8A: Food and Digestion

The mouth

• We chew the food in the mouth to break it into smaller pieces.
• Saliva in the mouth contains certain digestive juices. It mixes with the food with the help of the tongue. Saliva softens the food and makes it easy to swallow.
• Food is partly broken down by the process of chewing and by the chemical action of salivary enzymes (these enzymes are produced by the salivary glands and break down starches into smaller molecules).

The Gullet

• The gullet connects the mouth to the stomach.
• The chewed food moves down the gullet and goes into the stomach.
• No digestion takes place in the gullet.

The Stomach

• In the stomach, the food mixes with the digestive juices that are produced in the stomach.
• the food then moves to the small intestine.

The Small Intestine

• more digestive juices are added in the small intestine where the process of digestion is completed.
• The small intestine absorbs the digested food. Blood transports these nutrients to the rest of the body.
• The material that is not absorbed then moves to the large intestine.

The Large Intestine

• In the large intestine, water and some mineral salts are absorbed.
• The waste materials are passed out of the body through the anus as faeces.

The Anus

• Faeces are expelled from the body through the anus.

Nutrients                                   Use in the body                                                            Good Sources

Carbohydrate                            To provide energy                                        Cereals, bread, pasta, rice and potatoes

Protein                                    For growth and repair                                     Fish,meat, eggs, beans, dairy products

Fat                                      To provide energy. Also to store                                            Butter, oil and nuts  

                                           energy in the body and insulate

                                                it against the cold

Minerals                               Small amount for health                                   Salt,milk (for calcuim) and liver (for iron)

Vitamins                              Small amounts health                                              Dairy foods, fruit, vegetables

Fibre                               To provide roughage to help to keep    

                                      the food moving through the gut

Water                              Needed for cells and body fluids                                        Fruit juice, milk, water

If you have too little of a particular nutrient, we say that you have a deficiency in that nutrient. For example, fibre is needed to keep food moving through the intestines easily, and people who have a fibre deficiency in their diet may get constipation.

Too thin, too fat

Food is a store of chemical energy. If you look on the side of food packets you will see the food's energy content. This is usually measured in kilojoules, kJ. If we eat too little food, we will use up our store of fat and become too thin. If we eat too much food, especially foods rich in sugar and fat, we will increase our store of fat and become too fat.

It is important to balance the amount of food we eat with who we are and what we do. The amount of energy we need from our food depends on our age, our height and how much exercise we get.

For example, a one-year old baby needs 3850 kJ each day to continue to grow, whereas an adult Olympic swimmer in training needs 15,600 kJ each day. Someone who sits at a desk all day will need less food than their twin who climbs ladders all day to wash windows.

Digestion and enzymes

Our teeth break food down into small pieces when we chew. This is only a start to the process of digestion, as chewed pieces of food are still too large to be absorbed by the body. Food has to be broken down chemically into really small particles before it can be absorbed. Enzymes are needed so that this break-down happens quickly enough to be useful.

Enzymes

Take care - enzymes are not living things. They are just special proteins that can break large molecules into small molecules. Different types of enzymes can break down different nutrients:

• carbohydrase or amylase enzymes break down starch into sugar

• protease enzymes break down proteins into amino acids

• lipase enzymes break down fats into fatty acids and glycerol.

The inside wall of the small intestine needs to be thin, with a really big surface area. This allows absorption to happen quickly and efficiently. If the small intestine had a thick wall and a small surface area, a lot of digested food might pass out of the body before it had a chance to be absorbed.

To get a big surface area, the inside wall of the small intestine is lined with tiny villi (one of them is called a villus). These stick out and give a big surface area. They also contain blood capillaries to carry away the absorbed food molecules.