Chapter 3: Nutrients in the Body
3.6 The Large Intestine
We have now reached a fork in the digestive road. We could follow the uptake of the digested compounds into the enterocyte or we could finish following what has escaped digestion and is going to continue into the large intestine. Obviously from the title of this section we are going to do the latter. As we learned previously, fiber is a crude term for what has survived digestion and has reached the large intestine.
The ileocecal valve is the sphincter between the ileum (hence ileo– in ileocecal valve), and the large intestine. This name should make more sense as we go through the anatomy of the large intestine.
The large intestine consists of the colon, the rectum, and the anus. The colon can be further divided into the cecum (hence the –cecal in ileocecal valve), ascending colon, transverse colon, descending colon, and sigmoid colon as shown below.
The large intestine is responsible for absorbing the remaining water and electrolytes (sodium, potassium, and chloride) in chyme. By removing water, the unabsorbed chyme is converted into a more solid form (feces) which is then excreted via defecation. The large intestine contains large amounts of microorganisms like those shown in the figure below.
The large intestine can also be referred to as the gut. There are a large number of microorganisms found throughout the gastrointestinal tract that collectively are referred to by a variety of names: flora, microflora, biota, or microbiota. Technically, microbiota is the preferred term because flora means “pertaining to plants”. There are 10 times more microorganisms in the gastrointestinal tract than cells in the whole human body[1]. As can be seen in the figure below, the density of microorganisms increases as you move down the digestive tract.[2] The number of microorganisms is reported in “colony-forming units (cfu)” per milliliter (cfu/mL). One cfu is approximately one bacterium.
As described in the fiber sections, there are two different fates for fiber once it reaches the large intestine. The fermentable, viscous fiber is fermented by bacteria. Fermentation is the metabolism of compounds by the microorganisms in the gut. An example of fermentation is the utilization of the oligosaccharides raffinose and stachyose by microorganisms that results in the production of gas, which can lead to flatulence. Additionally, some bile acids are fermented by microorganisms to form secondary bile acids that can be reabsorbed. These secondary bile acids represent approximately 20% of the total bile acids in our body. Fermentable fibers can also be used to form short-chain fatty acids that can then be absorbed and used by the body.
Conversely, the non-fermentable, non-viscous fiber is not really altered and will be a component of feces, that is then excreted through the rectum and anus via defecation. This process involves both an internal and external sphincter that are shown in Figure 3.63 above.
Probiotics & Prebiotics
Recently there has been increased attention given to the potential of a person’s microbiota to impact health. This is because there are beneficial and non-beneficial bacteria inhabiting our gastrointestinal tracts. Thus, theoretically, if you can increase the beneficial, or decrease the non-beneficial bacteria, there may be improved health outcomes. In response to this, probiotics and prebiotics have been identified/developed. A probiotic is a live microorganism that is consumed, and colonizes in the body as shown in Figure 3.66.
A prebiotic is a non-digestible food component that selectively stimulates the growth of beneficial intestinal bacteria. It can be used as food for intestinal bacteria because it is not broken down in the small intestine. An example of a prebiotic is inulin (this is not the same as, or related to, the hormone insulin that you may be familiar with), which is shown in the figure below.
The net result is the same for both prebiotics and probiotics, an improvement in the beneficial/non-beneficial bacteria ratio.
The following video does a nice job of explaining and illustrating how probiotics work. The National Center for Complementary and Integrative Health (NCCIH) website is a good source of information if you have further questions on the topic.
Video Link: Probiotics (3:40)
Web Link: NCCIH: Probiotics
Some common examples of probiotic foods are sauerkraut, kimchi, kefir, and yogurts containing live cultures. If you’re wondering if your yogurt contains live cultures, check the label. Look for “live culture” or “active culture”.
It should be notes that the claims companies have made about their probiotic products have come under scrutiny. Dannon settled with the US Federal Trade Commission to drop claims that its probiotic products will help prevent colds or alleviate digestive problems, as seen in the top link below. General Mills also settled a lawsuit that accused them of a falsely advertising the digestive benefits of Yo-Plus, a product it no longer sells, as seen in the second link.
Some examples of prebiotics include the previously mentioned inulin, fructose-containing oligosaccharides and polysaccharides, and resistant starch. These are found in a number of foods including onions, leaks, sprouted whole grains, seeds, and berries.[3]
Resistant starch is so named because it is a starch that is resistant to digestion. As a result, it arrives in the colon to be fermented.
- Guarner F, Malagelada J. (2003) Gut flora in health and disease. The Lancet 361(9356): 512. DOI:10.1016/S0140-6736(03)12489-0 ↵
- DiBaise J, Zhang H, Crowell M, Krajmalnik-Brown R, Decker, et al. (2008) Gut microbiota and its possible relationship with obesity. Mayo Clin Proc 83(4): 460. DOI:10.4065/83.4.460 ↵
- Gut Health 101: Top Prebiotic and Probiotic Foods https://www.betternutrition.com/checkout/prebiotic-probiotic-foods-lists ↵
Defecation: Elimination of solid, indigestible waste.
Fermentation is the process by which some types of bacteria or yeast break down carbohydrates to make ATP in the absence of oxygen. Some types of fermentation produce molecules that are toxic to bacteria. Different fermenting microbes produce different molecules through fermentation, including alcohol, carbon dioxide, and acids. The ethanol in wine, beer, and other alcoholic beverages and the carbon dioxide that causes bread to rise before baking are both products of fermentation. When fermentation is being used as food preservation, the fermentation product is usually lactic acid.
Together, all of the chemical reactions that take place inside cells, including those that consume or generate energy, are referred to as the cell’s metabolism.
Oligosaccharides are carbohydrate molecules of middling size, made of 3-10 monosaccharides joined together.
Insulin is a hormone released from the pancreas that brings blood glucose levels down. Insulin sends a signal to the body’s cells to remove glucose from the blood by transporting it into different organ cells around the body and using it to make energy. In the case of muscle tissue and the liver, insulin sends the biological message to store glucose away as glycogen.
Polysaccharides are polymers consisting of chains of monosaccharide or disaccharide units joined together. Starches and fibers are the two main groups of polysaccharides.
