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Recent research has revealed that the large intestine and its resident bacterial population have key roles to play in determining our health and wellbeing. It is much more than just a waste storage facility.

Structural features

In an average adult, the large intestine is about 1.5m long and 5cm wide. It consists of the caecumappendixcolon and rectum.

The ileocaecal valve controls the entry of material from the last part of the small intestine called the ileum.

The human appendix has no known function and is thought to be a remnant from a previous time in human evolution.

What does the large intestine do?

The 4 major functions of the large intestine are:

  • reabsorption of water and mineral ions such as sodium and chloride
  • formation and temporary storage of faeces
  • maintaining a resident population of over 500 species of bacteria
  • bacterial fermentation of indigestible materials.

By the time partially digested foodstuffs reach the end of the small intestine (ileum), about 80% of the water content has been absorbed. The colon absorbs most of the remaining water.

As the remnant food material moves through the colon, it is mixed with bacteria and mucus, and formed into faeces for temporary storage before being eliminated.

It has been estimated that there are about 500 species of different bacteria found inhabiting the adult colon. Most of these bacteria can only survive in oxygen-free environments and are referred to as anaerobes. These bacteria ferment some of the undigested food components, converting them into short-chain fatty acids (SCFAs) and releasing gases like carbon dioxidehydrogen and methane. SCFAs such as acetic, propanoic and butyric acids then serve as an energy source for the bacteria as well as the cells lining the colon.

CH3COOH
acetic acid

CH3CH2COOH
propanoic acid

CH3CH2CH2COOH
butyric acid

Gut bacteria and health

The resident bacterial species in the large intestine form complex inter-relationships with themselves as well as their human host. It is now thought that, rather than being a non-harmful coexistence, it is in fact a symbiotic relationship where each gains benefit from the other.

Recent research has revealed that gut bacteria perform a host of useful functions apart from fermenting undigested macronutrient material. These include interacting with the immune system, producing vitamins such as vitamin K, stimulating the release of hormones involved in storage of fats and influencing mood and our feeling of wellbeing.

This high level of activity, which impacts on our health and wellbeing, has led some researchers to regard the gut bacteria as a body organ in its own right rather than a population of bacteria that just happen to live in the gut.

In addition to the importance of the ‘bacterial organ’, researchers now believe that the network of interconnected nerve cells lining the large intestine has a key role to play in food intake and its digestion. This enteric nervous system is now often referred to as the ‘second brain’. It is capable of directing messages to the brain as well as controlling the release of hormones that influence the movement of food down the gut, feelings of wellbeing and the sensations of being hungry or of being full.

This new field of scientific research – known as neurogastroenterology – is helping to explain how the ‘second brain’ influences the body’s immune response. This will lead to a greater understanding of how diseases such as inflammatory bowel disease develop and how they can be prevented.

    Published 5 September 2011