In the past, researchers looked at the relationship between individual foods and nutrients and health. Today, there is interest in looking at how combinations of foods and beverages, or dietary patterns, impact health. This summary of a NEL review presents what we know about different healthy eating patterns and the amounts, variety or combination of different foods and drinks, and how often they are eaten may affect body weight.

Conclusion

There is insufficient evidence on a relationship between adherence to a Mediterranean-style or vegetarian diet pattern and incidence of type 2 diabetes. There is limited, inconsistent evidence that adherence to a Mediterranean-style, DASH or modified DASH, or Nordic dietary pattern results in improved glucose tolerance and insulin resistance.

What the Research Says

• Four types of dietary patterns were identified using other methods to measure how well participants followed a specific pattern and relate it to risk of getting type 2 diabetes: (1) a Mediterranean-style pattern, (2) a DASH or modified DASH pattern, (3) a vegetarian pattern, and (4) a Nordic pattern.
• Overall, there were too few studies, too many different dietary patterns, and the study components were too different to compare the results across studies.
• A reduced chance of getting type 2 diabetes was found in a Mediterranean-style diet combined with olive oil and/or nuts, and in a vegetarian diet compared to a non-vegetarian diet where Black participants had a higher rate of type 2 diabetes than non-Blacks.
• Five out of eight studies were done outside of the United States. Only three out of eight articles reported the race/ethnicity of the participants and, of those, only one study showed results based on race/ethnicity.
• Limitations of the studies include:
  • All of the randomized controlled trials (RCTs) included participants with different health issues.
  • Too few studies looked at dietary patterns and the incidence of type 2 diabetes to be able to say whether a specific pattern or patterns protect against getting type 2 diabetes. However, two patterns studied (one Mediterranean-style and one vegetarian) showed a reduced chance of getting type 2 diabetes.
  • Too few studies looked at impaired glucose tolerance and/or insulin resistance that affect risk of type 2 diabetes, and the results were too different to identify a consistent pattern.
  • It is difficult to list the specific foods and beverages because there were too few studies looking at several different patterns that were defined differently.

Background

The goal of this systematic review project is to identify patterns of food and beverage intake that promote health and prevent disease. Historically, most dietary guidance has been based on research conducted on individual food components or nutrients. Dietary patterns are defined as the quantities, proportions, variety, or combination of different foods, drinks, and nutrients (when available) in diets, and the frequency with which they are habitually consumed. Different methods of analyses are used to assess dietary patterns including index or score, cluster or factor, reduced rank regression, in addition to other methods, to examine the relationship between adherence to dietary guidelines/recommendations or specific dietary patterns of a population and outcomes of public health concern. The objective of this systematic review was to assess the relationship between patterns of food and beverage intake identified using methods other than index or score, factor or cluster, or reduced rank analyses, and risk of type 2 diabetes.

Conclusion Statement

There is insufficient evidence on a relationship between adherence to a Mediterranean-style or vegetarian diet pattern and incidence of type 2 diabetes. There is limited, inconsistent evidence that adherence to a Mediterranean-style, DASH or modified DASH, or Nordic dietary pattern results in improved glucose tolerance and insulin resistance. (Grade IV-Not Assignable – Incidence of type 2 diabetes; Grade: III-Limited-Glucose tolerance and insulin resistance)

Methods

Literature searches were conducted using PubMed, Embase, Navigator (BIOSIS, CAB Abstracts, and Food Science and Technology Abstracts), and Cochrane databases to identify studies that evaluated the association between dietary patterns defined using methods other than index factor or cluster analysis and body weight status. Studies that met the following criteria were included in the review: conducted in subjects aged 2 to 18 years; randomized controlled trials, non-randomized controlled trials, or quasi-experimental studies; subjects from countries with high or very high human development (based on the 2011 Human Development Index); subjects who were healthy or at elevated chronic disease risk; published in English in a peer-reviewed journal.

The date range was unlimited. The results of each included study were summarized in evidence worksheets (including a study quality rating) and an evidence table. A group of subject matter experts were involved in a qualitative synthesis of the body of evidence, development of a conclusion statement, and assessment of the strength of the evidence (grade) using pre-established criteria including evaluation of the quality, quantity, consistency, magnitude of effect, and generalizability of available evidence.

Findings

• Four types of dietary patterns were identified using other methods of assessing dietary exposure related to type 2 diabetes risk: (1) a Mediterranean-style pattern, (2) a DASH or modified DASH pattern, (3) a vegetarian pattern, and (4) a Nordic pattern.
• Overall, there were too few articles and the dietary patterns and study characteristics were too varied to compare across studies.
• A favorable association was found in a Mediterranean-style diet combined with olive oil and/or nuts, and in a vegetarian diet compared to a non-vegetarian diet with incidence of type 2 diabetes higher in Black versus non-Blacks.
• Five out of eight studies were conducted outside of the United States with only three out of eight articles reported race/ethnicity and, of those, only one study reported results based on race/ethnicity.
• Limitations of the studies include:
  • All of the randomized controlled trials (RCTs) included different at-risk populations.
  • Too few articles examined a relationship between dietary patterns and the endpoint outcome of incident type 2 diabetes to draw a conclusion, although the two patterns studied (one Mediterranean-style and one vegetarian) showed a favorable effect.
  • Too few articles assessed the intermediate outcomes of impaired glucose tolerance and/or insulin resistance. The results related to impaired glucose tolerance and/or insulin resistance were too mixed to identify a consistent pattern.

Discussion

It is difficult to synthesize the results from the studies in this review because there were too few studies and they examined different dietary patterns or patterns that were operationalized differently. The studies included a predominantly Caucasian population with varied baseline health status.