A diet rich in animal protein promotes the establishment of intestinal bacteria that produce hydrogen sulfide, a very toxic gas that increases the risk of colorectal cancer.
It has long been established that regular consumers of red meats and deli meats are more at risk of developing colorectal cancer than people whose diet consists mainly of foods of plant origin.
This increase has been attributed to a combination of several factors, including the presence of nitrites/nitrates, oxidative stress caused by heme iron in red meat or the production of carcinogenic molecules during cooking.
Studies show that large carnivores are also at higher risk for overweight and type 2 diabetes, two important risk factors for colorectal cancer.
A smell of sulfur
Another consequence of a high meat intake is to significantly alter the composition of the gut microbiome.
First, diets high in meats are generally deficient in dietary fibre, which negatively impacts the diversity of this microbiome by reducing colonization of the gut by “friendly” species of bacteria that use these fibers to produce metabolites. anti-inflammatories.
Second, animal protein sources are generally richer in amino acids with a sulfur atom (methionine, cysteine), which favors the implantation of bacteria which reduce these sulfur atoms to hydrogen sulphide (H2S).(1)
In addition to being foul (rotten egg smell), this gas is also extremely toxic to the cells lining the inside of the colon due to its ability to directly attack the DNA of these cells and create inflammatory conditions. conducive to the development of cancer.
H2S might therefore represent another factor contributing to the increased risk of colorectal cancer observed in heavy meat eaters.
Increased risk of cancer
To further explore this possibility, researchers at Harvard University examined the association between a diet that promotes bacterial sulfide production and the incidence of colorectal cancer in 215,000 men and women participating in three large cohorts: the Health Professionals Follow-up Study (1986-2014), the Nurses’ Health Study (1984-2016), and the Nurses’ Health Study II (1991-2017).
By collecting stool samples from a representative subgroup of these cohorts, they first determined the type of diet that was most closely associated with the presence of sulphide-producing bacteria: on the one hand, a high consumption of red meats, deli meats, fries and soft drinks; on the other, a low intake of fruits, whole grains, legumes, green and cruciferous vegetables.(2)
They subsequently showed that greater adherence to this prosulfide diet was associated with an approximately 30% increase in the risk of colorectal cancer, particularly the distal forms of the disease (sigmoid colon and rectum).
These results are in agreement with another recent study showing that a high intake of red meats and soft drinks was associated with a higher presence of sulphide-producing bacteria and an increased risk of distal colon cancer.(3)
It is important to mention that this increased risk of cancer is not observed for other sulfur-rich foods, cruciferous vegetables in particular. In these vegetables, the sulfur is in the form of glucosinolates, molecules that are transformed into isothiocyanates by digestion.
Far from being harmful, isothiocyanates have on the contrary one of the strongest anticancer activities of the plant kingdom and have been repeatedly associated with a reduction in the risk of several cancers, including colorectal cancer.
It is therefore not the sulfur as such, but the source of this sulfur in the proteins of red meat that influences the establishment of H2S-producing bacteria.
♦ (1) Magee EA et coll. Contribution of dietary protein to sulfide production in the large intestine: an in vitro and a controlled feeding study in humans. Am. J. Clin. Nutr. 2000; 72:1488-94.
♦ (2) Wang Y et coll. Association between the sulfur microbial diet and risk of colorectal cancer. JAMA Netw Open 2021; 4: e2134308.
♦ (3) Nguyen LH et coll. Association between sulfur-metabolizing bacterial communities in stool and risk of distal colorectal cancer in men. Gastroenterology 2020; 158: 1313-1325.
