What is the question?

• The question is: What is the relationship between dietary patterns consumed and risk of certain types of cancer?

What is the answer to the question?

• Dietary patterns: Breast cancer
  • Moderate evidence indicates that dietary patterns rich in vegetables, fruits, and whole grains, and lower in animal-source foods and refined carbohydrates, are associated with reduced risk of postmenopausal breast cancer. The data regarding these dietary patterns and premenopausal breast cancer risk point in the same direction, but the evidence is limited as fewer studies include premenopausal breast cancer.
• Dietary patterns: Colorectal cancer
  • Moderate evidence indicates that dietary patterns higher in vegetables, fruits, legumes, whole grains, lean meats and seafood, and low-fat dairy; and low in red and processed meats, saturated fat and sugar-sweetened beverages and sweets relative to other dietary patterns are associated with lower risk of colon and rectal cancer. Moderate evidence also indicates that dietary patterns that are higher in red and processed meats, French fries, potatoes, and sources of sugars (e.g., sugar-sweetened beverages, sweets and dessert foods) are associated with a greater colon and rectal cancer risk. 
• Dietary patterns: Lung cancer
  • Limited evidence suggests that dietary patterns containing more frequent servings of vegetables, fruits, seafood, grains and cereals, legumes and lean vs. higher fat meats and lower fat or non-fat dairy products may be associated with lower risk of lung cancer, primarily among former smokers and current smokers. 
• Dietary patterns: Prostate cancer
  • Limited evidence suggests no relationship between dietary patterns and risk of prostate cancer. 

Why was this question asked?

• 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.

How was this question answered?

• 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 systematic review updates existing systematic reviews conducted by the 2015 Dietary Guidelines Advisory Committee.
• 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.

What is the population of interest? 

• For the intervention/exposure, children through older adults, age 2 years and older
• For the outcome, children through older adults, age 2 years and older

What evidence was found? 

• Dietary patterns: Breast cancer
  • This review identified 26 articles that met inclusion criteria.
  • Most studies reported dietary patterns were related with lower risk of post-menopausal breast cancer.
    • Dietary patterns were higher in vegetables, fruits, and whole grains, and lower in animal products and refined carbohydrates.
    • Alcohol was not consistently included within the dietary patterns 
    • Few studies reported results for premenopausal breast cancer risk.
    • Studies differed in dietary pattern methods, dietary intake assessment, and duration of follow-up.
    • Key limitations of the studies include not accounting for key confounders or possible changes in dietary intake over follow-up.  
  • The 2020 Committee updates and concurs with the conclusions drawn by the 2015 Committee.
• Dietary patterns: Colorectal cancer
  • This review identifies 24 articles that met inclusion criteria.
  • Most studies reported dietary patterns were related to lower risk of colorectal cancer.
    • Dietary patterns were higher in vegetables, fruits, legumes, whole grains, lean meats and seafood, and low-fat dairy, and lower in red and processed meats, saturated fat, sodas, and sweets.
    • Alcohol was not consistently included within the dietary patterns. 
    • Results were more consistent in men, and for total colorectal cancer risk.
    • Studies differed in dietary pattern methods, dietary intake assessment, and duration of follow-up.
    • Key limitations in the study design and conduct of included articles were identified. This includes a lack of accounting for key confounders or possible changes in dietary intake over follow-up.
  • The 2020 Committee updates the conclusions drawn by the 2015 Committee. 
• Dietary patterns: Lung cancer
  • This review identified 8 articles that met inclusion criteria.
  • Most studies reported dietary patterns were associated to lower risk of lung cancer, but had several limitations.
    • Dietary patterns had more frequent servings of vegetables, fruits, seafood, grains and cereals, legumes and lean vs. higher fat meats and lower fat or non-fat dairy products. 
    • Protective effects of the patterns were more consistent among participants who were former smokers and current smokers than among participants who were never smokers. 
    • Alcohol was not consistently included within the dietary patterns.
    • Many limitations in the study design and conduct of included articles were identified. This includes a lack of accounting for key confounders or possible changes in dietary intake over follow-up.
  • The 2020 Committee updates and concurs with the conclusions drawn by the 2015 Committee.
• Dietary patterns: Prostate cancer
  • This review identified 8 articles that met inclusion criteria.
  • Most studies reported no significant associations between dietary patterns and risk of prostate cancer.
  • Many limitations in the study design and conduct of included articles were identified. This includes a lack of accounting for key confounders or possible changes in dietary intake over follow-up.
  • The 2020 Committee updates an existing review from the 2015 Committee, which did not draw a conclusion about this relationship.

How up-to-date is this systematic review?

• This review searched for studies from January, 2014 to January, 2020, and updated existing systematic reviews that included evidence from January, 2000 to January, 2014.

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 certain types of cancer?

Conclusion statements and grades

• Dietary patterns: Breast cancer
  • Moderate evidence indicates that dietary patterns rich in vegetables, fruits, and whole grains, and lower in animal-source foods and refined carbohydrates, are associated with reduced risk of postmenopausal breast cancer. The data regarding these dietary patterns and premenopausal breast cancer risk point in the same direction, but the evidence is limited as fewer studies include premenopausal breast cancer. (Grade: Moderate - Postmenopausal breast cancer risk, Limited – Premenopausal breast cancer risk)
• Dietary patterns: Colorectal cancer
  • Moderate evidence indicates that dietary patterns higher in vegetables, fruits, legumes, whole grains, lean meats and seafood, and low-fat dairy; and low in red and processed meats, saturated fat and sugar-sweetened beverages and sweets relative to other dietary patterns are associated with lower risk of colon and rectal cancer. Moderate evidence also indicates that dietary patterns that are higher in red and processed meats, French fries, potatoes, and sources of sugars (e.g., sugar-sweetened beverages, sweets and dessert foods) are associated with a greater colon and rectal cancer risk. (Grade: Moderate)
• Dietary patterns: Lung cancer
  • Limited evidence suggests that dietary patterns containing more frequent servings of vegetables, fruits, seafood, grains and cereals, legumes and lean vs. higher fat meats and lower fat or non-fat dairy products may be associated with lower risk of lung cancer, primarily among former smokers and current smokers. (Grade: Limited)
• Dietary patterns: Prostate cancer
  • Limited evidence suggests no relationship between dietary patterns and risk of prostate cancer. (Grade: Limited)

Methods 

• A literature search was conducted using 4 databases (PubMed, Embase, Cochrane, CINAHL) to identify articles that evaluated the intervention or exposure of dietary patterns consumed and risk of breast, colorectal, lung, and prostate cancer. 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. 

Summary of the evidence

• Dietary patterns: Breast cancer
  • This systematic review update includes 26 studies that examined the relationship between dietary patterns and risk of breast cancer, met inclusion criteria, and were published between January 2014 and January 2020:
    • Three studies were randomized controlled trials,
    • Twenty-one were prospective cohort studies, and 
    • Two studies were nested case-control studies.
  • The studies were heterogeneous, in terms of which methods were used to identify or assess dietary patterns, how dietary intake was assessed, and duration of follow-up. However, despite this heterogeneity, the body of evidence was consistent in the types of foods and beverages examined in a number of the patterns, particularly in those studies that reported statistically significant associations with lower risk of breast cancer. 
    • In a number of studies, dietary patterns that included vegetables, fruits, and whole grains, and that were lower in animal products and refined carbohydrates, were associated with reduced risk of postmenopausal breast cancer. 
    • Alcohol was not consistently included within the patterns found to be inversely associated with breast cancer risk. 
    • Few studies reported results for premenopausal breast cancer risk.
    • The studies were direct and generalizable, in that the populations, interventions, comparators, and outcomes of interest in the included studies were directly related to the systematic review question, and were applicable to the U.S. population.
    • The body of evidence had several risks of bias, particularly in the observational studies, including lack of adjustment for all key confounders, assessment of a dietary pattern only once at baseline or in the first few years of follow-up, and a lack of accounting for possible changes in dietary intake that may have occurred over follow-up.
    • This systematic review updates and concurs with the conclusions drawn by the 2015 Committee.
• Dietary patterns: Colorectal cancer
  • This systematic review update includes 24 studies that examined the relationship between dietary patterns and risk of colorectal cancer, met inclusion criteria, and were published between January 2014 and January 2020:
    • Two studies were randomized controlled trials,
    • Twenty-one studies were prospective cohort studies, and
    • One study was a nested case-control study.
  • The studies were heterogeneous, in terms of which methods were used to identify or assess dietary patterns, how dietary intake was assessed, and duration of follow-up. However, despite this heterogeneity, the body of evidence was consistent in the types of foods and beverages examined in a number of the patterns, particularly in those studies that reported statistically significant associations with lower risk of colorectal cancer. 
    • In a number of studies, dietary patterns that included vegetables, fruits, legumes, whole grains, lean meats and seafood, and low-fat dairy, and that were lower in red and processed meats, saturated fat, sodas, and sweets were associated with lower risk of colorectal cancer.
    • Alcohol was not consistently included within the patterns found to be inversely associated with colorectal cancer risk. 
    • Results were more consistent in men, and for total colorectal cancer risk.
  • The studies were direct and generalizable, in that the populations, intervention, comparators, and outcomes of interest in the included studies were directly related to the systematic review question and were applicable to the U.S. population.
  • The body of evidence had several risks of bias, particularly in the observational studies, including lack of adjustment for all key confounders, assessment of a dietary pattern only once at baseline or in the first few years of follow-up, and a lack of accounting for possible changes in dietary intake that may have occurred over follow-up.
  • This systematic review updates the conclusions drawn by the 2015 Committee. The 2020 Committee determined that the body of evidence included in this update was consistent with that considered by the 2015 Committee, with the exception of alcohol. Because alcohol was not consistently part of the patterns found to be significantly associated with lower colorectal cancer risk, and in some cases, were part of cases associated with increased risk, “moderate alcohol” was removed from the conclusion statement.
• Dietary patterns: Lung cancer
  • This systematic review update includes 7 prospective cohort studies and one nested case-control study that examined the relationship between dietary patterns and risk of lung cancer, met inclusion criteria, and were published between January 2014 and January 2020. 
  • Though the body of evidence had some inconsistencies in direction and magnitude of effect, most studies reported significant associations between adherence to a dietary pattern and lower risk of lung cancer. 
    • In several studies, dietary patterns containing more frequent servings of vegetables, fruits, seafood, grains and cereals, legumes and lean vs. higher fat meats and lower fat or non-fat dairy products were associated with lower risk of lung cancer
    • The protective effects of the patterns were more consistent among participants who were former smokers and current smokers than among participants who were never smokers. 
    • Alcohol was not consistently included within the patterns found to be inversely associated with lung cancer risk.
  • Most studies had large analytic sample sizes with a sufficient number of lung cancer cases occurring over follow-up to examine associations. However, the width of confidence intervals indicates some degree of imprecision within the body of evidence.
  • The studies were direct and generalizable, in that the populations, intervention, comparators, and outcomes of interest in the included studies were directly related to the systematic review question, and were applicable to the U.S. population.
  • The body of evidence had several risks of bias, including lack of adjustment for all key confounders, assessment of dietary pattern only once at baseline or in the first few years of follow-up, and a lack of accounting for possible changes in dietary intake that may have occurred over follow-up.
  • This systematic review updates and concurs with the conclusions drawn by the 2015 Committee.
• Dietary patterns: Prostate cancer
  • This systematic review update includes 7 prospective cohort studies and one nested case-control study that examined the relationship between dietary patterns and risk of prostate cancer, met inclusion criteria, and were published between January 2014 and January 2020. 
  • Though the direction and magnitude of effect across the body of evidence was inconsistent, most studies reported no significant associations between adherence to a dietary pattern and risk of prostate cancer. 
  • Most studies had large analytic sample sizes with a sufficient number of prostate cancer cases occurring over follow-up to examine associations. However, the width of confidence intervals indicates some degree of imprecision within the body of evidence.
  • The studies were direct and generalizable, in that the populations, exposures, comparators, and outcomes of interest in the included studies were directly related to the systematic review question, and were applicable to the U.S. population.
  • The body of evidence had several risks of bias, including lack of adjustment for all key confounders, assessment of a dietary pattern only once at baseline or in the first few years of follow-up, and a lack of accounting for possible changes in dietary intake that may have occurred over follow-up. 
  • This systematic review updates the review done by the 2015 Committee, which did not draw a conclusion regarding the relationship between dietary patterns and the risk of prostate cancer due to limited evidence from a small number of studies with wide variation in study design, dietary assessment methodology and prostate cancer outcome ascertainment. The 2020 Committee determined that, based on the 8 additional studies in their update, limited evidence is now available to suggest no relationship between dietary patterns and risk of prostate cancer.