In biology, humans are known as consumers which means that all the molecules, minerals, vitamins and water needed to survive must be eaten. Plants, on the other hand, are known as producers. The process of photosynthesis allows them to be able to assemble, or produce, the nutrients they need from carbon dioxide, water and minerals from the ground.

Even though food contains nutrients, it must first be digested to allow them to be absorbed into the body. The process of digestion can be thought to have three distinct phases: ingestion, digestion and egestion. These words are a bit peculiar and can be translated as

• ingestion – eating
• digestion – breaking down food into its constituent molecules
• egestion – getting rid of any undigested food as faeces.

Eating is a complex activity. First of all, how do we know when to eat and when to stop? What types of things make food appetising to us? What makes us feel hungry and what makes us feel full?

Feeding is controlled by a part of the brain called the hypothalamus. It takes signals from nerves that monitor the body and relay information on things like the level of sugar in the blood and the amount of food in the stomach. If these levels fall below what is normal, the hypothalamus stimulates the sensation of hunger and the desire to eat. These signals are switched off when the stomach is full and nutrients are detected in the bloodstream. We also become conditioned to eat at regular times of the day.

Certain drugs, like amphetamines, can suppress these feelings of hunger. They were once used as slimming pills but scientists soon realised the dangerous side-effects. Some unscrupulous slimming aids still contain drugs similar to amphetamine but these should always be avoided.

In humans, eating also plays an important part in how people interact with each other. Mealtimes provide opportunities to reinforce relationships in families and celebrate special occasions.

The digestive system is effectively a long tube that runs down the centre of the body. Food molecules need to move from the inside of the digestive system and get absorbed into the blood stream. To achieve this, the large, insoluble, food molecules must first be broken down into molecules small enough to pass through the intestine wall. This is the process of digestion.

Digestion also means that the nutrients in the food can be reassembled into the molecules that the body requires. For example, a piece of beef contains proteins that make up muscles in a cow. Once digested, the amino acids in the protein are released and they can be reassembled to make human proteins such as haemoglobin for red blood cells or in the growth of new skin cells and hair.

Digestion allows your food molecules to be absorbed and reassembled in the form that the body requires.

Faeces (or poo) contains any parts of the diet, like fibre, that cannot be digested. It is given its dark colour by bile which contains wastes from the breakdown of red blood cells and a large portion (75%) of faeces is living bacteria from the large intestine.

It is estimated that the average time food stays in the digestive system is around 24 hours. This is made up of 3-4 hours in the stomach, 6 hours in the small intestine, 6 hours in the large intestine and then finally 6 hours in the rectum waiting to be egested.