The recent focus on the interplay between the intestinal microbiota and human health has demonstrated a wide range of correlations greater than we ever imagined. It is already known that many different microorganisms inhabit our body in a harmonious way. Especially in the gut, those microorganisms protect us against pathogens and “teach” our immune system how to distinguish between “beneficial” and “harmful” agents.
In this relationship we are entrusted with the role of supplying them with energy. Depending on the energy source available, this community of microorganisms can respond in a way that can be beneficial or detrimental to our health.
Carbohydrates, especially dietary fibers, are widely studied for their ability to beneficially modulate the intestinal microbiota. Proteins are less discussed but are still very important macronutrients that can also be used as an energy source for the gut microbiota.
Most of the time our organism is very well adapted to protein digestion and amino acids absorption. But, depending on several factors, those compounds may not be properly digested or absorbed. They continue through the gastrointestinal tract until they reach the colon.
The excessive load of proteins and amino acids that reach the small and large intestines can shape the bacterial community; this community can secrete enzymes such as proteases and peptidases that will influence the intestinal barrier function and permeability. Increased gut permeability is often associated with inflammatory bowel diseases and other intestinal disorders. Moreover, these changes in gut permeability can be associated with the proliferation of potentially pathogenic bacteria like E. coli, Salmonella, Shigella and Campylobacter genus.
Such changes can lead to a pH alteration and generation of sulfur and N-nitroso compounds, heterocyclic amides and indoles, all substances that can present detrimental and cytotoxic activity. For example, the fermentation of L-carnitine, an amino acid largely found in meats, often produces trimethylamine which is absorbed and oxidized in the liver generating thimethylamine N-oxide (TMAO). High levels of TMAO in the blood can be associated with cardiovascular diseases.
On the beneficial side, bacterial fermentation of tryptophan, an essential amino acid, can produce serotonin, an important neurotransmitter and substrate for melatonin production, as well as indole compounds that positively modulate immune system response.
Therefore, it is important to note that protein consumption is essential for human health and that problems only start when protein digestibility or quantity are not adequate, leading to a build-up in the colon, with consequent increase in fermentation and generation of harmful metabolites.
References
– Corpet, D.E., Yin, Y., Zhang, X., Remesy, C., et al., Colonic protein fermentation and promotion of colon carcinogenesis by thermolyzed casein. Nutrition and Cancer, 1995, 23, 271-281
– Agus, A., J. Planchais, and H. Sokol, Gut microbiota regulation of tryptophan metabolism in health and disease. Cell host microbe, 2018. 23(6): p. 716-724
– Pasini, E., et al., Pathogenic gut flora in patients with chronic heart failure. JACC Heart Fail, 2016. 4(3): p. 220-7
– Ma, N., et al., Contributions of the interaction between dietary protein and gut microbiota to intestinal health. Current Protein and Peptide Science, 2017, Vol. 18 No. 8
