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 cardiovascular disease?
Conclusion statements and grades:
Dietary Patterns: Children
Limited evidence suggests that dietary patterns consumed by children and adolescents reflecting higher intakes of vegetables, fruits, whole grains, fish, low-fat dairy, legumes, and lower intake of sugar-sweetened beverages, other sweets, and processed meat, are associated with lower blood pressure and blood lipid levels, including low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, and triglycerides later in life. (Grade: Limited)
Dietary Patterns: Adults
The 2020 Dietary Guidelines Advisory Committee conducted a systematic evidence scan and confirmed that the conclusion drawn by the 2015 Dietary Guidelines Advisory Committee generally reflects the current state of science: Strong and consistent evidence demonstrates that dietary patterns associated with decreased risk of cardiovascular disease are characterized by higher consumption of vegetables, fruits, whole grains, low-fat dairy, and seafood, and lower consumption of red and processed meat, and lower intakes of refined grains, and sugar-sweetened foods and beverages relative to less healthy patterns. Regular consumption of nuts and legumes and moderate consumption of alcohol also are shown to be components of a beneficial dietary pattern in most studies. Randomized dietary intervention studies have demonstrated that healthy dietary patterns exert clinically meaningful impact on cardiovascular risk factors, including blood lipids and blood pressure. Additionally, research that includes specific nutrients in their description of dietary patterns indicate that patterns that are lower in saturated fat, cholesterol, and sodium and richer in fiber, potassium, and unsaturated fats are beneficial for reducing cardiovascular disease risk. (2015 Dietary Guidelines Advisory Committee Grade: Strong)
Diets Based on Macronutrient Distribution: Children
No evidence was available to determine the relationship between diets based on macronutrient distribution consumed by children or adolescents and concurrent or future development of cardiovascular disease. (Grade: Grade Not Assignable)
Diets Based on Macronutrient Distribution: Adults
Limited evidence suggests non-energy restricted diets based solely on macronutrient distribution with either carbohydrate, fat, and/or protein proportions outside of the Acceptable Macronutrient Distribution Range, are neither beneficial nor detrimental regarding risk of cardiovascular disease in adults, primarily among those at high-risk, such as those with overweight, obesity or features of metabolic syndrome. (Grade: Limited)
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 cardiovascular disease. 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:
One-hundred ninety articles were identified that met inclusion criteria and examined the relationship between dietary patterns and/or diets based on macronutrient proportion and risk of cardiovascular disease (CVD).
Four included articles, all from prospective cohort studies (PCSs) published between January 2014 and October 2019, examined the relationship between dietary patterns consumed by children and CVD.
Two of the articles used index or score analyses to examine dietary patterns.
Two of the articles examined the dietary patterns identified with factor and cluster analyses.
Most of the studies examined intermediate CVD outcomes in childhood, although one study reported on incidence of CVD in adulthood.
This body of evidence updates an existing systematic review from the 2015 Dietary Guidelines Advisory Committee, which found insufficient evidence in pediatric populations published between 1980 and 2013 that met inclusion criteria on dietary patterns and CVD and therefore, was unable to form a conclusion statement at the time.
One-hundred forty-nine articles examined dietary patterns consumed by adults and CVD.
These articles represent new evidence published since an existing systematic review that included articles published between January 1980 and 2013, which was reviewed by the 2015 Committee.
A systematic evidence scan was conducted to identify and examine newly published evidence, and determine whether a full systematic review update was warranted.
Based on results from the systematic evidence scan, the 2020 Committee determined that the newly 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 articles were identified that met inclusion criteria and examined diets based on macronutrient distribution consumed in childhood and risk of CVD across the lifespan.
Forty-nine included articles examined diets based on macronutrient distribution consumed by adults and CVD outcomes, met inclusion criteria, and were published between January 2000 and October 2019.
Nineteen articles came from randomized controlled trials (RCTs) and 30 articles came from PCSs.
When describing and categorizing studies included in these reviews, 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 studies enrolled participants who were overweight or obese, or exhibited features of metabolic syndrome.
The majority of RCTs (n=11) reported no significant effects of macronutrient distributions on intermediate CVD outcomes, such as LDL cholesterol.
Although results from several RCTs (n=8) reported significantly improved intermediate CVD outcomes, diets compared between studies were heterogeneous with macronutrient distributions inconsistently above or below the AMDR and dependent upon the comparison of interest (e.g., fat within vs. above the AMDR).
Many PCSs reported no significant associations across specified macronutrient distributions and CVD mortality endpoint outcomes.
Among the PCSs (n=9 of 30) that also reported dietary patterns, the majority reported that diets with energy derived from total fat intakes above the AMDR were associated with increased CVD risk, and generally reflected lower diet quality.
Numerous limitations were identified across the body of evidence:
Vastly different study designs and diet assessment approaches were used to examine macronutrient distributions. Few studies evaluated macronutrient distribution in the context of dietary patterns in relation to CVD.
Foods and food groups consumed as part of the diet pattern, were inconsistently assessed and reported, thereby limiting meaningful conclusions regarding nutrient density and overall nutritional quality.
The gradient between proportions compared within and across studies varied widely.
Although many studies compared proportions that were distinctly different, some compared only slight differences in macronutrient content (e.g., 42.0% vs 43.7% fat), thereby reducing the specificity of the impact.
Due to the variability in methodology used to estimate macronutrient intake and/or adjust for total energy, confidence in the accuracy of reported proportions of energy falling outside the AMDR is low.
Several studies focused on a particular macronutrient of interest, such as “high-protein” or “low-carbohydrate” intake, but the proportion for that macronutrient was within the AMDR.