Human Nutrition and Digestion

Human nutrition is the provision of essential materials (in the form of food) required by organisms to support life. For these nutrients to be useful, they must be broken down and absorbed, a process known as digestion. This entire process is fundamental to providing energy and the building blocks for growth and repair.

### The Balanced Diet

A balanced diet provides all essential nutrients in the correct proportions to maintain good health. There are seven key components:

### The Human Digestive System

The digestive system, or alimentary canal, is a long tube running from the mouth to the anus, where food is broken down and absorbed.

1. Ingestion and the Mouth:

Food enters the body through the mouth (ingestion). Mechanical digestion begins as the teeth chew food, increasing its surface area. Simultaneously, chemical digestion starts as the salivary glands release saliva containing the enzyme salivary amylase, which begins breaking down starch into simpler sugars.

2. Oesophagus:

The chewed food (bolus) is pushed down the oesophagus by waves of muscle contractions known as peristalsis.

3. Stomach:

The stomach is a muscular sac that performs both mechanical and chemical digestion. Its muscular walls churn the food (mechanical digestion). Glands in the stomach wall secrete gastric juice, which contains:

* Hydrochloric acid (HCl): This creates a highly acidic environment (pH 1.5-2.5) that kills harmful bacteria and provides the optimal pH for pepsin.

* Pepsin: A protease enzyme that begins the chemical digestion of proteins into smaller polypeptides.

4. Small Intestine:

This is where most chemical digestion and all absorption of digested food occurs. It is divided into the duodenum and the ileum.

* In the duodenum (the first part), the acidic chyme from the stomach is mixed with secretions from the pancreas and liver.

* Pancreatic Juice: Contains pancreatic amylase (digests remaining starch), trypsin (a protease that digests proteins), and lipase (digests lipids).

* Bile: Produced by the liver and stored in the gall bladder, bile is released into the duodenum. It is not an enzyme but performs a vital function called emulsification. Bile breaks down large fat globules into smaller droplets, increasing the surface area for the enzyme lipase to act upon.

* The ileum is the final and longest section of the small intestine. Its primary role is absorption. It is highly adapted for this function with:

* A very long length (several metres).

* A folded inner surface covered in millions of tiny, finger-like projections called villi.

* The epithelial cells of the villi have their own micro-projections called microvilli.

These adaptations create an enormous surface area for efficient absorption. Digested nutrients (glucose, amino acids, fatty acids, glycerol) are absorbed through the villi walls. Glucose and amino acids pass into the bloodstream (capillaries), while fatty acids and glycerol enter the lacteal, which is part of the lymphatic system.

5. Assimilation:

Once absorbed into the bloodstream, the soluble food molecules are transported to the body's cells. This process, where cells use the nutrients, is called assimilation. For example, glucose is used in respiration, and amino acids are used to synthesize new proteins.

6. Large Intestine and Egestion:

Undigested food, mainly fibre, passes into the large intestine. Its main function is the absorption of water from the remaining material. The leftover solid waste, called faeces, is stored in the rectum before being expelled from the body through the anus. This removal of undigested waste is called egestion.