ISSA SN Chapter 13: Digestion and Absorption
ISSA SN Chapter 13: Digestion and Absorption

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    Chapter Goals:

    • Describe and define the terms rating to digestion.
    • Know how digestion occurs in the human body.
    • Know the digestive system’s anatomy.
    • Find the enzymes used for digestion. 


    Eating for survival just requires us to at some random food, drink liquid, and then rest. If you eat for the extension of your life and the improvement of athletic performance, then you require an understanding of digestion. 

    Our digestive system starts at the mouth and then ends with the anus. It is a strong muscular tube that is lined with thick epithelium that has specialized cells that differ depending on the part of the system you look at. The digestion system is the connection the body has with the external environment. Digestion is that process where we break down useful biomolecules for obtaining the needed energy for life and the building blocks for that life.

    The main functions of the digestive system:

    • The reception, mastication, and the transport of the substances we take in and the waste products that occur. 
    • The secretion of acids, mucus, enzymes for digestion, the bile, and any other materials we need for the breakdown of ingested food. 
    • Digestion for the food stuff that we take in.
    • Storage for the products of waste temporarily. 
    • Excretion of the waste and some non-waste material sometimes. 
    • Some particular auxiliary functions.

    The Digestive System – Physical Components

    The system is going to be made up of several anatomically different structures that compose he gut and some organs that attach to the gut. They work to provide some functions of the whole digestive process.


    Food is going to enter the whole digestive system through the mouth first. It has four main functions. 

    First, it will physically break down the food through the chewing process and this will reduce the size of the ingested food.

    Second, it is going to mix the food together with saliva to moisten it and that prepares it for the swallowing to go further in digestion. The saliva serving the role of lubrication for the journey of the food down the esophagus and eventually to the stomach. The saliva also has a protein for digestion that is called ptyalin. This is used in the breakdown of carbohydrates we consume. 

    We swallow our food mass an this goes through our pharynx and then the esophagus. Thorough chewing is vital for the process of digestion to occur well. 

    Third, we have the mouth regulating the temperature of the food by the use of either heating or cooling the food. the operation of the enzymes and other digestive things is dependent on this temperature regulation throughout the body. 

    Fourth, the last major function of the mouth is the constant initiation of swallowing when the bolus is ready. 


    The esophagus is an extension between the pharynx and the stomach. It is used for transportation of food and water that travels to the stomach. The contractions of the muscle to move the food through the system is involuntary, so we have no control over it. It travels at around three inches per second. The esophageal sphincter is a ring like muscle that relaxes to let the food finally enter into the stomach. 

    If the sphincter were to weaken, or to malfunction, the acidic content of the stomach is possible to shoot up the esophagus and this produces a sensation that we call heartburn. 

    It is best to practice sitting up and trying not to fill the stomach to much with giant meals. 


    This is the muscular sac that is around the size of two quarts and it varies a tad in this size based on the person’s actual size. Its function is to store and then gradually release food into the small intestine when said food is ready to be released. 

    The stomach secretes substances for aiding in the breakdown of food. The mucus will act to protect and lubricate the stomach wall to offer a buffer for the secretion of acids. hydrochloric acid is one of the more important acids in our stomach. This acid functions to aid in the digestion of proteins. It does this by catalyzing the action of pepsin. 

    Gastrin is another hormone involved in the regulation of the activity in the gastric system. It stimulates the release of gastric acid. 

    The macronutrients we take in will leave the stomach at differing times due to their rate of digestion. One thing that will enter the bloodstream the fastest will be the liquids containing carbohydrate drinks. These are used for sports drinks and they are able to replenish the body’s main energy source quicker. When the stomach has a mixed amount of carbs and proteins, it will take longer for the stomach to empty. The stomach takes an average of 1 – 4 hours depending on the kind of food we take in. 

    The intestines are seen as the main location that the body absorbs some nutrients, but really, there are some that we get through the stomach only. These are the things we may absorb in the stomach: water, alcohol, and vitamins like niacin, for one. 

    Water being able to be absorbed by the stomach is actually super helpful in replenishing water and nutrients lost while exercising. And they will often contain glucose and fructose for the fast absorption of those associated with the liquids. This will help to spare the supplies of glycogen in the body while exercising. 

    The stomach is still going to be the main start of the process of digestion into smaller parts. Turning complex molecules like our proteins, into the amino acids for further use. We know this process as hydrolysis. This is going to be continued in the intestines. 

    The Small Intestine

    This organ stretches to around 12 feet total in length and it is put into three regions. These are the duodenum, jejunum, and the ileum. 

    The duodenum connects to the stomach and makes up the first part of the small intestine. It has some small absorption occurring here, but it is mostly for continued breakdown of the storage and food. the remaining food from here will move to the jejunum and ileum next.

    The small intestine has many of its enzymes coming from the pancreas, and some from the intestinal cells. These are things like maltase, lactase, peptidase, and the nucleosidases. Bile from our bladder is added in to help the fat digestion.

    The villi in the small intestine work to provide a unique surface for the specialized cells to complete the absorption. They increase the surface area like many of the similar things in the body. 

    The rest of the small intestines will work to absorb many proteins, carbs, and vitamins and minerals. 

    Large Intestine and Rectum

    This is going to be the part of the intestine that is 3 feet in length. They join together at a place called the cecum, where the ileum and the large intestine meet. In these parts we see the final absorption of any water left, some minerals, and also some of the vitamins here. 

    The live microbes live and exist here. They produce some of the vitamins that are taken in through their own metabolism. This is like the vitamin K. the large intestine may also be called the colon. It is used to store the waste products of digestion also. 

    The proper stimulation of the colon causes the emptying of the contents into the rectum. This triggers defecation. The more fiber there is in the diet, the softer the feces will be and the easier it is to defecate. 


    This is found along the small intestine close to the stomach and it is an accessory organ for the gut. The pancreas is used to produce some important secretions in the absorption and digestion process. It works on the things in the small intestine primarily. Some example enzymes include lipase, amylase, trypsin, and nuclease. 

    Hormonal secretions in the form of insulin and glucagon work to control the metabolism of carbohydrates. 

    Liver and Gall Bladder

    Digestion is not done until the nutrients have been put into the liver and released then to the bloodstream. We connect the intestines to the liver through the portal vein. So, the nutrients are delivered via the liver essentially. 

    Factors Affecting Digestion

    For us to get the most out of our meals, we should:

    Eat slow and make sure that we chew our food really well.

    Keep our posture upright and avoid eating in a laid down position.

    Eat many meals of moderate size instead of very few larger meals.

    Eat when we are calm, as the nervousness may affect the movement of the parts in our digestive system and even cause disturbances throughout. 

    Allow a lot of time for the occurrence of digestion. Strenuous activity is avoided right after you eat. 

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    Avoid the foods that you know will irritate your stomach like the hot and spicy food, or some alcohols.

    Consult with a physician when the digestive system has some disorders present or possibly suspected. 

    When you consume multi-nutrient liquid meal style products and protein powders, you should drink them slow, so the nutrients are recognized by the system and ingested properly. 

    About Digestive Supplement Aids

    Supplements for digestive enzymes have been around for a lot of time and are appearing more now in the products for sports nutrition. These digestive enzymes will act as catalysts for the breakdown of food parts. This of course will speed up the processes, as all enzymes do. Under standard conditions, the healthy digestive system is sufficient for this and will adjust to the increases in food consumption according to the diet itself. 

    The digestive enzymes come in the form of plant and animal based supplements and can even derive from the microbial commercial produced ones. Bromelain is also increased in popularity due to use as a healing and inflammation reductions aid. 

    Lactose Intolerance

    This is a condition that is seen by many people, upwards of millions. It requires medical supervision and a special diet or even the use of medically given digestive enzyme supplements. 

    Lactose deficiency is when the small intestine makes lower levels of lactase and are unable to digest much lactose.

    Lactose malabsorption may be caused from a lactose deficiency. Here, the undigested lactose passes to the colon where the bacteria might break down the undigested lactose and create an upset of fluid and gas in the body.

    Intestinal Microbials

    There are billions of microscopic bacteria, fungi, and other kinds of microorganisms all living within the intestinal tract. 

    Probiotics are live organisms in the body that may benefit the body as oppose to the pathogens that might cause adverse effects. Some approved uses of probiotics are:

    Helping to support the gastrointestinal health.

    Promotion of bodily regularity.

    It could promote a favorable flora in the gut.

    It can aid in digestion, and this is through the absorption of nutrients.

    For some species, they can help to reduce symptoms of diarrhea. 

    ISSA SN Chapter 13: Digestion and Absorption 1
    ISSA SN Chapter 13: Digestion and Absorption 2
    ISSA SN Chapter 13: Digestion and Absorption 3

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