Biology B1 - Human Health and Diet


Human Health and Diet

A balanced diet

  • Carbohydrates - simple sugars such as glucose - high energy source
  • Fats - fatty acids and glycerol - hight energy source 
  • Proteins - amino acids - growth and repair

Proteins are also used as an 'emergency' energy source if a diet has insufficient carbohydrates and fats. A balanced diet will also include nutrients that do not provide energy, such as:

  • minerals, such as iron, to make the haemoglobin needed in red blood cells vitamins,
  • such as vitamin C, which prevents a disease called scurvy
  • fibre, which prevents constipation
  • water.

A balanced diet is different for different people and will vary according to age, gender, physical activity and religion (certain foods may be banned). Some people choose not to eat particular things - vegetarians and vegans avoid animal products, while others may be allergic to certain foods.

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Human Health and Diet

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.

A diet leading to people becoming underweight or overweight may be caused by the person having low self-esteem and a poor self-image, which may themselves be influenced by body weight. 

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Human Health and Diet

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 (kg) ÷ Height (m)²

under 18.5 -  Underweight 

between 18.5 and 24.9 - Normal

between 25 and 29.9 - Overweight

between 30 and 39.9 - Obese 

over 40 - Very Obese

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Human Health and Diet


Teenagers are growing quickly and need plenty of protein in their diet, but people in developing countries may not get enough. This is called protein deficiency.

Protein deficiency

Kwashiorkor is a disease caused by protein deficiency. Symptoms include a badly swollen abdomen.

Kwashiorkor is common in developing countries because people may not get enough protein in their diet. The reasons why are complex, but include:

  • overpopulation (too many people for the land and resources available)
  • limited investment in farming methods, for example few tractors or artificial fertilisers.
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Human Health and Diet

Estimated average daily requirement

You can calculate the estimated average daily requirement (EAR) for protein using this formula:

EAR = 0.6 × body mass (kg)


A person weighing 60kg

EAR = 0.6 × 60 = 36 g of protein

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Human Health and Diet


The EAR is an estimated daily amount only. It is for an average person with a particular body mass, but some people may need more protein.

  • A growing teenager will need more protein in their diet than an adult with the same body mass.
  • Pregnant women will also need more protein in their diet, as will new mothers who are breast feeding their baby.

The body does not store proteins but it can store fats and carbohydrates:

  • fats are stored around organs and under the skin as adipose tissue
  • carbohydrates are converted to fats, or stored as glycogen in the liver.

Proteins from plants are ‘second class’ proteins. Proteins from animals are ‘first class’ proteins.

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