Mechanical and chemical digestion in the mouth
Many people believe the digestive process begins when food enters our mouth, but this couldn't be further from the truth. Many aspects of our lifestyle regulate gut function, from how you regularly schedule meals to your exercise and sleep routines. In many ways, proper gut function is contingent on a healthy lifestyle.
The autonomic nervous system plays a major role in our ability to digest our food. When we are stressed or physically active, the autonomic nervous system activates the sympathetic nervous system which diverts blood and nervous system activity towards our muscles to help promote muscular contraction. When we are about to eat, the parasympathetic nervous system is activated, diverting blood and nervous system activation towards the organs of digestion to promote the digestive process.
Many factors go in to anticipatory responses of digestion. Regularly scheduled meals build this anticipatory bump in your digestion. In other words, keeping a strict meal schedule will help you better digest your food. If you normally eat in a chaotic manner, you won't get these benefits.
The mere smell of food has beneficial effects on your digestion. You've likely experienced an increase in saliva production when you smell something delicious. This is, in fact, stimulating digestion. Saliva contains antimicrobial enzymes as well as digestive enzymes that begin the process of breaking down your food.
Once food enters your mouth and you begin chewing, the saliva mixes with the mechanically broken down food and forms what is called food bolus. By mechanically churning the food and mixing it with your saliva, digestive enzymes have an easier time breaking down your food. It also lubricates the bolus so it can more easily slide down your esophagus into your stomach.
The food passes from the esophagus into the stomach through the lower esophageal sphincter, or LES. The LES allows food to pass into the stomach while preventing back flow after you've stopped eating. Along with the diaphragm, the LES acts as a reflux barrier such that food and gastric ensymes don't back up in to the esophagus and damage it.
Once food enters the stomach, the strong muscular contractions of the organ continues to mechanically break down the bolus. Furthermore, gastric acid and protein digesting enzymes called proteases are released to continue chemical digestion. There is a thick mucus layer in the stomach which also contains bicarbonate that protects the stomach wall from being digested by the acid and enzymes.
Mechanical and chemical digestion in the stomach
Food generally spends 2-4 hours in your stomach while the gastric acid and proteases act on the bolus, converting it into something called chyme. The low pH of gastric acid creates an environment where proteases work at maximum efficiency.
Once adequately broken down , the stomach contracts to empty its contents through the pyloric sphincter and into the duodenum: The first part of the small intestine. Ultimately, the time it takes for your stomach to empty depends on the size of the meal as well as your glycemic response to that meal. Essentially, if blood glucose gets too high, signals are sent to the stomach to slow down emptying.
When food enters the duodenum, a series of events shifts the pH of the chyme and the focus of digestion. The gallbladder contracts to release its stored bile, which helps emulsify fats, making them small enough so that fat digesting enzymes called lipases can efficiently break them down. (Bile does more, but we'll save that for another time)
The pancreas also releases bicarbonate to increase the pH of the chyme, other lipases, a protein digesting enzyme called trypsin, and amylase, which breaks down carbohydrates. These enzymes function more efficiently in a more alkaline environment, so efficient digestion in the duodenum is dependent on adequate bicarbonate to increase pH.
Digestion and absorption in the small intestine
The small intestine is structurally different than the rest of the gut, owing to its primary function of absorbing the nutrients from the food we eat. While both the stomach and colon have an inner and outer mucus layer, the small intestine lacks the thicker, attached inner mucus layer. The inner layer is thicker and attached in the stomach and large intestine, which would interfere with digestion in the small intestine.
Food enters the small intestine in the duodenum, an 8 inch section attached directly to the stomach. For the most part, most absorption of nutrients begins in the duodenum. Though, water, alcohol, and some fat soluble substances such as aspirin and other NSAIDs can be absorbed in the stomach.
Iron and folate are absorbed in the duodenum. The next section, the jejunum, is approximately 8 ft long and absorbs the majority of nutrients from our food. Water-soluble nutrients such as sugars, amino acids, and water-soluble vitamins are absorbed directly into the bloodstream via the portal vein, which goes directly to the liver. Fats and fat-soluble vitamins are packaged in to tiny little packages called chylomicrons and enter the lymphatic system.
The final part of the small intestine, the ileum, is approximately 9 ft long and absorbs the remainder of nutrients. The ileum is also the area where bile acids are re-absorbed into the bloodstream and sent to the liver for recycling. This process also causes the secretion of antimicrobial peptides to prevent overgrowth of bacteria in to the small intestine.
Bacterial counts are very low in the small intestine, but ramp up dramatically in the colon. Since the ileum is directly adjacent to the colon, separated only by the ileocecal valve, the secretion of antimicrobial peptides prevents small intestinal bacterial overgrowth(SIBO). The presence of bile acids in the ileum indicates that the ileocecal valve will open soon.
Motility patterns in the small intestine are different than elsewhere in the gut. Segmentation and peristalsis predominate after feeding, while the migrating motor complex moves things along and cleans up the gut during periods of fasting.Segmentation is a form of motility that acts to mix the chyme with enzymes and break it down further. Peristalsis slowly moves the chyme along the small intestine towards the colon, or large intestine.
The migrating motor complex, or MMC, functions as a housekeeping system for the gut during longer periods of fasting. It moves bacteria and leftover food fragments from the small intestine towards the colon to help limit bacterial growth in the small intestine.The MMC typically kicks in 3 hours after the small intestine is empty, and thus, varies based on meal size.
The MMC can begin in either the small intestine or gastric antrum, the last part of the stomach. Enzymes from the stomach as well as bile are also released during the MMC, helping to clean up any leftover mess. Muscular contraction progresses along the length of the small intestine until it reaches the colon. The MMC repeats every 90 minutes and is interrupted by feeding, particularly fats.
Impairment of the MMC is believed to be a major contributor to small intestinal bacterial overgrowth. In addition to high meal frequency, other factors can impair the MMC. This includes stress, autoimmunity caused by bacterial toxins, and narcotics/
The colon: The final act in digestion and absorption
The colon, or large intestine, is the terminal portion of the digestive tract. It's made up of the appendix, cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anus.
Food enters the colon from the ileocecal valve. The colon is different than other portions of the digestive tract with respect to the enzymes that catalyze digestion. While other areas of the gut produce enzymes to aid digestion, the enzymes in the colon are actually created by the resident bacteria that live there, called the microbiome.
These enzymes help break down components of food that we can't digest, or convert excess nutrients in to other nutrients. Dietary fiber is fermented by these microbes to generate short chain fatty acids(SCFAs), which are important for providing energy for the cells that line the colon as well as impacting host metabolism and circadian rhythms. SCFAs are slightly acidic, so the pH of the colon is more acidic than the small intestine, generally slightly acidic to neutral(~7.0).
The microbiome also converts the ~5% of bile acids that aren't re-absorbed in the ileum in to secondary bile acids. These secondary bile acids act as signaling moecules, but can be damaging to the colon at high doses. Most conversion of bile acids in the colon occurs in the cecum.
Members of the microbiome also have access to typically digestible food that exceeds our ability to digest and absorb it. For example, we have a limited ability to absorb sulfur-based amino acids. The remainder enter the colon and are converted to hydrogen sulfide, which has beneficial effects at low doses but negative effects at high doses.
Furthermore, members of the microbiome produce micronutrients such as vitamin k, thiamine, and riboflavin that contribute to host nutrient status. These nutrients, as well as water and electrolytes, are absorbed in to the blood through the colon. A properly functioning colon is essential for proper stool consistency, and colonic dysfunction is often the underlying cause of diarrhea or constipation.
Circadian rhythms play a pivotal role in colonic motility. Colonic motility generally increases early in the day as well as after large meals. Rapid motility leads to diarrhea, while slow motility leads to constipation. When you hit the sweet spot, the chyme entering the colon is liquidy, but becomes more solid along the course of the colon, leaving the anus mostly solid.
Feces is approximately 75% water, with the solid portion being mostly dead bacteria, remaining indigestible carbohydrate, and protein and fat that comes from shedding of the cells in the gut. The brown color of feces is dictated by the presence of bile pigments that are transformed by the microbiome.
Green feces typically contains untransformed bile pigments and bilirubin, while yellow feces is due to infection. Pale or gray feces can indicate poor bile flow, whereas black or red feces indicates blood in the stool. Foul smelling stool that smells like sulfur typically occurs in those on diets high in the sulfur-based amino acids methionine and cysteine.
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