ISSA SN Chapter 13: Digestion and Absorption

Yet to sign up for the ISSA nutrition cert? My site visitors get a special discount here. You can even get this nutrition cert for free with the purchase of their personal trained cert (an excellent combo).

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 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 lined with thick epithelium with 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 the process of breaking down useful biomolecules to obtain the needed energy for life and the building blocks for that life.

The main functions of the digestive system:

  • The reception, mastication, and transport of the substances we take in and the waste products that occur. 
  • The secretion of acids, mucus, enzymes for digestion, bile, and any other materials we need to break down ingested food. 
  • Digestion for the foodstuff 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 will comprise several anatomically different structures that compose the 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, reducing the size of the ingested food.

Second, it will mix the food with saliva to moisten it, preparing it for swallowing to go further in digestion. The saliva serves 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, which goes through our pharynx and 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 use of either heating or cooling the food. The operation of the enzymes and other digestive things depends 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 to transport food and water that travels to the stomach. The muscle contractions to move the food through the system are involuntary, so we have no control over it. It travels at around three inches per second. The esophageal sphincter is a ring like a muscle that relaxes to let the food finally enter into the stomach. 

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

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


This is the muscular sac around the size of two quarts and varies a tad in this size based on the person’s actual size. It stores and then gradually release food into the small intestine when it is ready to release. 

The stomach secretes substances to aid 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 regulating the activity of 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 can 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 where the body absorbs some nutrients, but 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 absorbed by the stomach is 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 spare glycogen supplies in the body while exercising. 

The stomach will still be the main start of the digestion process into smaller parts. Turning complex molecules, like our proteins, into 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 mostly for a 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 nucleosidases. Bile from our bladder is added in to help 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 similar things in the body. 

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

Large Intestine and Rectum

This will be part of the intestine that is 3 feet long. 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 some vitamins. 

Exclusive PTP CPT Offers

Gold Standard Cert
NASM Gold Standard Personal Trainer Certification - Save 25&percnt off
Most Popular Cert
ISSA - Most Popular Online Personal Trainer Certification 3 Certs for
Best Study Materials
TA - Trainer Academy - Best Study Materials for Personal Trainer Certification Online - See MVP discount
A Good Option
ACE Certification- A Good Fitness Course Online Option - 25&percnt off
A Good Option
NCSF Certification - A Good Option - Save 25%
Best CPT for you?
Best CPT For You? Take the Personal Trainer Certification Online quiz and Get a Personalized Recommendation Just for You

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

Proper colon stimulation 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 control carbohydrates’ metabolism. 

Liver and Gall Bladder

Digestion is not done until the nutrients are put into the liver and released into 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 slowly, and make sure that you chew your 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 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. 

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

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 slowly so the system recognizes the nutrients and ingests them properly. 

About Digestive Supplement Aids

Supplements for digestive enzymes have been around for a long 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 will speed up the processes, as all enzymes do. Under standard conditions, a 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 microbial commercially produced ones. Bromelain is also increased in popularity due to its use as a healing and inflammation reduction aid. 

Lactose Intolerance

This condition 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 lowers lactase levels and cannot digest much lactose.

Lactose malabsorption may be caused by 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

Bills of microscopic bacteria, fungi, and other kinds of microorganisms live within the intestinal tract. 

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

Helping to support gastrointestinal health.

Promotion of bodily regularity.

It could promote 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 4
ISSA SN Chapter 13: Digestion and Absorption 5
ISSA SN Chapter 13: Digestion and Absorption 6

Tyler Read - Certified Personal Trainer with PTPioneer

Tyler Read

All Posts

PTPioneer Editorial Integrity

All content published on PTPioneer is checked and reviewed extensively by our staff of experienced personal trainers, nutrition coaches, and other Fitness Experts. This is to make sure that the content you are reading is fact-checked for accuracy, contains up-to-date information, and is relevant. We only add trustworthy citations that you can find at the bottom of each article. You can read more about our editorial integrity here.

Ask me a question and I will reply ASAP

Find the best Cert for you

Get The Sectret Cheat Sheet For The ISSA Exam


Get The Sectret Cheat Sheet For The CSCS Exam


Get The Sectret Cheat Sheet For The ACSM Exam


Get The Sectret Cheat Sheet For The ISSA Nutritionist Exam


Get The Sectret Cheat Sheet For The NCSF CPT Exam


Get The Sectret Cheat Sheet For The NASM CNC Exam


Get The Sectret Cheat Sheet For The NASM PES Exam


Get The Sectret Cheat Sheet For The NASM CES Exam


Get the top 5 Tips for Passing the ACE CPT


Get the top 5 Tips for Passing the NASM CPT


Get The Sectret Cheat Sheet For The NSCA CPT Exam


Get The Sectret Cheat Sheet For The ACE Exam


Get The Sectret Cheat Sheet For The NASM Exam