Not All Plant-Based Diets Equal for CRC Risk Reduction

Megan Brooks

December 05, 2022

Maintaining a diet rich in healthy plant foods and low in unhealthy plant foods is associated with a lower risk for colorectal cancer (CRC) in men, although the strength of the association may vary by race and ethnicity and tumor site, new research shows.

The "take-home message is that improving the quality of plant foods and reducing animal food consumption can help prevent colorectal cancer in men," Jihye Kim told Medscape Medical News.

The findings suggest that "not all plant-based diets are the same with regard to colorectal cancer protection/risk," said Kim, a professor in the College of Life and Sciences at Kyung Hee University in South Korea.

The study was published online in BMC Medicine.

Healthy vs Unhealthy Plant-Based Diets

The researchers investigated the risk for CRC in association with three plant-based dietary patterns defined by a priori indices: an overall plant-based diet index (PDI), a healthful plant-based diet index (hPDI), and an unhealthful plant-based diet index (uPDI).

All three indices negatively weigh animal foods but weigh plant foods differently depending on their nutritional quality.

Examples of foods contained in the hPDI include whole grains, fruits, vegetables, vegetable oils, nuts, legumes, tea, and coffee. Foods in the uPDI include refined grains, fruit juices, potatoes, and added sugars.

They calculated the PDI, hPDI, and uPDI using data from quantitative food frequency questionnaires provided by 79,952 men (mean age, 60 years at baseline) and 93,475 women (mean age, 59 years at baseline) in the Multiethnic Cohort Study.

During a mean follow-up of about 19 years, 4976 participants developed CRC.

The plant-based diet indices were significantly inversely associated with the risk for CRC in men.

Researchers found that men with the greatest adherence to PDI and hPDI had a 24% (hazard ratio [HR], 0.76) and 21% (HR, 0.79) lower risk for CRC, respectively, compared with men with the lowest adherence. No significant association was found between the risk for CRC in men and uPDI.

None of the plant-based diet indices was significantly associated with the risk for CRC in women, which could be due to different dietary habits between men and women, the researchers say.

In general, women consume more plant foods and less animal foods than men do, they point out.

In addition, women in the Multiethnic Cohort Study consumed higher amounts of healthy plant foods and lower amounts of less healthy plant foods compared with men, so they may not have further benefits with high scores of plant-based diet indices. Also, men are at higher risk for CRC than women are in general.

These findings suggest that the benefits from plant-based diets may vary by sex, race and ethnicity, and anatomic subsite of tumor.

In men, the inverse association of overall PDI was greater in Japanese American, Native Hawaiian, and White groups than in African American and Latino groups, and for left colon and rectum tumors than for right colon tumors. The decreased risk with the hPDI was suggested across racial and ethnic groups and was observed for all tumor subsites.

"It was interesting to see that the association of a plant-based diet with CRC varied by race and ethnicity. It is not clear why. It could be nondietary lifestyle factors or genetic factors," Kim told Medscape Medical News.

"We should investigate that more in the future," Kim added.

By way of limitations, it's possible that residual or unmeasured confounding might exist despite adjustment for key CRC risk factors, the researchers say. However, the subgroup analyses suggest that the impact of residual confounding due to body mass index, smoking status, and alcohol consumption was "minimal," they note.

Another limitation is that the analysis was based on diet measured only at baseline, but dietary habits might change over time.

Overall, the findings "support that improving the quality of plant foods and reducing animal food consumption can help prevent colorectal cancer," Kim and colleagues say.

Th e study was supported by the National Research Foundation of Korea (NRF) and the US National Cancer Institute and National Institutes of Health. The authors have declared no relevant conflicts of interest.

BMC Medicine. Published online November 29, 2022. Full text

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