Process of Food Digestion – An Interesting Story
How food is converted into nutrients
When food is solid, we chew it. We swallow it when it’s liquid. Then it goes down our throats. It’s interesting to learn what happens to food throughout the digestive tract and how our food choices affect our health, fitness and well-being.
When we chew our food in the mouth, it is broken into small pieces and mixed with saliva. Saliva is a colorless watery liquid that is always present in our mouth. It contains enzymes that digest food. These enzymes in saliva convert insoluble starches into water-soluble substances.
Actually, this is the process of digestion. Enzymes convert many complex food substances into simpler substances that can then be absorbed by the body and used for its needs. These enzymes are made by various organs/glands. Finally, what remains in the digestive tract after the food has been processed is of no use to the body & is excreted as stool or feces.
When we smell food, our mouths water. Saliva begins to flow down from the three pairs of salivary glands—one pair under and in front of each ear, another pair at the back of the lower jaw, and the third pair under the tongue.
Normally we produce about 8 to 10 cups of saliva in our mouth in a day.
The enzyme in this saliva breaks down carbohydrates into simpler soluble sugar forms.
The more we break our food down by chewing, the better. Saliva can then act on the food more quickly. Thorough chewing supports the digestive process. (It is said that each bite of food should be chewed 32 times, once for each tooth).
The food then enters our throat through the esophagus. The small flexible lid in our esophagus, called the epiglottis, closes automatically when we swallow our food. It prevents food from passing through the adjacent windpipe.
The stomach is an important pouch-shaped organ. It constantly contracts and relaxes, stirring up the food inside. The lining of the stomach secretes many enzymes.
These enzymes help break down proteins so the body can absorb nutrients. These nutrients are then used by the body for body repair or growth, or as fuel (energy).
There are a large number of glands in the gastric mucosa. One of the enzymes called renin converts protein from milk into soft curds. Another enzyme called pepsin breaks down the long protein chains into smaller, water-soluble units called peptones.
Stomach also makes a large amount of hydrochloric acid. This is the same acid that we see in the chemistry lab. This acid performs many tasks as follows:
1) It weakens proteins by breaking some of their bonds.
2) It dissolves minerals from various foods we eat
3) It kills bacteria that enter our stomach with food.
Food stays in the mouth for a few minutes but in the stomach for hours. The maximum secretion of renin, pepsin and hydrochloric acid occurs about two hours after eating. Food digestion at this time continues very actively.
Digestion in the stomach is essentially the breakdown of proteins into simpler peptone units with the help of two enzymes and hydrochloric acid.
Outflow valve of the stomach, which remains closed most of the day, occasionally opening, allowing a very small amount of semi-digested/digested food to pass further into the small intestine. This valve opens and closes automatically. It allows partially digested, semi-liquid, mushy food to pass into the small intestine.
Small bowel process:
The small intestine is a long tube that processes semi-digested food from the stomach.
The upper part of the small intestine is called the duodenum and is about 25 cm long. There are three main juices that digest food in the small intestine. Bile, a bitter substance, comes from the liver.
The second comes from the pancreas and the third from the small intestine. Juice from the small intestine trickles in many places along the way. The small intestine is quite long, about 5 to 6 times longer than your own body size. It is properly folded in the abdomen. Most of the digestion takes place in this small intestine.
The digestion process is a bit complicated. Pancreatic juices contain many enzymes and hormones. These help break down peptones (derived from proteins) into individual amino acids.
Pancreatic juice also digests fats and carbohydrates. Carbohydrates are converted into soluble glucose by pancreatic juice. Similarly, lactose (present in milk) is also converted into soluble glucose.
Fats are digested into simpler substances with the help of a different type of liver secretion.
The liver is the chemistry laboratory of the human body.
Blood flows from the heart to the liver. The liver controls blood sugar levels and the storage of that sugar in the muscles. It takes amino acids from the blood and turns them into proteins and stores them.
It releases these proteins when the body needs them. It also destroys toxins and stores vitamins and minerals.
All food, which is now broken down into simple, mostly water-soluble substances, is absorbed by the body via the small intestine.
The inner lining of the small intestine has a series of tiny, finger-like projections called villi. These are suction organs that capture the digested materials and pass them to the blood. The blood transports these nutrients to various organs of the body through circulatory processes.
Some of the digested fats are also removed through another network called the lymphatic system. This system also empties its contents into blood somewhere near the throat.
Vitamins and minerals are often water soluble and after they are broken down from food when other nutrients are digested, they are not further broken down and absorbed into the blood. Vitamin A, which is fat-soluble, is absorbed in the small intestine in a similar way to fat itself.
Food that we eat eventually enters the blood. It is then carried to all parts of the body to meet their energy needs and body building and regulation of bodily functions.
The large intestine is adjacent to the small intestine and is tubular in shape. Its inside is smooth with no protrusions. It is placed upside down in the stomach in the shape of the English letter U.
As digested food moves through the intestines, water is absorbed through the walls and into the blood. The food becomes less liquid and hard. As the digested food breaks down, some substances are produced that carry a bad odor. The undigested food is then expelled from the body in the form of stool through the opening called the anus.
How is the digested food made usable?
Carbohydrates (starches and sugars) are broken down into simple sugars, mainly glucose, first in the mouth and then in the small intestine.
Glucose is absorbed directly into the bloodstream from the villi of the small intestine. It travels throughout the body and is used by the muscles as an energy source to do their work.
A small portion of glucose is converted to glycogen, which is the form in which glucose is stored in both the muscles (about two-thirds) and the liver (about one-third).
Some glucose is always circulating in the blood and the level is constant. If the sugar level rises, it is a sign of something wrong – like diabetes, for example.
Fats are finally broken down into glycerol and fatty acids via simpler intermediate forms. Some of these fatty acids, like glycerol, go directly to the blood and then to the liver. These fatty acids are either used for energy or sent to other parts of the body via the blood. They can then be used for energy or incorporated back into fats, which are transported through the blood for storage as adipose tissue.
Excess food that we consume in excess of what we need is mostly converted and stored in the form of fat. In the case of insufficient food intake (e.g. when fasting), fat deposits are used up first.
Proteins reach the bloodstream as amino acids, which are their building blocks. At various points in the body, these amino acids are taken up by the body’s organs and incorporated into various compounds.
Tissue proteins, enzymes, hormones, and so many other chemical compounds are proteins in nature. The liver itself produces and stores body proteins.
Vitamins and minerals are also stored in the liver. Whenever the body needs them, they are delivered to the body parts and organs.
Thousands of interconnected processes are constantly going on in our body. Food is digested and then absorbed in the small intestine. All of the ingested vital nutrients are then circulated to various parts of the body/organs via the blood. And the process goes on.
Thanks to Pradeep Mahajan