A balanced diet has many benefits for our health, it can help manage weight; build muscle, reduce fat, keep your heart healthy, help you sleep well, keep your skin and nails healthy, and provide a steady amount of energy through the day. What we sometimes forget about is the digestive system. The system that helps digest and absorb the nutrients we use to fuel every day activity and service our human bodies with the growth and repair of cells. Our bodies constantly recycle cells on a daily basis. Examples include, bone growth and repair, remember osteoblasts and osteoclast cells? Another example would be, skin cells, we renew skin cells between 2-4 weeks depending on age and this becomes longer and slower as we age.

To fuel all our everyday activities and physiological functions, our digestive system unlocks energy and nutients from our daily diet. It would make sense then that we keep the digestive system in the best condition we can to allow it to complete its very important role. Failure of part or parts of the system will reduce the amount of nutrition entering the body, reducing the ability of the body to replenish, repair or grow.

2 Important Functions

The two function of the digestive system are digestion and absorption.


Digestion, the first function of the digestive system and refers to the breakdown of fats; proteins, and carbohydrates into smaller soluble molecules. There are two types of digestion in the human body. Mechanical digestion Begins in the mouth as we chew food with our teeth and move it around with our tongue. This begins the process of digestion. The teeth cut; crush, and chew food, mixing it with saliva to create a bolus (a ball of food).

Mechanical digestion also occurs in the stomach as it churns food over while mixing it with enzymes for chemical digestion.

Chemical Digestion

Chemical digestion is completed using enzymes that digest broken up food into smaller, soluble molecules that the digestive system can absorb into the bloodstream.

Chemical digestion takes place at different points in the system, using different enzymes to complete its job. Each enzyme is specific in its role to breakdown fats; protein, or carbohydrates.

Amylase is the enzyme that breaks down carbohydrate into maltose. It is found in saliva in the mouth and later found in the small intestine. Amylase is produced by salivary glands and is known salivary amylase and is later produced in the pancreas, known as pancreatic amylase.

A unique Protease enzyme “Pepsin” begins the process of breaking down protein in the stomach. Pepsin is produced in the stomach.

Another protease enzyme “Trypsin” released from the pancreas into the small intestine further breaks down protein.

Lipase breaks down fats into oils known as fatty acids and glycerol in the small intestine. Lipase is produced in the pancreas.

Maltase breaks down maltose into glucose in the small intestine and is where maltase is produced.

Note: As nutrients move through the digestive system they get progressively smaller and readily soluble to be absorbed into the bloodstream.

The Right Environment for the Right Enzymes

Enzymes of the the digestive system require different Ph levels to work effectively.

Enzymes of the mouth require a more alkaline (higher) pH level to function. Enzymes of the stomach require a lower (acidic) pH level. Enzymes of the small intestine require a higher (alkaline) pH level.

The stomach has a low Ph level as a result of the production of (hydrochloic acid) to kill of microorganisms that could have been ingested with food and provides the right environment for pepsin to breakdown protein.


Once food has left the stomach and enters the small intestine, it will be acidic, to prevent this causing damage to the small intestine bile raises the pH level. Enzymes of the small intestine alo prefer a more alkaline (higher pH) level. To increase the pH level, bile, produced in the liver and stored in the gallbladder is secreted into the small intestine for two reasons:

Neutralise acid, providing an alkaline condition for the enzymes amylase, maltase, and lipase to function.

Emulsifies fats providing a large surface area for lipase to breakdown fats into fatty acids and glycerol.

Moving Food Through the Digestive System

Peristalis is the term used for the process of food moving through the digestive system. Two sets of muscles in the gut and digestive system walls work together to move food through the system. Circular muscles will reduce the diameter of the gut when contracted, while longitudinal muscles reduce the length of the gut when they contract. These contractions create a wave like movement to push food through the digestive system. Imagine how a caterpillar moves, this is a similar wave like movement.


The second function of the digestive system. Absorption is the movement of food molecules through the wall of the small intestine into the bloodstream. The small intestine is where absorption begins. The ileum is the largest section of the small intestine and can be 2-4 metres in length. The small intestine has a large surface area to give absorption of food molecules the best chance of transfer to the bloodstream quickly and efficiently.

The Villi

The small intestine is full of finger shaped structures known as villus (one single villi) and have 3 important features:

One cell thick: so that absorption is quick with a short distance for absorption by diffusion and active transport into the bloodstream. A large network of blood capillaries: to transport glucose and amino acids from the small intestine to the blood stream. Lacteals: An internal structure that transports fatty acids from the small intestine to the lymph Hepatic portal vein: transports absorbed food into the liver

Assimilation & Egestion

As food passes through the alimentary canal (the gut) food is digested and absorbed for use by all cells within the body for several processes such as respiration; movement, circulation, muscle repair and bone growth among many more. Undigested food is not used and will leave the digestive system.

Assimilation is the term used to describe the transfer of food for use in the body. For example, the transfer of glucose into energy and proteins into amino acids to build new proteins.

The liver plays an important role in assimilation as it converts glucose into a storable molecule called glycogen that can be stored in the liver and the muscles. And the conversion of amino acids into different proteins that the body will use to build and repair different cells. The liver also removes nitrogen from amino acids to form urea and releases energy from amino acids, a process called deamination,

Egestion is the term provided to passing faeces (waste product of digestion), leaving the digestive system by passing out of the body through the anus. By the time food and water reaches the end of the small intestine most of the contents will have been absorbed.

The remaining elements consist of water, bacteria, cells from the gut and indigestible substances such as cellulose from plant cells walls.

The large intestine is the location where any remaining water is absorbed to leave a waste product called faeces. The entrance of the large intestine is the colon and the final section of the large intestine is the rectum where faeces is stored before being ejected through the anus and out of the digestive system.

The Digestive Process


Chewing – (mastication) – mechanical breakdown Produces saliva containing salivary amylase to begin breakdown of carbohydrate Moistens food and protects teeth against decay


Produces a swallowing action from its muscular walls to move food out of the mouth and into the oesophagus.


Rhythmical involuntary muscular (wave like) contractions (peristalis) push food into the stomach.


Produces gastric juices containing hydrochloic acid (kills baceteria) and the enzyme pepsin to breakdown protein. pH level in the stomach is low (acidic).


Secretes pancreatic juice containing digestive enzymes to further the chemical digestion and absorption of nutrients:

Lipase – breaks down fat. Amylase – breaks down carbohydrate into glucose. Trypsin – breaks down protein into amino acids.


Produces bile acids enabling fats to mix with water (emulsification). Produces bile that will help neutralise acid from the stomach as it enters the small intestine.

Gall Bladder

Stores bile produced by the Liver for release into the small intestine.

Small Intestine

Receives bile juice from gallbladder and pancreatic juice from the pancreas. Primary site of digestion and absorption. Has a large surface area amplified by folded sections known as Villi, finger like projections for absorption of nutrients. Digested food passes through the thin wall of the small intestine into the bloodstream by a process called diffusion.

Large Instestine (colon)

Absorbs water, vitamins, and minerals. Contains bacteria which produce vitamins and help prevent infection of the intestine.


Stores faeces


Opening for the ejection of waste (faeces)


It is important for Personal Trainers and Fitness Instructors to understand the digestive system, its roles and functions. The digestive system unlocks energy; proteins, vitamins, and minerals for use in all bodily functions. The two functions of the system are absorption and digestion.

For further reading on the digestive system you can refer to your level 2 gym or level 3 personal trainer manual. If you are not studying with us and would like more information, please head over to our website for details on our personal trainer courses or online personal trainer course you.

If you are a qualified personal trainer or fitness trainer we deliver level 2 CPD awards for instructing kettlebellscircuit sessions, and suspended movement training.