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Topic last updated: 25 Oct 2021

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Diet and digestion

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The digestive system in action

The digestive system is a good example of many organs working together to carry out an important body function.

The process of digestion starts at the mouth. The food is broken down by both physical and chemical digestion through the mouth, stomach, duodenum and ileum. In the ileum, most of the food has been broken down into small soluble molecules. These move from your small intestine into your bloodstream by diffusion, which takes place down a steep concentration gradient. The concentration of glucose, amino acids, fatty acids and glycerol in the small intestine is much, much higher than the concentration in the blood. The villi of the small intestine provide a very large surface area where this diffusion can take.

Water is removed from the mixture of undigested food, enzymes and dead cells which remains as it passes through the colon. The waste material known as faeces is finally passed out of the body through the anus.

The human digestive system

Animation of the human digestive system

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The following animation illustrates how the absorption of different nutrients, such as glucose, fatty acids, glycerol or amino acids, take place during digestion. Absorption is a key process of digestion, as it is the phase in which our body obtains the nutrients from the food and distributes them wherever they are needed.

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Bacteria in the gut

Inside your digestive system, along with the enzymes, acid, bile, alkali, digested and undigested food there are trillions of bacteria - around 1013 of them, with a mass of about 1.5 kg! Many of these bacteria are very good for you but some of them can cause problems.

A healthy colony of bacteria in your digestive system seems to improve the health of your gut. Can you improve the balance of healthy bacteria and so improve the health of your digestive system?

  • Probiotics are foods, often special dairy products, which contain live bacteria. They are claimed to help colonise your gut with useful Lactobacillus or Bifidobacteria bacteria. The scientific evidence for this is not very strong.
  • Prebiotics act as food for the beneficial bacteria living in your gut. They are found in foods such as tomatoes, bananas, onions and asparagus as well as specially made dairy products. Prebiotics are often oligosaccharides which your body cannot digest but the beneficial bacteria can. The scientific evidence is growing that these foods really do have a positive effect on the numbers of 'good' bacteria in your gut and on overall gut health.

To assess your knowledge, have a go with the quick 3-question quiz below. To try again, click on the "Back to start" button.

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Chemical digestion
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