Diet and digestion

Food provides the energy needed by living things. Food undergoes both physical digestion and chemical digestion so that it can be absorbed into the bloodstream.

Carbohydrates and fats are high energy sources. Protein, meanwhile, is needed for growth and repair. Protein deficiency leads to diseases such as kwashiorkor.

A balanced diet

The table below summarises the three main nutrients in food, what they are made from, and why they are needed.

The main nutrients in food

nutrientmade frommain uses
potato, banana, pasta carbohydratessimple sugars such as glucosehigh energy source
cheese fatsfatty acids and glycerolhigh energy source
meat proteinsamino acidsgrowth and repair

Proteins are also used as an 'emergency' energy source if the diet has insufficient carbohydrates and fats in it.

A balanced diet will also include nutrients that do not provide energy, such as:

  • minerals, like iron, to make haemoglobin
  • vitamins, such as vitamin C, which prevents a disease called scurvy
  • fibre, which prevents constipation
  • water

Underweight, overweight

In warm weather, or when you are not doing much exercise, you do not need to eat as much food as when it is cold, or when you have exerted yourself physically. If you eat too much food without taking enough exercise, you will become overweight, with very overweight people being described as obese. Overweight people may suffer from health problems, including:

  • diabetes - an illness in which the body is unable to control the amount of sugar in the blood
  • arthritis - an illness in which the joints become worn, inflamed and painful
  • heart disease
  • breast cancer


Teenagers are growing quickly and need plenty of protein. But people in developing countries may not get enough protein in their diet. This is called protein deficiency. It can lead to a disease called kwashiorkor, which causes a swollen abdomen.

You can calculate the recommended daily average (RDA) intake of protein using this formula:

RDA in g = 0.75 × body mass in kg


What is the protein RDA for a person with a body mass of 60kg?

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RDA = 0.75 × 60 = 45g

Body Mass Index

The Body Mass Index (BMI) is a guide to whether someone is underweight, normal weight or overweight. You can calculate the BMI using this formula:

BMI = mass in kg ÷ (height in m)2


What is the BMI of a 1.7 metre-tall person with a body mass of 60kg?

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BMI = 60 ÷ (1.7)2 = 20.8

In this example, the person would be at the ideal weight for their height.

The meaning of BMI's

under 18.4underweight
between 18.5 and 24.9ideal weight
between 25 and 29.9over the ideal weight
between 30 and 39.9obese
over 40very obese

Physical digestion

The food we eat consists of large lumps of material. We must bite off small pieces and chew them up into even smaller parts, before swallowing them. When the food gets into our stomach it is broken down further as it is squeezed and moved around by the stomach’s muscular walls. This is physical digestion.

Food is moved through the digestive system by the contractions of two sets of muscles in the walls of the gut. One set runs along the gut, while the other set circles it. Their wave-like contractions create a squeezing action, moving down the gut.

Chemical digestion

The substances which our body needs cannot be absorbed into the blood until they have been broken down into small, soluble chemicals. This process is called chemical digestion, and it works with the help of enzymes. Once the food is completely digested, its molecules are small enough to pass through the wall of the small intestine by diffusion.

Digestive enzymes

Different enzymes digest different food molecules.

Nutrients and the enzymes that digest them

nutrientdigestive enzymes

Chemical digestion occurs in the mouth, stomach and small intestine. In the stomach, stomach acid helps the enzymes to work.

Bile - higher

Bile is an alkaline substance produced by the liver and stored in the gall bladder. It is secreted into the small intestine, where it emulsifies fats. This is important, because it provides a larger surface area in which the lipases can work.

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