Colorectal Cancer, Plant-Based Eating, and the Challenges of Dietary Research
By Suzanne Morris, DVM, MWC
In addition to being National Kidney Month, March is also Colorectal Cancer Awareness Month.[1,2] In the US, colon cancer is the third leading cause of cancer-related death.[3] Globally, the incidence of colorectal cancer (CRC) in individuals aged < 50 years is increasing.[3,4]
Whereas kidney health organizations recommend plant-based diets, health organizations are more hesitant when it comes to recommending plant-based diets for CRC prevention or management. Many of the health organization guidelines relating to diet and CRC prevention mention that there is a possible benefit to increasing fruit and vegetable intake while decreasing animal product consumption. But they add that there is insufficient evidence to support this as a recommendation and refer to ongoing studies. For example, the Centers for Disease Control (CDC) states that studies are underway to evaluate the effect of diet on CRC prevention and then refers to the general medical recommendation to increase fruit, fiber, and vegetable intake while decreasing fats for chronic disease risk prevention.[5] Similarly, the American Cancer Society advises that eating a diet higher in fruits, vegetables, and whole grains “probably” reduces the risk of CRC, but that there are conflicting study data on fiber intake effects and refers to ongoing studies.[6] The National Institutes of Health/National Cancer Institute (NIH/NCI) categorizes CRC prevention and risk factors according to supporting data. Hence, smoking and excessive alcohol intake are considered to have “adequate” supporting evidence as risk factors, whereas the benefit of an adult starting a diet higher in fruits, fiber, and vegetables and lower in fat and meat has “no reliable evidence.” The NIH/NCI notes that although studies have found associations between meat and or fat intake and CRC risk, those findings have been mixed.[7]
Indeed, the limitations of the studies that the NIH/NIC references are often encountered when studying diets in various populations.[8] Some of the problematic factors cited by the NIH/NIC include questionnaire validity, differences in the ages of studied populations, varied meat preparations, and vegetable intake.[7] This raises the challenges of nutritional research study design and the various (confounding) factors to consider when interpreting diet study data. For example, randomized controlled trials are the gold standard in medical research but can be difficult to control when evaluating an entire dietary pattern, as opposed to the (more easily controllable) intake of a single nutrient. Also, changing a participant’s diet usually requires modifications to other aspects of their diet. So, for example, if increasing protein intake also affects carbohydrate intake, which is the factor in the study results—the increased protein or the altered carbohydrate intake?[8]
In the meantime, there are ongoing studies of plant-based diets as a means of preventing CRC (NCT03615599, NCT04753359). These may provide more evidence upon which to base CRC-specific dietary guidelines in the future.
References
1. National Institute for Diabetes and Digestive and Kidney Diseases (NIDDK). National Kidney Month 2023. March 2023. Accessed March 8, 2023. https://www.niddk.nih.gov/health-information/community-health-outreach/national-kidney-month
2. National Colorectal Cancer Alliance. Raise awareness during National Colorectal Cancer Awareness Month. Accessed March 13, 2023. https://www.ccalliance.org/
3. NIH/National Cancer Institute. Colorectal Cancer Prevention (PDQ®) – Patient. Updated June 10,2022. Accessed March 20, 2023. https://www.cancer.gov/types/colorectal/patient/colorectal-prevention-pdq
4. Patel SG, Karlitz JJ, Yen T, Lieu CH, Boland CR. The rising tide of early-onset colorectal cancer: a comprehensive review of epidemiology, clinical features, biology, risk factors, prevention, and early detection. Lancet Gastroenterol Hepatol. 2022;7(3):262-274. doi:10.1016/S2468-1253(21)00426-X
5. Centers for Disease Control. What can I do to reduce my risk of colorectal cancer? Reviewed February 23, 2023. Accessed March 21, 2023. https://www.cdc.gov/cancer/colorectal/basic_info/prevention.htm
6. American Cancer Society. Can colorectal cancer be prevented? Updated June 9, 2020. Accessed March 28, 2023. https://www.cancer.org/cancer/colon-rectal-cancer/causes-risks-prevention/prevention.html
7. NIH/NCI. Colorectal Cancer Prevention (PDQ®)–Health Professional Version. https://www.cancer.gov/types/colorectal/hp/colorectal-prevention-pdq#_1014
Weaver CM, Miller JW. Challenges in conducting clinical nutrition research. Nutr Rev. 2017;75(7):491-499. doi:10.1093/nutrit/nux026