The composition of our gut microbiota is affected by different factors: age, genetics, environment and diet. In particular, the relationship between diet and microbiota is a subject of interest for science and the microbiota seems to react easily to dietary changes: switching from animal-based to plant-based diet or from a low-fiber, high-fat diet to the opposite can make a significant difference. It is therefore relevant to question the influence of the different components of a diet on the microbiota, starting with the main macronutrients.
Are carbs good for gut microbiota?
Carbohydrates or sugars include a wide variety of different compounds, ranging from simple molecules (glucose, fructose, for example) to long, complex chains (like starch). The quality and type of carbohydrates we eat may impact the activity and composition of the gut microbiota.
For example, the Western diet is characterized by high amounts of sugar, in particular an excessive consumption of fructose. The latter will be associated with an increased presence of Enterobacteriaceae and Escherichia coli, which are characterized by the presence of lipopolysaccharides on their membrane. This results in an increase in lipopolysaccharide levels, which can induce a mild state of inflammation and promote insulin resistance. As a result, a change in blood sugar levels is observed, leading to overweight and obesity.
And regarding more complex molecules, starch is one of the main components of carbohydrates in our diet. It is found, for example, in pasta, rice or bread. Some of the starch consumed are resistant to digestion. Called “resistant starch” (RS), it is found, for example, in legumes or whole grains. It has been shown that a diet enriched with RS can be associated with an increase in certain species of bacteria and it seems that the composition of the microbiota itself is linked to the potential fermentation of certain nutrients.
Fiber and complex carbohydrates
The consumption of plant-fibers, made of complex carbohydrates, have also an impact on the gut microbiota. A diet rich in fiber will be associated with a greater diversity of microbiota species. This richness in fibers brings also a better ratio between Bacteroïdes and Firmicutes.
Specific enzymes, that the human body does not possess, are required in order to digest and metabolized fibers. But the gut microbiota is composed of species such as Bacteroïdes thetaiotaomicron and Bacteroïdes ovatus, which possess these enzymes in significant quantities. When non-digestible carbohydrates, mainly in the form of fiber, are ingested, the microbiota ferments them. One of the main fermentation products are short-chain fatty acids (SCFA), such as acetate, propionate or butyrate. Much attention is given to their possible role as “signaling molecules” between the microbiota and the host. Among the processes that may be mediated by SCFA are the regulation of metabolism, appetite, body composition and modulation of the immune system.
What about protein-rich foods?
Studies showed that an excessive meat consumption is linked to an increased incidence of intestinal cancer. This increased incidence is also linked to the gut microbiota’s activity:
- The consumption of animal proteins is associated with an increase in the number of Bacteroïdes.
- Some of the nutrients found in red meat, a low-calcium diet or choline can promote the growth of specific species. And it is precisely the levels of these species in the microbiota that appear to be associated with the incidence of colon cancer.
In addition, the microbiota is able to metabolize L-carnitine, an amino acid found in red meat. The product of this metabolism is trimethylamine (TMA) which, if it reaches the liver, undergoes a chemical transformation and becomes TMA N-oxide. This compound may be associated with the acceleration of atherosclerosis and, therefore, an increased risk of cardiovascular diseases.
Dietary fat and their effects
Diets rich in fat can also modulate the composition of the microbiota. Theories about the mechanisms by which fatty acids may modulate the microbiota involve, for example:
- to act at the level of the cell membrane
- to interfere with energy production
- to inhibit the activity of certain enzymes
- to inhibit the growth of certain types of bacteria.
If the quantity of fats consumed has an impact, the type of fatty acids also plays a role. A diet high in saturated fats would increase intestinal permeability to molecules such as lipopolysaccharides. Once in the liver, lipopolysaccharides may exert effects associated to the development of liver diseases, such as non-alcoholic fatty liver disease and non-alcoholic steatohepatitis (NAFLD and NASH).
What about prebiotics?
A prebiotic is defined as “a substrate selectively used by host microorganisms, providing a health benefit”. Several molecules are considered as prebiotics. They can be non-digestible carbohydrates, inulin, fatty acids, polyphenols, as well as oligosaccharides such as fructo-oligosaccharides (FOS), galacto-oligosaccharides (GOS), or oligosaccharides contained in breastmilk (HMO), for example.
The main known prebiotics are oligosaccharides (FOS, GOS) but the microbiota may also selectively use undigested fibers and polysaccharides as prebiotics (inulin for example). They are the main modulators of microbiota composition and functionality. In the case of a low-fiber diet, the microbiota will seek nutrients elsewhere and even uses the glycoproteins of the lining of our intestinal wall. By eroding the gut lining, this inevitably reduces its effectiveness as a barrier. Thus, increasing the amount and carefully choosing the type of fiber to consume are simple strategies that can be easily implemented to improve health in many different situations through the microbiota.
Another source of prebiotic is linoleic acid, an essential fatty acid. Conjugated linoleic acid (CLA) refers to a group of substances that correspond to different forms of linoleic acid. The main source of CLA for humans is milk and dairy products, such as yogurt. The consumption of CLA is associated to a favorable body composition and the prevention of certain cardiovascular risk factors. It is also considered a prebiotic.
Polyphenols are also considered prebiotics. Approximately 90-95% of the polyphenols in food are not absorbed in the small intestine, but reach the colon where they are metabolized by the microbiota.
This post is written on the basis of the book “Microbiota”.
“Microbiota” is published in english by the Danone Institute Italy and Danone Institute International. It gathers a mix of historical, anthropological and scientific concepts explaining why research on gut bacteria, from ancient texts to the most recent scientific evidence, is a sector of great interest for science. Thanks to a Q&A structure, 7 recognized experts explain several topics considering the different ages and conditions in life. The book is available in pdf and e-book format (on the Danone Institute International website)