to prevent reinjury or recurrence, and implementing programs to return people to their original health and function and to prevent long-term problems.3 An example of secondary prevention would be a change in diet and exercise that would help lower the hemoglobin A1C (A1C) in a patient who's been diagnosed with diabetes.

Prediabetes is a risk factor for cardiovascular disease. (See Resources about diabetes.) However, risk factor modification, including diet, is known to reduce the progression of prediabetes to diabetes.2 Until the past decade, the typical dietary advice has been to lose 7% to 10% of body weight by eating a well-balanced diet (along with increased physical activity) based on the Diabetes Prevention Trial, a multicenter, randomized controlled trial (RCT; N = 3,234) conducted from 1996 to 2001 in the United States.4

Emerging evidence now points to the MD as a healthy eating option to help patients reduce their risk of developing T2DM. For those who already have diabetes, this diet improves glycemic control and provides cardiovascular protection.2,5

What constitutes the MD?

The MD doesn't have a single definition; the studies discussed in this article used variations of the MD found in the countries surrounding the Mediterranean Sea.5 The term MD is often used to describe the consumption of common healthy foods of the region.

Trichopoulou and colleagues compared various ingredients of the MD and the impact each part of the diet has on health and reduction of total mortality.6 (See How do components of the MD compare?) Certain parts of the diet have a more positive impact on health than others. For example, the high intake of plant foods such as vegetables, fruits, nuts, and legumes (fleshy, colorful plant seeds such as beans, peas, and lentils) has the largest impact.7-9

Olive oil is the staple fat used for cooking in this diet.8 Butter, margarine, and other solid fats are rarely used. Solid fats are high in saturated and trans fats, which damage arteries and can contribute to dyslipidemia.7-9 Reducing saturated fat improves insulin sensitivity, although this hasn't been well studied in individuals with diabetes.10 Olive oil is a good source of monounsaturated fat, or healthy fat, that can help improve cholesterol levels when used instead of saturated fats.7-9

Although the MD was traditionally part of the lifestyle of poor rural residents in the MD region before the mid-1960s, regular physical activity was also integral to this lifestyle. The MD is much more than a dietary plan; the physically active lifestyle of these residents is an important factor in the recommendations for spreading the traditional MD to non-MD countries.6

Mattioli and colleagues conducted a review of 17 research trials and defined the MD as the eating habits found in populations living around the Mediterranean Sea (Crete and southern Italy) after World War II. During that time, food was in short supply and nutrition was “characterized by a relatively high consumption of inexpensive and [unprocessed] foods such as cereals, vegetables, legumes, nuts, fish, fresh fruits, and olive oil as the principal source of fat. The consumption of meat was low, whereas the consumption of milk, dairy products, and wine was low–moderate.”5

The components of the traditional MD are like those in other modern healthy diet trends (such as the Healthy Eating Index or the Dietary Approaches to Stop Hypertension) with two unique elements: moderate intake of red wine during meals and abundant fat intake, provided that the fat comes from virgin olive oil, tree nuts, and/or fatty fish.5,11

Trichopoulou and colleagues stated that future research is needed to clarify whether red wine should be distinguished from other types of alcohol intake as part of the MD's health benefits.6 Gepner and colleagues studied 54 individuals in Israel who had T2DM. They compared A1C levels between two groups: Group one drank 150 mL of dry red wine with dinner, and group two drank mineral water. Both groups were advised to adhere to the MD, without caloric restriction. They found that those who drank red wine had an A1C of 6.6 ± 0.9, and those who drank mineral water had an A1C of 6.9 ± 1.4.12

Primary prevention

A meta-analysis by Bloomfield and colleagues (N = 7,447) showed that individuals who adhered to the MD had a 30% decrease in the risk of developing T2DM.13 More recently, Dinu and colleagues published a meta-analysis review of 10 articles that revealed a 17% drop in the incidence of diabetes for individuals following the MD.14

Secondary prevention

In a systematic review of 20 RCTs and 3,073 participants with T2DM, Ajala and colleagues compared the MD with other dietary interventions (such as low-fat, high-glycemic index, American Diabetes Association [ADA], European Association for the Study of Diabetes, and low-protein diets) that lasted 6 months or longer.15 They found a weighted mean difference in A1C of −0.47% in favor of the MD. The MD was also effective in helping patients lose weight.15

Huo and colleagues conducted a meta-analysis of RCTs to explore the effects of the MD on glycemic control, weight loss, and cardiovascular risk factors in 1,178 patients with T2DM.16 The MD and control diets with which they were compared weren't consistent across the studies. The control diets were a low-fat diet; usual dietary habits; nonrestricted-calorie, low-carbohydrate diet; the 2003 ADA diet; or a high-carbohydrate diet. Compared with control diets, the MD led to greater reductions in A1C, fasting plasma glucose, fasting insulin, body mass index, and body weight.16

How do components of the MD compare?

Recently, Esposito and colleagues published a meta-analysis of the MD for patients with T2DM.17 They found that the MD is associated with better glycemic control in patients with T2DM (A1C reduction ranging from 0.3% to 0.47%). They propose that the MD may protect patients from T2DM due to its anti-inflammatory and antioxidative effects. The protective nutrients in the diet, such as fiber, vitamins, minerals, and polyphenols, as well as the absence of proinflammatory nutrients such as saturated and transfatty acids, refined sugars, and starches, are thought to account for the diet's beneficial effects.17 The evidence supports the use of the MD to help patients reduce the risk of T2DM.

What nurses can do

Due to the increasing number of patients diagnosed with T2DM, nurses need to be prepared to teach patients about healthy food choices. Many nurses are the “first responders” to newly admitted or clinic patients who may be challenged by needing to balance diabetes management with that of other complex medical problems.18

Completely changing one's diet can be overwhelming. Nurses can suggest patients make small changes to their diet to gradually incorporate more MD elements. For example, they can encourage patients to use more olive oil rather than butter or margarine. Individualize teaching and nutrition plans based on each patient's needs.19 Advise them to consume more fruits and vegetables as snacks instead of processed or prepackaged items.7

Extra virgin and virgin olive oils are high in monounsaturated fatty acids and can be used in cooking and baking as an alternative to butter and other oils, which are higher in saturated and trans fats.7,20 Teach patients to choose whole wheat instead of white bread, pasta, and rice and to use these in moderation.7

Encourage them to use vinaigrette dressings as a vegetable dip instead of sour cream-based dips. Another healthy choice is to sprinkle on seeds, nuts, and vinaigrette dressings instead of using cheese, croutons, and creamy dressings.7,21,22

Incorporating more legumes, vegetables, and fruit into a diet plan with less meat is consistent with the MD-style diet plan. Consuming a moderate amount of poultry and fatty fish such as salmon and tuna are recommended as well as consuming a small amount of lean meat.7

A simple, easy-to-understand tool is the MD plate method, which can be useful when low literacy issues are a concern.22 Refer patients to a registered dietitian nutritionist and to diabetes self-management education classes for more in-depth clarification of what constitutes healthy eating.2 Finally, encourage all patients to maintain regular physical activity in conjunction with making healthy diet choices.2,6,10,19

Knowledge is power

Nurses can familiarize themselves with the MD and its impact on the health of patients with prediabetes and T2DM. Encourage patients to progressively adjust their diets to include more of the items in the traditional MD.2,6,10,19 All nurses play an important role in teaching patients how to reduce their risk for developing T2DM and improving glycemic control in patients with preexisting T2DM.

Startling statistics1

The CDC estimates that from 2009 to 2012, 86 million U.S. adults age 20 or older had prediabetes, or 37% of the U.S. population. For those age 65 or older, the proportion with prediabetes rose to 51%. The CDC's findings are based on fasting glucose or A1C levels. Additionally, 12.6% of adults age 20 or older already had diabetes.

Resources about diabetes


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3. Institute for Work and Health. What researchers mean by...primary, secondary and tertiary prevention.
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7. MedlinePlus. MD diet review. 2016.
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15. Ajala O, English P, Pinkney J. Systematic review and meta-analysis of different dietary approaches to the management of type 2 diabetes. Am J Clin Nutr. 2013;97(3):505–516.
16. Huo R, Du T, Xu Y, et al. Effects of Mediterranean-style diet on glycemic control, weight loss and cardiovascular risk factors among type 2 diabetes individuals: a meta-analysis. Eur J Clin Nutr. 2015;69(11):1200–1208.
17. Esposito K, Maiorino MI, Bellastella G, Chiodini P, Panagiotakos D, Giugliano D. A journey into a Mediterranean diet and type 2 diabetes: a systematic review with meta-analyses. BMJ Open. 2015;5(8):e008222.
18. U.S. National Library of Medicine. Metabolic syndrome. Medline Plus. 2017.
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20. MedlinePlus. Dietary fats explained. 2017.
21. MedlinePlus. Salads and nutrients. 2017.
22. Watts SA, Stevenson C, Adams M. Improving health literacy in patients with diabetes. Nursing. 2017;47(1):24–31.
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