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Risk for colorectal cancer (CRC) was increased in people who consumed larger amounts of foods friendly to sulfur-metabolizing bacteria, an analysis of the Health Professionals Follow-up Study and Nurses' Health studies suggested.

Participants in the highest quintile of adherence to this so-called sulfur microbial diet -- where junky food like french fries, red and processed meats, and low-calorie drinks feature highly; and fruits, leafy vegetables, and whole grains are often nowhere to be found -- had a 27% higher risk of CRC compared to those in the lowest quintile, after adjusting for various factors (HR 1.27, 95% CI 1.12-1.44, P<0.001), reported Andrew T. Chan, MD, MPH, of Massachusetts General Hospital and Harvard Medical School in Boston, and colleagues.

Anatomical subsite analysis showed that distal CRC was positively associated with a greater adherence to the diet (HR 1.25, 95% CI 1.05-1.50, P=0.02), while proximal colon cancer was not (HR 1.13, 95% CI 0.93-1.39, P=0.19), the group wrote in .

"We performed this study because there is strong evidence linking diet to colorectal cancer risk and emerging evidence supporting a key role of the gut microbiome in colorectal cancer," Chan told MedPage Today. "However, specific data to show if the influence of diet on cancer risk was mediated through its influence on the gut microbiome was lacking."

A that developed a dietary score associated with enrichment of sulfur-metabolizing gut bacteria demonstrated an increased risk for distal CRC with such a diet, though it only involved men.

"The next step will be to examine if diet could be modified in such a way as to encourage the development of a gut microbiome that has less sulfur-reducing bacteria to lower the risk of colorectal cancer," Chan said.

For the current study, Chan and colleagues analyzed data on 214,797 healthcare professionals from the (1986-2014; 51,529 men), the (1984-2016; 121,700 women), and the (1991-2017; 116,429 women). Individuals with inflammatory bowel disease, CRC, or other cancers (except for nonmetastatic skin cancer) were excluded.

From 2012 to 2014, stool samples were provided by 519 participants from the studies. This subsample (307 men and 212 women) was used to develop the sulfur microbial diet.

The primary outcome assessed CRC incidence. Overall, there were 3,217 (1.5%) incident cases over an average follow-up of 26 years (5,278,048 person-years).

Using a "food-frequency questionnaire" provided to participants, the researchers examined dietary patterns involving the 43 foods most associated with sulfur-metabolizing gut bacteria on genomic sequencing.

Analyses adjusted for the following risk factors: body mass index (BMI), race, prior endoscopy or physical examination, smoking status (including pack-years), physical activity, CRC family history, energy intake, as well as use of aspirin, non-steroidal anti-inflammatory drugs, or menopausal hormonal therapy.

"Slightly stronger associations" linking the diet to CRC occurred among smokers and those who did not take aspirin regularly.

Participants with greater adherence to the diet tended to be younger, and had higher BMIs, less physical activity, and fewer endoscopies or physicals. Overall, most participants in the studies were white (89%), the average age was 51-57 years, and mean BMI was 24-26.

The analysis had several limitations, the researchers acknowledged, including recall bias and the fact that participants were all healthcare workers, which may limit the findings for the general public.

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