Background:
This important public health question was identified by the U.S. Departments of Agriculture (USDA) and Health and Human Services (HHS) to be examined by the 2020 Dietary Guidelines Advisory Committee.
The 2020 Dietary Guidelines Advisory Committee, Dietary Patterns Subcommittee conducted a systematic review to answer this question with support from the Nutrition Evidence Systematic Review (NESR) team.
The goal of this systematic review was to examine the following question: What is the relationship between dietary patterns consumed and risk of type 2 diabetes?
Conclusion statements and grades:
Dietary patterns: Children
Insufficient evidence is available to determine the relationship between dietary patterns consumed by children or adolescents and risk of type 2 diabetes. Grade: Grade Not Assignable
Dietary patterns: Adults
The 2020 Dietary Guidelines Advisory Committee reviewed newly published evidence using a systematic evidence scan and determined that the conclusion drawn by the 2015 Dietary Guidelines Advisory Committee generally reflects the current state of science: Moderate evidence indicates that healthy dietary patterns higher in vegetables, fruits, and whole grains and lower in red and processed meats, high-fat dairy products, refined grains, and sweets/sugar-sweetened beverages reduce the risk of developing type 2 diabetes. 2015 Dietary Guidelines Advisory Committee Grade: Moderate
Diets based on macronutrient distribution: Children
No evidence is available to determine a relationship between diets based on macronutrient proportion distribution consumed during childhood and risk of type 2 diabetes. Grade: Grade Not Assignable
Diets based on macronutrient distribution: Adults
Insufficient evidence is available to determine the relationship between macronutrient distributions with proportions of energy falling outside of the AMDR for at least one macronutrient and risk of type 2 diabetes, due to methodological limitations and inconsistent results. Grade: Grade Not Assignable
Methods:
Two literature searches were conducted using 3 databases (PubMed, Cochrane, Embase) to identify articles that evaluated the intervention or exposure of dietary patterns consumed and the outcomes of type 2 diabetes. A manual search was conducted to identify articles that may not have been included in the electronic databases searched. Articles were screened by two NESR analysts independently for inclusion based on pre-determined criteria
Data extraction and risk of bias assessment were conducted for each included study, and both were checked for accuracy. The Committee qualitatively synthesized the body of evidence to inform development of a conclusion statement(s), and graded the strength of evidence using pre-established criteria for risk of bias, consistency, directness, precision, and generalizability.
Dietary patterns were 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.
Diets based on macronutrient distribution were examined when at least one macronutrient proportion was outside of the acceptable macronutrient distribution range (AMDR) for carbohydrate, fat, and/or protein, whether or not the foods/food groups consumed were provided.
Studies examining energy-restricted diets that induce weight loss or treat overweight and obesity for the purposes of treating additional or other medical conditions were excluded.
Summary of the evidence:
Dietary patterns: Children
One article from a prospective cohort study examined dietary patterns consumed during adolescence (retrospectively) and risk of type 2 diabetes.
Dietary patterns: Adults
Fifty-two articles examined dietary patterns consumed by adults and risk of type 2 diabetes.
These articles represent new evidence published since a systematic review completed by the 2015 Dietary Guidelines Advisory Committee.
A systematic evidence scan was conducted to identify and examine these articles, and determine whether a full systematic review update was warranted.
Based on results from the systematic evidence scan, the 2020 Committee determine that the recently published evidence was generally consistent with the body of evidence from the existing review, and a full systematic review update was not needed at this time. Therefore, the conclusion statement and grade from the existing review were carried forward.
Diets based on macronutrient distribution: Children
No articles were identified that met inclusion criteria and examined diets based on macronutrient distribution consumed during childhood and risk of T2D across the lifespan.
Diets based on macronutrient distribution: Adults
Twenty-three articles examined diets based on macronutrient distribution consumed by adults and risk of type 2 diabetes, met inclusion criteria, and were published between January 2000 and October 2019.
Two studies were RCTs, and 21 articles were prospective cohort studies.
When describing and categorizing studies included in this review, the Committee did not label the diets examined as “low” or “high,” because no standard definition is currently available for, for example, “low-carbohydrate” or “high-fat” diets. Instead, the Committee focused on whether, and the extent to which, the proportions of the macronutrients were below or above the AMDR.
Most of the articles examined distributions in which the proportion of energy from carbohydrate was below the AMDR; fat was above the AMDR; and protein was within the AMDR in at least one of the exposure groups compared.
Across studies, aspects of diet quality i.e., the foods/food groups consumed as part of the diet, were reported with limited detail such as “animal-based” macronutrient distributions.
Among studies that provided the context of foods/food groups, diets based on macronutrient distributions with proportions outside of the AMDR tended to have higher amounts of saturated fat, trans fat, and/or animal-based sources of protein and fat, such as processed meat, red meat, butter, and cheese as well as refined grains, SSB’s, and lower-fiber cereals/breads.
Numerous limitations were identified that prevent adequate assessment across this body of evidence:
Several studies did not directly test differences in macronutrient proportions in the context of a constant dietary pattern
The gradient between macronutrient proportions compared within and across studies varied. Several studies compared distinct proportions between groups (e.g., 33.4% vs. 47.5% carbohydrate), whereas others were much closer in proximity relative to one another (e.g., 41.0% vs. 45.0% carbohydrate) or to the AMDR limit (e.g., 44.9% vs. 45% carbohydrate)