Food Digestion -How does our body break down food?
For some reasons, the only pathway we are aware of when it comes to the breakdown of food is that which is obvious to us – we eat and excrete faeces and the cycle continues to repeat itself. What exactly happens between the period when we take in food and when the food degenerates and comes out of our bodies as faeces?
Food is divided into six classes – carbohydrates, proteins, fats and oil, vitamins, minerals, and water. Each of these foods follows different pathways in their breakdown and absorption into the body.
A singular meal may contain these classes of food in different but adequate proportions; during the breakdown process, however, food is digested according to its class. We will look at the digestion of three of these classes: carbohydrates, proteins, and lipids.
Where does digestion occur?
Digestion takes place in the alimentary canal or GIT which begins in the mouth and extends down to the anus. The gastrointestinal tract (GIT) consists of the mouth, oesophagus, stomach, small intestine, large intestine, cecum, rectum, and anus.
The chance of digestion occurring at a particular location depends on the availability of enzymes responsible for breaking down a particular class of the food. For example; the saliva contains salivary amylase that acts on carbohydrates but does not contain any enzyme for protein digestion, explaining why protein digestion does not begin in the mouth.
Many of the foods we eat contain carbohydrates; rice, yam, cereals, etc. The end product of carbohydrate breakdown is simple sugars like glucose, fructose, and galactose. Digestion of protein begins in the mouth and ends in the small intestine.
In the Mouth
Saliva contains an enzyme called ptyalin or salivary amylase which is activated by little amounts of sodium chloride. Ptyalin acts on amylase and amylopectin bonds in carbohydrate to release maltose, maltotriose, etc. while the food remains in the mouth.
In the oesophagus
The oesophagus does not have digestive enzymes that act on carbohydrates; hence the bolus of food passes down continually into the stomach. In the oesophagus, however, mucus is present which aids lubrication and free movement of the food bolus.
In the stomach
The Ph of the stomach is acidic; this acidic environment deactivates amylase enzyme that has been swallowed with the bolus of food. This implies that in the stomach, no digestion of carbohydrate occurs.
In the Small Intestine
The small intestine is divided into the duodenum, jejunum, and the ileum. The pancreas secretes pancreatic juice into the small intestine, and digestive secretions also come from the liver and gall bladder into the intestine. Pancreatic juice also contains amylase, hence the breakdown of carbohydrates continues in the duodenum.
Amylase degrades glycogen and starch to maltose and dextrin. Other enzymes like sucrose break down sucrose to glucose and fructose, and lactase acts on lactose converting it to galactose and glucose.
Unlike carbohydrates, protein digestion begins in the stomach and not in the mouth. When food containing protein gets to the stomach, it causes the pyloric cells to secrete a hormone called gastrin. This hormone is responsible for stimulating the fundic cells of the stomach to secrete hydrochloric acid.
Due to the nature of proteins, they are denatured by the hydrochloric acid; making them easier to digest.
An enzyme called pepsin acts on proteins, breaking them down to proteases and peptones; rennin converts caseinogens to casein
Trypsin and chymotrypsin further act on these proteases and peptones, converting them to dipeptides, tripeptides, and polypeptides.
The dipeptides, tripeptides and polypeptides are acted upon by dipeptidases, tripeptidases and aminopeptidase and are converted amino acids.
These amino acids are then absorbed and used by the body.
As fat moves from the mouth through the oesophagus to the stomach, it is warmed and softened to prepare it for digestion. Bile salt is responsible for a process known as emulsification, which reduces the surface tension and increases the surface area of fats making them easily digestible.
In the mouth
Saliva contains an enzyme called lingual lipase secreted by the lingual glands of the mouth. This enzyme breaks down triglycerides into fatty acids and glycerol.
In the stomach
Gatric lipase in the stomach also breaks down triglycerides to fatty acids and glycerol
In the small intestine
Pancreatic juice contains enzymes like pancreatic lipase, phospholipase, colipase, etc. that also acts on lipids converting them to simpler substances like fatty acids and glycerol which can be absorbed into the body.
The phase that follows food digestion is absorption and it occurs majorly in the small intestine.