“Emerging evidence supports a possible role for overall dietary patterns that, in totality, emphasize habitually consuming fruits, vegetables, grains, and low-fat dairy and reducing red meat and alcohol intake,” wrote the study authors.
An umbrella review published in JAMA Network Open displayed evidence of an association between lower colorectal cancer (CRC) risk and higher intakes of dietary fiber, dietary calcium, and yogurt as well as lower intakes of alcohol and red meat.
Importantly though, the investigators indicated that further research into specific foods for which evidence remains suggestive, including other dairy products, whole grains, processed meat, and specific dietary patterns, is still necessary.
“Emerging evidence supports a possible role for overall dietary patterns that, in totality, emphasize habitually consuming fruits, vegetables, grains, and low-fat dairy and reducing red meat and alcohol intake,” wrote the study authors, who were led by Sajesh K. Veettil, PhD.
In this study, researchers searched MEDLINE, Embase, and the Cochrane Library from database inception to September 2019 and graded the evidence of prospective observational studies that assessed the association of dietary patterns, specific foods, food groups, beverages, macronutrients, and micronutrients with the incidence of CRC. Evidence of association was graded according to established criteria and categories, including convincing, highly suggestive, suggestive, weak, or not significant.
Of a total of 9954 publications, 222 full-text articles (2.2%) were evaluated for eligibility, and 45 meta-analyses (20.3%) that described 109 associations between dietary factors and CRC incidence were selected for assessment in the current study.
“It has to be noted that the criteria in this article did not explicitly account for biologic plausibility as part of the determination for association,” the authors added.
Of the 109 associations, 35 (32.1%) were nominally statistically significant using random-effects meta-analysis models. Overall, 17 associations (15.6%) showed large heterogeneity between studies (I2 > 50%), while small-study effects were found for 11 associations (10.1%). Notably, excess significance bias was not detected for any association between diet and CRC.
In the primary analysis, investigators identified 5 convincing (4.6%), 2 highly suggestive (1.8%), 10 suggestive (9.2%), and 18 weak associations (16.5%) between diet and CRC; there was no evidence for 74 associations (67.9%). Ultimately, there was conclusive evidence of a correlation between intake of red meat (high vs low) and alcohol (≥4 drinks/day vs 0 or occasional drinks) with the incidence of CRC. Additionally, an inverse association of higher versus lower intakes of dietary fiber, calcium, and yogurt with CRC risk was identified.
Following sensitivity analyses, the evidence for convincing associations remained robust.
“Prudent dietary patterns, characterized by higher intakes of vegetables, fruit, whole grains, and low-fat dairy products and lower intakes of alcohol and meat products, are often accompanied by a healthier lifestyle; the converse is true for Western-type dietary patterns,” the authors noted. “Hence, the suggestive association observed for these dietary behaviors could be influenced by the totality of a healthy or unhealthy lifestyle, in addition to combination(s) of protective or harmful dietary factors.”
Importantly, this study is not without limitations, including that the review excluded dose-response meta-analyses because the data needed for predictive interval estimation and assessment of small study and excess significant bias effects were not available in the articles evaluated. Moreover, randomized clinical trials are scarce in research on associations between diet and cancer due to the cost, the long follow-up time required for cancer end points, and ethical concerns; therefore, the current umbrella review was restricted to meta-analyses of prospective observational studies.
Veettil SK, Wong TS, Loo YS, et al. Role of diet in colorectal cancer incidence. JAMA Network Open. 2021;4(2):e2037341. doi: 10.1001/jamanetworkopen.2020.37341