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Dietary patterns and risk of colorectal cancer (CRC) are clearly linked, according to an umbrella review of meta-analyses of prospective observational studies.

Overall, 24 (68.6%) of the associations deemed significant pointed to protective effects for certain dietary patterns, including adherence to a healthy diet, a Mediterranean diet, a pesco-vegetarian diet, or a semi-vegetarian diet, reported Nathorn Chaiyakunapruk, PharmD, PhD, of the University of Utah College of Pharmacy in Salt Lake City, and colleagues.

The evidence was especially persuasive for an association of both red meat intake (high versus low) and alcohol (≥four drinks/day vs zero/occasional) with CRC incidence, as well as for an inverse risk association of higher versus lower intakes of dietary fiber, calcium, and yogurt. The evidence remained robust after sensitivity analyses, they stated in .

Suggestive evidence also emerged for positive associations between higher intakes of processed meat and even a moderate intake of alcohol (more than one to three drinks/day) and CRC incidence.

In terms of specific foods, there was also a protective association with whole grains, legumes, non-fermented milk, fruits and vegetables, and micronutrients, such as both supplemental and dietary calcium, zinc, magnesium, vitamin A, vitamin B₆, folic acid, vitamin D, and vitamin E.

The remaining significant associations (31.4%) suggested a higher risk with adherence to an unhealthy diet or a Western diet as well as with increased intake of processed meat, pork, eggs, and heme iron.

"More research is needed on specific foods for which evidence remains suggestive, including other dairy products, whole grains, processed meat, and specific dietary patterns," the authors noted.

They assessed 45 eligible meta-analyses of observational studies describing 109 associations between dietary factors and CRC incidence.

The investigators ranked the quality of the evidence according to five classifications: convincing, highly suggestive, suggestive, weak, and non-significant.

On random-effects meta-analysis models, 32.1% of the associations were "nominally statistically significant," with large heterogeneity between studies for 15.6% of the associations (I² >50%) and small-study effects for 10.1%.

The primary analysis identified five (4.6%) convincing, two (1.8%) highly suggestive, 10 (9.2%) suggestive, and 18 (16.5%) weak associations between diet and CRC, while for 74 (67.9%) associations there was no evidence.

The authors noted that while the etiology of CRC is multifactorial, with both genetic and environmental factors involved, such as excess adiposity, poor diet, and physical inactivity, likely play an important role in occurrence and progression.

Andrew T. Chan, MD, MPH, of Harvard University and Massachusetts General Hospital in Boston, said that the umbrella study provides a detailed and thorough synthesis of available evidence linking dietary factors with CRC risk.

"Based on the results, I think the link between high intake of red meat and alcohol and an increased risk of colorectal cancer is convincing," said Chan, who was not involved in the research. "This should provide greater justification for healthcare providers to recommend consuming red meat and alcohol only in moderation."

Furthermore, the study supports the benefit of a higher intake of fiber, calcium, and yogurt in reducing CRC risk, he stated. "Given other potential health benefits associated with these foods or nutrients, there is now even greater reason to consider eating them," Chan said.

And that intake should most likely start sooner rather than later: A reported a 30% reduction in CRC risk with a lifelong vegetarian diet, while a recent found a healthy diet had no impact on overall survival in people already battling metastatic CRC.

Study limitations included the exclusion of dose-response meta-analyses due to unavailable data, the long follow-up time required to reach cancer endpoints, and ethical concerns. As a result, the study was confined to meta-analyses of prospective observational studies. Also, some recall bias may have been present in the self-reporting of diet, and the authors did not conduct subgroup analysis by sex, age group, or cancer site because of insufficient data to grade evidence quality for most of the exposures. Dietary associations with CRC may differ according to sex and tumor location, they pointed out.

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