Jump to

Key points

  • 'Obese' is a medical term used to describe a person with a high excess of body fat.
  • A person is described as obese if their body mass index (BMI) is greater than 30 kg/m².
  • BMI is just one way to measure obesity.
  • An obese person is at greater risk of , heart disease and some types of cancer.

Obesity and exercise


A school fitness adviser explains what obesity is and the steps that can be taken to avoid it

Can you answer these questions based on the video?

1. Body mass index - BMI - is calculated using a person's height and ......?

2. According to the video, which two words describe how a diet should be?

  1. A person's height and weight.
  1. Balanced and healthy.

What is obesity?

A person is said to be obese if they have such a high excess of body fat that their health might be affected.

Health workers use body mass index (BMI) to assess obesity. They measure the height and mass of a person and then use the equation below to calculate their BMI:

BMI = mass (kg) ÷ height² (m²)

For example, if a person has a mass of 60 kg and a height of 1.65 m:

BMI = 60 kg ÷ 1.65²

BMI = 22 kg/m²

A person is overweight if their BMI is between 25 kg/m² and 30 kg/m². A person with a BMI over 30 kg/m² is said to be obese.

Therefore, the person in this example can be described as being a healthy weight.

A graph with weight in kg on the vertical axis; height in cm on the horizontal axis. The categories used are underweight, healthy weight, overweight, obese.
A graph that illustrates variations in weight

How might someone become obese?

Energy balance is the relationship between energy input (calories consumed through food and drink) and energy output (calories used by the body for our energy requirements).

This relationship dictates whether weight is lost, gained, or remains the same.

A person might become obese by eating food that supplies more energy than they need.

A 13-year-old girl needs about 9,000 kilojoules (KJ) of energy each day. Boys the same age needs around 10,000 KJ/day. Adult women need roughly 8,400 KJ/day and adult men need roughly 10,500 KJ/day.

If people consistently eat food that supplies more energy than this, without taking extra exercise, they might eventually become obese.

three sets of scales. the first shows energy consumed and expended are equal - a balanced weight. the second shows energy consumed is more than energy expended. this shows a weight gain. The third shows energy expended is greater than consumed. this shows a weight loss.

Health problems related to obesity

Obesity increases the risk of life-threatening health problems, including:

  • Heart disease
  • Stroke
  • Type 2 diabetes
  • Some types of cancer, including cancers of the breast, bowel and liver

A person can prevent obesity by eating sensible amounts of nutritious food and exercising regularly.

Test your knowledge

Quiz - Health and obesity