Plant-based diet key to CRC prevention?

Increased consumption of plant-based foods is associated with a lower risk of developing colorectal cancer (CRC) in men, according to the Multiethnic Cohort study.

The association was observed particularly with diets rich in healthy plant foods with low less-healthy plant food content and was not seen in women. “The strength of the association [in men varied] by race and ethnicity and anatomic subsite of tumours,” the researchers said.

Participants in this large multi-ethnic population (n=79,952 [men; mean age 60 years] and 93,475 [women; mean age 59 years]) completed a self-administered, comprehensive questionnaire that included a detailed dietary assessment. Researchers evaluated the correlation between CRC risk and quintiles of three plant-based diet indices: overall plant-based diet index (PDI), healthful PDI (hPDI), and unhealthful PDI (uPDI). [BMC Med 2022;20:430]

During an average follow-up of 19.2 years, 4,976 incident CRC cases were identified.

Men in the highest PDI quintile had lower CRC risk than those in the lowest quintile (hazard ratio [HR], 0.76; p_trend<0.001). This was similarly seen for hPDI (HR, 0.79; p_trend<0.001) but not for uPDI (HR, 1.08; p_trend=0.19).

When evaluating individual plant food components, consuming more whole grains (HR per SD, 0.95) and legumes (HR per SD, 0.91) was associated with lower CRC risk.

 

Analyses by race, ethnicity, tumour subsite in men

The inverse association for PDI was stronger in Japanese-American, Native Hawaiian, and White groups (combined) than in the combined African-American or Latino groups (p<0.001).

“This pattern of association may be attributable to the differences in nondietary lifestyle risk factors among racial and ethnic groups,” said the researchers. There were more African-American than Japanese-American and White men in the study who were obese and less physically active.

On subsite-specific analysis, the inverse association for PDI was stronger for left colon and rectum tumours than for right colon tumours (p_{heterogeneity}=0.005). This could be due to the greater exposure of the left colon and rectum than the right colon to cytotoxic and genotoxic damages owing to the longer transit time and faecal mass storage before elimination. [Gastroenterology 2020;159:241-256.e13; Nutrients 2019;11:1579]

For hPDI, the inverse association was observed across all racial and ethnic groups (p_{heterogeneity}=0.91, but only significant in Japanese-American and White men) and in all tumour subsites (p_{heterogeneity}=0.16).

For uPDI, no significant association was found in any racial or ethnic group (p_{heterogeneity}=0.91). On subsite-specific analysis, the trend for increased cancer risk was observed for tumours of the rectum but not the right or left colon (p_{heterogeneity}=0.048).

 

Less risk in women?

In women, no significant associations were observed, be it in the overall assessment or in any of the race-, ethnicity-, or subsite-specific analyses.

The difference may have been driven by the varying dietary habits between men and women, the researchers noted. “Women consume more plant foods and less animal foods vs men in general. In our population, women consumed greater amounts of healthy plant foods and less amounts of unhealthy plant foods vs men, and they might not have further benefits with higher scores of plant-based diet indices.”

Moreover, CRC risk is generally higher in men than in women, hence the greater benefit of plant-based diets in men observed in the study. [CA Cancer J Clin 2020;70:145-164]

 

Quality matters

“[Taken together,] these findings emphasize the potential importance of the quality of plant foods on the prevention of CRC and suggest that the benefits from plant-based diets may vary by sex, race and ethnicity, and anatomic subsite of tumour,” said the researchers.

The findings also suggest that reducing animal food consumption, or careful selection of animal food choices, can help prevent CRC. “It might be helpful to consider the healthiness and quality of animal foods for future research on the association of plant-based diets with CRC risk,” they added.

The researchers also called for further investigation on potential interactions between genetic and environmental factors to ascertain the racial and ethnic differences in the diet-CRC correlation.