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.
The table below summarises the three main nutrients in food, what they are made from, and why they are needed.
|nutrient||made from||main uses|
|carbohydrates||simple sugars such as glucose||high energy source|
|fats||fatty acids and glycerol||high energy source|
|proteins||amino acids||growth 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:
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:
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
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?
BMI = 60 ÷ (1.7)2 = 20.8
In this example, the person would be at the ideal weight for their height.
|between 18.5 and 24.9||ideal weight|
|between 25 and 29.9||over the ideal weight|
|between 30 and 39.9||obese|
|over 40||very obese|
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.
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.
Different enzymes digest different food molecules.
Chemical digestion occurs in the mouth, stomach and small intestine. In the stomach, stomach acid helps the enzymes to work.
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.