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 growth, size, body composition, and/or risk of overweight or obesity?
Conclusion statements and grades:
Dietary patterns: Children
Limited evidence suggests that dietary patterns consumed by children or adolescents that are lower in fruits, vegetables, whole grains, and low-fat dairy while being higher in added sugars, refined grains, fried potatoes, and processed meats are associated with higher fat-mass index and BMI later in adolescence. (Grade: Limited)
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 dietary patterns emphasizing vegetables, fruits, and whole grains; seafood and legumes; moderate in dairy products (particularly low and non-fat dairy) and alcohol; lower in meats (including red and processed meats), and low in sugar-sweetened foods and beverages, and refined grains are associated with favorable outcomes related to body weight, (including lower BMI, waist circumference, or percent body fat) or risk of obesity. Components of the dietary patterns associated with these favorable outcomes include higher intakes of unsaturated fats and lower intakes of saturated fats, cholesterol, and sodium. (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 distribution consumed during childhood and growth, size, body composition, and risk of overweight/obesity. (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 growth, size, body composition, and risk of overweight/obesity, due to methodological limitations and inconsistent results. (Grade: Grade not assignable)
Methods:
Two literature searches were conducted using 3 databases (PubMed, Cochrane, and 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 in diets, and the frequency with which they are habitually consumed.
Diets based on macronutrient distribution were considered to be those with at least one macronutrient proportion 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:
Eighty-eight articles were identified that met inclusion criteria and examined the relationship between dietary patterns and/or diets based on macronutrient proportion and growth, size, body composition, and/or risk of overweight or obesity.
Twelve articles from prospective cohort studies were identified that examined dietary patterns consumed by children and growth, size, body composition, and/or risk of overweight or obesity, met inclusion criteria, and were published between January 2014 and October 2019.
Dietary patterns were assessed using a variety of methods including factor/cluster analysis, indices or scores, latent class analysis, and reduced rank regression
Outcome measures varied across studies and included incidence of overweight/obesity, fat mass, lean mass, body mass index (BMI), central adiposity, and weight/height.
Despite variability in methods, dietary patterns in childhood or adolescence that generally reflect poorer diet quality tended to associate with higher fatmass index and BMI later in adolescence. However, the findings should be interpreted with caution due to several limitations.
Across the body of evidence, the direction of significant findings were mixed with relatively small and inconsistent magnitude.
Most of the studies assessed diet once at baseline with methods that were not necessarily valid, reliable, or applicable in children.
Fifty-four articles examined dietary patterns consumed by adults and growth, size, body composition, and/or risk of overweight or obesity. The 2020 Committee determined with a systematic evidence scan 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.
No studies identified met inclusion criteria that examined diets based on macronutrient distribution consumed by children or adolescents and growth, size, body composition, and/or risk of overweight or obesity.
Thirty-one articles examined diets based on macronutrient distribution consumed by adults and growth, size, body composition, and/or risk of overweight or obesity, met inclusion criteria, and were published between January 2000 and October 2019
Twenty-two articles came from randomized controlled trials (RCTs) and nine articles came from 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.
Foods or food groups consumed as part of the diet were not reported consistently.
Results across studies were inconsistent, with the majority of studies reporting no significant association between diets based on macronutrient distribution and growth, size, body composition, and/or risk of overweight or obesity.
Some studies reported diets based on macronutrient distributions with proportions of energy from carbohydrate and/or fat within or close to AMDRs compared to those that were outside AMDRs significantly associated with favorable fat-mass, BMI, waist circumference, and/or change in weight. Notably, these results were closely linked to better quality of the diet where reported, typically consisting of vegetables, fruit, whole-grains, vegetable oils, nuts, pulses, and/or lean meat or fish
Numerous limitations that prevent adequate assessment across this body of evidence were identified:
Several studies did not directly test the difference in macronutrient proportions in the context of various dietary patterns during energy balance.
Although statistically significant relationships were reported, the gradient between macronutrient distributions was relatively narrow within studies (e.g., 45.3% vs. 43.8% carbohydrate) and between studies
Due to the variety of methods used to estimate macronutrient intake and adjust intake for total energy, the confidence in the reported proportions of energy falling outside the AMDR is low.
Several studies reported to be examining one particular macronutrient of interest, such as “high-protein” or “low-carbohydrate” intake, but the proportion for that nutrient was within the AMDR.