What is Nutrition?

The process of consuming nutrients to utilize them for various biological purposes is known as nutrition. The substances absorbed from food are known as nutrients. They serve different important functions in the body, such as repairing wounds and damages, generating energy for metabolism, and regulating biochemical reactions.

What are the processes involved in nutrition?

Digestion can occur in two ways depending on the type of mechanism involved. It can be physical or mechanical. The former involves the physical breakdown of food, while the latter involves the chemical breakdown of food in the digestive system.

  1. Ingestion is the initial mechanical step of digestion. In this, the food enters the body through the mouth, where it is chewed and swallowed.
  2. It is followed by the chemical digestion of food into smaller particles.
  3. Then, the absorption of the nutrients produced by the digestion of food takes place.
  4. Next is assimilation. It is the process in which tissues use the nutrients for various biological functions.
  5. Egestion is the last step in which the waste products formed after digestion are removed from the body.

Types of Nutrients


The nutrients that are required by the body in large amounts are known as macronutrients. They are formed by the joining of hydrocarbons. They provide energy for metabolism in the form of ATP when their chemical bonds break down. The classes of macronutrients are explained below:

  • Carbohydrates such as glucose, sucrose, and glycogen can be obtained from vegetables, fruits, milk, and grains. Carbohydrates can be in the form of monosaccharides containing only one sugar unit. They can be in the form of disaccharides containing two monosaccharide units joined through a glycosidic bond. They can also be in the form of polysaccharides containing several monosaccharide units joined by glycosidic bonds.
  • Lipids are nutrients that are insoluble in water. They can be obtained from dairy products, oil, meat, and processed foods. Lipids are triglycerides containing glycerol and three fatty acids. They can be in the form of phospholipids containing glycerol, two fatty acids, and phosphate groups. They can also be in the form of sterols containing a hydrocarbon chain attached to a four-ringed structure.
  • Proteins can be obtained from dairy products, seafood, plant products, and meat. They are made up of amino acids joined through peptide bonds. They regulate multiple biological processes in the form of hormones and enzymes. Proteins are also involved in the repair of cells and tissues.
  • Water is a macronutrient that contains hydrogen and oxygen, not carbon. It is involved in the transportation of nutrients and waste products in the body. It is also involved in the maintenance of the homeostasis of temperature.


The nutrients that are involved in several functions but are needed by the body in small amounts are known as micronutrients. They act as cofactors and coenzymes in biochemical reactions. The classes of micronutrients are listed below:

  • Minerals such as sodium, iron, and potassium are the inorganic compounds which are required by the body. The minerals required by the body in very small amounts (few milligrams) are known as trace minerals. And the minerals required by the body in large amounts (hundreds of milligrams) are known as macrominerals.
  • The body requires thirteen vitamins. These vitamins can be water-soluble or fat-soluble. Vitamin B complex and vitamin C are water-soluble, while vitamins A, K, E, and D are fat-soluble.

Minerals maintain fluid balance, the transmission of nerve impulses, muscle contraction, and many other functions in the body. Vitamins help in the process of energy production. They are involved in the formation of red blood cells and bone tissues. They also help in the functioning of the nervous system and the immune system.


The process of breakdown of the consumed food particles in the digestive system is known as digestion. It results in the formation of small molecules that can be dissolved in water. In humans, the digestion of food releases nutrients that are absorbed in the body. Digestion is a catabolic process that is divided into two steps:

  • Mechanical digestion: It involves the breakdown of food particles through physical methods. The chewing of food by teeth is an example of mechanical digestion.
  • Chemical digestion: It involves the breakdown of molecules using enzymes.

Human digestive system

The human digestive system is composed of organs involved in the consumption and digestion of food, absorption of nutrients, and excretion of waste products. The path followed by the food marks the digestive tract. The organs of the digestive system jointly form the gastrointestinal system. They are

  • Mouth
  • Salivary glands 
  • Pharynx
  • Esophagus
  • Stomach
  • Small intestine (divided into duodenum, jejunum, and ileum)
  • Large intestine (divided into colon, cecum, rectum, and anus)
  • Liver
  • Pancreas
  • Gall bladder
An image shows the anatomy of the human digestive system in which the mouth is connected to the stomach through a long tube-shaped esophagus. The liver, gall bladder, and pancreas are present near the stomach. The stomach is connected to the coiled tube-like small intestine. The small intestine is joined to the large intestine that continues into rectum and anus.
CC-BY | Image Credits: https://www.niddk.nih.gov

Transport of food through the gastrointestinal tract

The path taken by food during digestion is known as the gastrointestinal tract. The steps involved in the transportation of food through this tract are listed below:

  1. The mouth is the entry point of food, from where it reaches the digestive tract. The movement of food inside the mouth is facilitated by the saliva secreted by the salivary glands. The saliva moistens the food.
  2. The process of swallowing allows the food to reach the pharynx. The epiglottis covers the trachea (windpipe) to ensure that the food enters the esophagus only.
  3. The esophagus is also known as the food pipe. Here, the food travels through the peristaltic movement.
  4. The peristaltic movement occurs by the contraction of the muscles of the walls of the food pipe.
  5. At the end of the esophagus, the lower esophageal sphincter relaxes and allows the food to pass into the stomach. This sphincter also prevents the backward movement of food.
  6. In the stomach, the food mixes with the digestive juice, which helps in its breakdown. It produces chyme that is transported to the small intestine.
  7. The digestive juices from the liver, small intestine, and pancreas mix with chyme in the small intestine and digest it further.
  8. The nutrients released after digestion are absorbed by the walls of the small intestine.
  9. The undigested food and other waste products of digestion travel to the large intestine through the peristaltic movement.
  10. The large intestine absorbs the excess water and produces stool that is transported to the rectum again through the peristaltic movement.
  11. The stool is stored in the rectum and is later excreted from the body through the anus.

Chemical digestion of food

The nutrients present in food, such as carbohydrates, proteins, and fats, are broken down into smaller units in the digestive tract and absorbed by the walls of the small intestine. Vitamins and minerals are also absorbed in the small intestine.

Digestion of carbohydrates

  • The salivary amylase released by the salivary glands and the pancreatic amylase released by the pancreas break the glycosidic bonds of polysaccharides, producing glucose and maltose.
  • The maltase enzyme digests maltose into glucose molecules.
  • The lactase enzyme gives glucose and galactose by breaking down lactose molecules.
  • The sucrase enzyme gives fructose and glucose by breaking down sucrose molecules.
  • Glucose, galactose, and fructose are absorbed by the small intestine.
A flowchart represents the mechanism of breakdown of carbohydrates. The image has three flowcharts. The first one shows the breakdown of polysaccharides starch and glycogen into disaccharide maltose by amylase. Maltose is converted to glucose by maltase. The second flowchart shows the breakdown of disaccharide sucrose into glucose and fructose (monosaccharides)  by the sucrase enzyme. The third flowchart shows the breakdown of disaccharide lactose into glucose and galactose (monosaccharides) by the lactase enzyme.
Representation of breakdown of carbohydrates

Digestion of proteins

  • The pepsin enzyme in the stomach digests proteins to produce polypeptides.
  • In the duodenum, the pancreatic enzymes trypsin and chymotrypsin also produce polypeptides from proteins.
  • The enzymes exopeptidases and dipeptidases produce amino acids by breaking down polypeptides.
  • The small intestine secretes the enterokinase enzyme to activate these enzymes.
An illustration represents breakdown of proteins. The image shows the folded structure of a protein that converts into a long polypeptide chain resembling a chain of beads. The polypeptide further breaks down into a tripeptide containing three amino acids and a dipeptide containing two amino acids. They are represented as three and two beads, respectively. These peptides again break down to form amino acids.
Representation of protein breakdown

Digestion of fats

  • The lingual lipase enzyme secreted in the mouth breaks down fats into diglycerides.
  • In the small intestine, fats are converted into fatty acids, triglycerides, monoglycerides, and diglycerides by the action of bile secreted by the liver and the pancreatic lipase enzyme secreted by the pancreas.

Role of digestion in nutrition

The process of digestion is essential for the breakdown of very large nutrients that the body cannot use in its original form. The breakdown process releases small-sized nutrients that can be easily absorbed through the small intestine.

Common Mistakes

Most of the students assume that only vitamins and minerals are nutrients. However, there are a total of six classes of nutrients, including water.

Students may assume that water is not a macronutrient because it does not contain carbon. In truth, water is a significant macronutrient that the body requires in large quantities for regulating multiple biochemical reactions.

Context and Applications

  • Bachelors of Science in Human Biology
  • Bachelors of Science in Biotechnology
  • Bachelors of Science in Food and Nutrition
  • Bachelors of Science in Dietetics
  • Masters of Science in Food Biotechnology
  • Masters of Science in Nutrition and Dietetics
  • Digestive system
  • Human anatomy
  • Types of nutrition
  • Modes of nutrition

Practice Problems

Q1: Identify the correct definition of nutrition.

(a) Nutrition is the process of the breakdown of food to produce nutrients.

(b) Nutrition is the process of consuming nutrients in the form of food to carry out the body functions.

(c) Nutrition is the process of the breakdown of macronutrients to produce micronutrients.

(d) None of the above

Correct option: (b)

Q2: Identify the correct definition of digestion.

(a) Digestion is the process in which the breakdown of macronutrients produces micronutrients.

(b) Digestion is the process in which waste products are removed from the body.

(c) Digestion is the process in which large and complex food particles break down to produce smaller particles.

(d) Digestion is the process by which food enters the body.

Correct option: (c)

Q3: What is the importance of nutrition in humans?

(a) Body growth

(b) Energy production

(c) Cell repair

(d) All of the above

Correct option: (d)

Q4: Which of the following is not a macronutrient?

(a) Carbohydrates

(b) Water

(c) Minerals

(d) Proteins

Correct option: (c)

Q5: Which of the following enzymes breaks down polysaccharides during digestion?

(a) Amylase

(b) Trypsin

(c) Pepsin

(d) Lactase

Correct option: (a)

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