Evidence is strong that consuming high amounts of dietary fiber protects against colorectal cancer. Previous research has suggested that fiber may play a role in colon cancer prevention due to its interaction with trillions of bacteria in our gut.
Now, a study adds to that evidence by focusing on advanced colorectal adenoma, a non-cancerous tumor that has the potential to develop into cancer.
The study, published in The American Journal of Clinical Nutrition, suggests that a high-fiber diet promotes healthy gut bacteria and its byproducts.
Gut microbiota are the microorganisms that live in our digestive tracts – in our stomach, intestines, and colon. We have about 10 trillion human cells in our body, but we have way more – about 100 trillion – microorganisms residing in our gut. A growing body of research is showing that these microorganisms are important to our health – from training our immune system, to producing vitamins and fighting off harmful bacteria.
In the study, approximately 700 Chinese patients over 50 years old completed food surveys asking what they ate over the past five years. About half were diagnosed with advanced colorectal adenomas.
Researchers collected fecal samples to test for molecules known as short-chain fatty acids. When gut bacteria digest fiber it produces fatty acids such as butyrate. Lab studies suggest butyrate may protect against colon cancer several ways, such as by promoting programmed cell death.
The researchers found that those with high intake of vegetables had high amounts of butyrate, and lower risk of advanced adenoma. In general, patients with advanced colorectal adenoma had significantly lower amounts of butyrate and acetate than those in the healthy group.
They then compared the gut microbiota communities in the two groups and found that the adenoma group had a drastically different gut microbiota composition than the healthy patients. Butyrate-producing bacteria such as Clostridium, Roseburia, and Eubacterium, were significantly lower in the advanced colorectal adenoma group than in the healthy group.
When the investigators divided a subset of the participants into those with low-fiber or high-fiber intake, the quantity of butyrate-producing bacteria was far greater in the high-fiber healthy group than in their counterparts who ate a low-fiber diet or the high-fiber group with adenomas. More research is needed to understand this.
The authors hypothesize that changes in the intestinal microenvironment, whether because of diet (high fiber/low fiber) or other factors, may influence the composition of gut microbiota and its byproducts, which might affect the development of advanced colorectal adenoma.
Have the study authors ruled out the reverse causal explanation? Is it possible that the adenoma group exhibits low levels of butyrates because the adenomas themselves affect the population of butyrate-producing bacteria?
That is, could the chain of causation be low fiber -> adenomas -> low butyrate-producing bacteria, instead of the hypothesized low fiber -> low butyrate-producing bacteria -> adenomas?
how much fiber do you need per day to have this benefit?
Great piece of work, with valuable finding, need more exploration.