Our gastrointestinal (GI) tract is an ecosystem – a collection of microorganisms that differ in number, type, and function throughout it. There are approximately 10 times as many microorganisms in it (approximately 100 trillion) as there are cells in the body. While most of the microbes are bacteria, the gut can also harbor yeasts, single-cell eukaryotes, viruses, fungi, archaea, and small parasitic worms. The majority is found within the large bowel where they contribute to digesting food, fermentation of undigested food components, especially carbohydrates/fiber, absorbing and synthesizing nutrients like phytonutrients, and to fecal bulk. Some of the most commonly found or recognized genera of gut bacteria in adults are Bifidobacterium, Lactobacillus, Bacteroides, Clostridium, Escherichia, Streptococcus and Ruminococcus.
Although bacteria account for most of the mass of the microbiota, viruses are actually the most abundant inhabitants. We tend to think of viruses as harmful, but that’s not always the case. The viruses found in the GI tract are primarily bacteriophages, meaning that they infect gut bacteria cells but they don’t necessarily harm them. Rather, they have a symbiotic relationship. Viruses can quickly transfer genes — beneficial genes. So, if new bacteria are introduced to your gut, either through diet or probiotics, the viral cells can help the bacteria thrive by transferring the genetic code.
Functions of the Microbiota
The human GI microbiome has extensive functions, such as:
- Development of immunity
- Defense against pathogens
- Host nutrition including the production of short-chain fatty acids (SCFA)
- Host energy metabolism
- Synthesis of vitamins and fat storage
- Influence on human behavior
All these functions make it an essential ecosystem of the body, without which we would not function correctly. Microbiota, meaning microorganisms of the microbiome, stimulate the immune system, break down potentially toxic food compounds, and synthesize certain vitamins and amino acids, including the B vitamins and vitamin K. For example, the key enzymes needed to form vitamin B12 are only found in bacteria, not in plants and animals.
How Diet Affects Our Microbiota
Not only does diet influence the microbial composition of the microbiota, but the microbiota also influences the nutritional value of food. Though specific bacteria vary, they share many of the same genes. For example, we do not have the ability to produce the enzymes required to break down some components found in food like insoluble fiber, resistant starch, and certain sugars. However, GI bacteria do have that ability and are absolutely essential for proper digestion. Bacteria enable us to eat a diverse diet and receive a broad range of micronutrients and phytochemicals — the bioactive non-nutrient plant compounds in fruits, vegetables, grains, and other plant foods — that have been linked to reductions in the risk of major chronic diseases .
Digestion of Our Dietary Insoluble Fiber, Resistant Starch and Some Types of Sugar
Many sugars are quickly absorbed in the upper part of the small intestine, but more complex carbohydrates like insoluble fiber, resistant starch, and some types of sugar are not as easily digested and may travel lower to the large intestine. Sugars like raffinose, for example, is a sugar found in sprouts, beans, cabbage, and broccoli, and we can’t digest most of it. Similarly, lactose, the milk sugar, does not undergo digestion if we lack the required digestive enzyme.
Fiber that contains cellulose, hemicellulose, lignin, pectin, mucilage, and gum is called insoluble fiber and our digestive system lacks the enzymes to break these food components down. Starches that do not respond to our digestive enzymes are called resistant starches. These starches react instead much like fiber in our GI tract, helping us feel full and slowing the absorption of nutrients.
In our colon, the microbiota help to break down these compounds with their digestive enzymes. The fermentation of indigestible fibers causes the production of SCFA which can be used by our body as a nutrient source, and which also plays an important role in muscle function and possibly the prevention of chronic diseases, including bowel disorders and type 2 diabetes, and reduces the risk of inflammatory diseases, heart disease, certain cancers, and obesity, among others.
What are Short-Chain Fatty Acids (SCFA)?
Short-chain fatty acids (SCFA) are the main metabolic products of anaerobic bacterial fermentation in the large intestine of non-digestible carbohydrates that become available to the gut microbiota. They represent the major flow of carbon from the diet through the microbiome to the host. In addition to their important role as fuel for intestinal epithelial cells — tissues that line the outer surfaces of organs and blood vessels throughout the body, as well as the inner surfaces of cavities in many internal organs — SCFAs stimulate immune cell activity, help maintain normal blood levels of glucose and cholesterol, and modulate different processes in the GI tract and in other tissues such as adipose and immune tissues. In addition to playing an essential role in the maintenance of microbiome health, they are important signaling molecules in the microbiota-host interaction and have been shown to modulate leukocyte development, survival, and function . Considering that, it is important to note that these molecules also play an important role in the maintenance of intestinal homeostasis — the ability or tendency to maintain internal stability in an organism to compensate for environmental changes — as changes in their concentrations are associated with the development of inflammatory diseases such as inflammatory bowel disease, gout, and arthritis.
How are SCFA Created and How do They Work
In addition to family genes, environment, and medication use, our diet plays a large role in determining what kinds of microbiota live in the GI tract. All of these factors create a unique microbiome from person to person. A high-fiber diet, in particular, affects the type and amount of microbiota in the intestines.
Dietary fiber can only be broken down and fermented by enzymes from microbiota living in the colon. SCFA are released as a result of fermentation and this lowers the pH of the colon, inhibiting the growth and activity of pathogenic bacteria, which in turn determines the type of microbiota present that would survive in this acidic environment. The lower pH limits the growth of harmful bacteria.
In short, the production of SCFA is encouraged through the intake of high-fiber foods in our diets, such as fruits, veggies, legumes, and whole grains.