Gut flora’s primary benefit to the host is the gleaning of energy from the fermentation of undigested carbohydrates and the subsequent absorption of short chain fatty acids. The most important of these are butyrates, metabolized by the colonic epithelium; propionates by the liver; and acetates by the muscle tissue.
Intestinal bacteria also play a role in synthesizing vitamin B and vitamin K as well as metabolizing bile acids, sterols and xenobiotics.[ Cummings, J.H.; MacFarlane, G.T. (1997). “Role of intestinal bacteria in nutrient metabolism”. Clinical Nutrition 16: 3–9.]
The human body carries about 100 trillion microorganisms in its intestines, a number ten times greater than the total number of human cells in the body. The metabolic activities performed by these bacteria resemble those of an organ, leading some to liken gut bacteria to a “forgotten” organ. It is estimated that these gut flora have around a hundred times as many genes in aggregate as there are in the human genome.[ Junjie Qin; et al (2009). “A human gut microbial gene catalogue established by metagenomic sequencing”. Nature 464 (7285): 59–65]
Somewhere between 300 and 1,000 different species live in the gut, with most estimates at about 500 [see Wikipedia]. However, it is probable that 99% of the bacteria come from about 30 or 40 species [Beaugerie L, Petit JC (April 2004). “Microbial-gut interactions in health and disease. Antibiotic-associated diarrhoea”. Best Pract Res Clin Gastroenterol 18 (2): 337–52]
Fungi, protozoa, and weird stuff called Archaea also make up a part of the gut flora, but little is known about their activities.
Research suggests that the relationship between gut flora and humans is not merely commensal (a non-harmful coexistence), but rather a mutualistic relationship. Though people can survive without gut flora, the microorganisms perform a host of useful functions, such as fermenting unused energy substrates, training the immune system, preventing growth of harmful, pathogenic bacteria, [Guarner F, Malagelada JR (February 2003). “Gut flora in health and disease”. Lancet 361 (9356): 512–9.] regulating the development of the gut, producing vitamins for the host (such as biotin and vitamin K), and producing hormones to direct the host to store fats.
There is a negative side too: in certain conditions, some species are thought to be capable of causing disease by producing infection or increasing cancer risk for the host. Remember I am talking about normal flora doing this.
Obesity
Using mice as models, it has been demonstrated that alterations on microbiota composition due to the administration of different antibiotics increases the fat content of tissues, without affecting bodyweight.
Moreover, weight loss properties of microbiota can be transferred. In an interesting study reported in the NY Times, March 2013, mice were investigated for a potential link between gastric bypass surgery and alterations in gut flora, fattened-up mice were divided into two groups. The test group underwent gastric bypass surgery while the control group received sham surgery. After the sham surgery, the controls were further divided into two groups: One received a fatty diet; the other a weight-loss diet.
In the test group, the microbial populations quickly changed following bypass surgery, and the mice lost weight. This may be the real mechanism that enables bypass surgery to work.
Next, the researchers transferred intestinal contents from each of the groups into other mice, which lacked their own intestinal bacteria. The animals that received material from the bypass mice rapidly lost weight; stool from mice that had the sham operations had no effect. [New York Times March 27, 2013]
Some Oliogsaccharide Pre-Biotic content of foods:
- Raw chicory root (64.6%)
- Raw Jerusalem artichoke (31.5%)
- Raw dandelion greens (24.3%)
- Raw garlic (17.5%)
- Raw leek (11.7%)
- Raw onion (8.6%)
- Cooked onion (5%)
- Raw banana (1%)