#58 Summary of animal nutrition

A balanced diet contains suitable proportions of each group of nutrients – carbohydrates, fats, proteins, minerals, vitamins, water and fibre – and the correct amount of energy.

• Eating food containing more energy than you can use up causes weight increase, which can lead to obesity. Children who do not get enough food may suffer from energy protein malnutrition, in which they do not grow properly and have little energy.

• Digestion is the breakdown of large molecules of food into small ones, so that they can be absorbed through the wall of the alimentary canal.

• Mechanical digestion breaks down large pieces of food to small ones. It is done by the teeth, the muscles in the wall of the alimentary canal and bile salts. Chemical digestion breaks down large molecules to small ones. It is done by enzymes.

• Mammals have four types of teeth – incisors, canines, premolars and molars – each with their own functions.

• Digestion begins in the mouth, as teeth grind food into smaller pieces, and amylase digests starch to maltose.

• Protein digestion begins in the stomach, where pepsin digests proteins to polypeptides. Rennin is present in young mammals, and clots milk protein. Hydrochloric acid kills bacteria and provides a low pH for the action of pepsin.

• Pancreatic juice flows into the duodenum. It contains enzymes that digest starch, proteins and lipids, and also sodium hydrogencarbonate to partly neutralise the acidity of food coming from the stomach.

• Bile also flows into the duodenum. It contains bile salts, which emulsify fats, making it easier for lipase to digest them. 

• The lining of the small intestine is covered with villi, giving it a very large surface area, which helps to speed up absorption. Cells on the surface of the villi make enzymes, which complete the digestion of food. The villi contain blood capillaries to absorb glucose, amino acids, water, vitamins and minerals, and lacteals to absorb fatty acids and glycerol. 

• The absorbed nutrients are carried to the liver in the hepatic portal vein. Some are used in the liver, some are stored, and some are sent on in the blood to be delivered to cells all over the body.

• The colon absorbs more water from the food. In the rectum, the  undigested food is formed into faeces, which are eventually egested through the anus.