The human digestive system is not only one of the most intricate and complex systems in human physiology, but it is also one of the most fascinating. The science and engineering behind a system of tubes and holding tanks having the capability to squeeze and push food and liquids, extracting what is needed and discarding what it doesn’t, simply amazes us. Not to mention a majority of this biological plumbing system acts as our first line of defence against microbial invaders, which is is again mind blowing. To stand afar and try to understand this magnificent biological system would be ridiculous and would do it an injustice. So, we will now take a journey into one of the body’s most fascinating network of organs to better understand the realm of human digestion.
Overview of the Digestive Tract
The human digestive tract, from beginning to end, is approximately 30 feet long in a deceased adult human. It is shorter while we are alive due to the constant peristaltic activity and contracted smooth muscle in certain areas of the small and large intestine during digestion. The entire process of digestion begins in our mouths. As we start to chew, our teeth break down food into smaller, flatter pieces thus increasing surface area. As we do this, our salivary glands and tongue secrete digestive enzymes, namely lipase and salivary amylase1. This begins the hydrolysis (breakdown using water) of macronutrients.
Once in the first 10-15 inches of small intestine2 (called the duodenum), the pH of the food fluid is increased by bicarbonate to allow optimal pH for the next series of digestive enzymes. These enzymes, along with bile (to emulsify fat-soluble macronutrients), further degrade the partially broken-down foods until they are small enough to be taken up by certain cells within the surface of the intestines. These cells reside along tiny fiber-like protrusions in the cell wall called microvilli and have the ability to allow only particularly-selected micronutrients to pass through into the bloodstream, causing everything else to continue along the rest of the intestines.
The next section of the small intestine is called the jejunum and is about 8.2 feet long. This portion of the alimentary canal digests and absorbs simple sugars, fats, and amino acids and allows them to pass into the bloodstream. Finally, the last section of the small intestine is called the ileum and claims about 9.8 feet of the digestive tract. This portion transports vitamin B12, bile acids/salts, and other remaining nutrients from the microvilli to the bloodstream. The final destination of food takes us to the last part of the digestive tract, the large intestine, or colon. Although there are many sections of the large intestine, the colon primarily consists of the cecum, anal canal, and rectum. The main function of the colon is to absorb water and pass along food refuse to be excreted out of the body.
Along with extracting vital nutrients from food and absorbing it into our bodies, the intestinal tract is also a reservoir of friendly bacteria that acts as our first line of defence, containing a mucosal lining that is chalk full of probiotics and immunoglobulins3. Lysozyme from saliva and bile-containing enzymes also contribute to the intestine’s ability to house the first line of defence of our immune system.
Although enzymes are simple catalysts that speed up reaction rates, digestive enzymes in particular are catalysts that speed up the rate at which poly-macro and micronutrients get hydrolyzed. This is especially important because without these little helpers, it could take years to digest the next hot dog you eat. Although digestive enzymes pretty much have only one job for the most part, there are quite a few categories that these enzymes are put into relative to their function:
Proteases – these enzymes have the very important job of chopping down large proteins into smaller peptides and then ultimately individual amino acids.
Lipases – these special enzymes are involved in the cleaving off of the 3 fatty acid molecules from their glycerol backbone in triglycerides.
Amylases – almost as their names imply, amylases degrade starches and other carbohydrates into simple sugars.
Nucleases – these interesting enzymes do just what their names describe, split nucleic acid polymers into individual nucleotides.
The main storage facilities and excretion sites that you’ll find these types of enzymes in are exocrine glands of the mouth, stomach, pancreas, and small intestines. For example, pepsin and gastric lipases are found within the chief cells of the stomach. The ductal and acinar cells of the pancreas excrete a myriad of different proteases along with lipases, phospholipase, nucleases and amylase4, just to name a few. Within the microvilli of the small intestine there are enzymes that are bound to the hair-like villi of the intestinal walls called brush border enzymes. These include disaccharide-cleaving enzymes such as lactase, sucrase, and maltase. There are other proteases that reside along the brush border as well.
So, wherever these digestive enzymes hang out or are secreted, their main job is to exponentially speed up the process by which water literally dices up and breaks down food and prepares it for absorption into the body.
Just as the name implies, probiotics are microorganisms (namely various classes of bacteria) that reside along the human gastrointestinal tract, that have the potential for certain health benefits5. Live probiotics are typically found in fermented dairy and certain vegetables, although many supplement manufacturers have found new technologies to enrobe and encapsulate them for oral consumption. This is why the lyophilized forms of Bifidobacterium and Acidophilus are so popular in vitamin and health food stores. Some of these strains of “acid-loving” bacteria have even been shown to improve immune function by increasing the number and actions of certain lymphocytes6.
Although many of the claims to the benefits and safety of probiotics have been challenged7, it is advised to conduct one’s own research and talk to a health care practitioner before supplementation.
GI Tract Disorders
One of the most painful but common agonies in humans are gastrointestinal tract diseases and disorders. The majority of these conditions are rooted in inflammation, infection, or cancers. Of these diseases, gastroenteritis is by far the most common, which is basically inflammation of the stomach and small intestine. What causes inflammation along the gastric and alimentary canal are typically pathogens, autoimmune responses, and various forms of cancers. For example, ileitis is an inflammatory condition in which Lawsonia intracellularis infects certain sections of the Ilium. Diverticulitis is becoming more and more common and is the result of inflamed pouches that form within the intestinal wall. Once these parts of the intestines become inflamed, it becomes more and more difficult to eat as peristaltic activity irritates these inflamed sites causing extreme discomfort in many cases8. So, whether irritable bowel syndrome or appendicitis is at hand, if diarrhea, vomiting, bloody stools, constipation, etc. is observed, it might be time to consider seeing a health care professional to examine the potential of a GI tract disorder.
In a recent study lead by Dr. Noa Stettner focusing on nitric oxide (NO), a signalling molecule in biological systems, underwent research to observe its ability to alleviate gut inflammation and its positive effects on colon cancer. In the study, the treatment seemed to have a beneficial effect on colon cancer, which is known to be aggravated by gut inflammation. In mice with colonic tumors, intestinal inflammation subsided and their tumors decreased in number and size after receiving arginine (a NO booster) and other supplements9. So, consider looking for nitric oxide boosters such as arginine, ornithine, etc. in your next vitamin supplement, if GI tract health is an issue. Chron’s disease is a chronic immune-influenced GI tract disorder that effects millions of people across the globe. The very unfortunate fact of this matter is, there is very little understanding of its pathophysiology with no definite cure thus far. Up until recently, corticosteroids were the therapies of choice for many with not much else to directly alleviate the symptoms. Now, there have been new breakthroughs in the introduction of anti-TNFα (Tumor necrosis factor alpha) antibodies along with other biologicals that have been recently approved for this disease10, with other medications in the pipeline.
It is well known that many of us struggle with starches. The gas, the bloating, the discomfort in our bowels after ingesting some of our favorite carbs such as pizza, lasagna, etc. According to recent studies that have examined these starches at a molecular level, it turns out that it is actually the primary and tertiary arrangement of the starch molecules that has a great impact on their digestibility. Different starches from different types of foods from various countries have an impact on how the starches are digested11. There are now new technologies that are underway to manipulate these starches on a molecular level to allow different degrees of digestibility. So, perhaps one day there will be a particular brand of dough or rice that will allow us to enjoy our favorite Italian dishes once again!
It is no wonder that the complex plumbing system of the human body is in fact, a wonder. One can get lost in learning all about the dynamics of the human digestive system and its intricacies. Understanding exactly how this system works and what causes certain GI tract disorders and diseases leads to potential fixes and makes all of our lives just a little bit more comfortable. As science and technology advances, it is reassuring to know that this complex system of tubes, bags and muscles will be so well understood that the generations to come will suffer less and less and develop healthier digestive systems than ever before.
Author: Chad Brey, a California State University, Northridge alumnus, has since worked as a chemist for various analytical and research facilities such as Amgen, Baxter, and Nusil Technology. Since 1997 he has worked in the dietary supplement industry for companies such as Earthwise Nutrition (formerly known as Great Earth Vitamins) and has earned a number of certificates as an IACET-certified dietary supplement specialist. Chad has written dozens of technical articles on the specifics of how certain dietary supplements work. Chad has formulated and developed small and large molecules in research and development laboratories since 2003 and continues to consult others in R&D today.
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