Obesity is when a person is carrying too much body fat for their height and sex. A person is considered obese if they have a Body Mass Index (BMI) of 30 or greater. BMI is a measurement of your weight in kilograms divided by your height in metres squared. Obesity can happen when you eat more calories than you burn off over a period of time.

The rate at which you burn off calories from food and drink is known as your metabolic rate. This is often faster during growth spurts and puberty, but reaches a fairly steady rate by adulthood.

People who are very active generally have a higher metabolic rate than those who are inactive because they burn off calories faster through energetic activity. For example, a labourer working on a building site may need as many as 4,000-5,000 calories a day to keep an even weight. In contrast, an office worker who uses a car to get to work and does not exercise may only need 1,500 calories a day.

If the amount of calories provided by your daily food intake is more than the calories that you burn off, your body will store the extra energy as fat. This is the body's way of protecting itself in case of starvation. However, starvation in developed countries is extremely rare, and this insurance against "hard times" is hardly ever needed.

Most of us have more food than we need, and much of it is higher in calories than the human body was originally designed to cope with. Fast foods, high calorie snacks and large portions all mean it is easy to take in more energy than we need. Obesity has now become one of the most serious medical problems of the Western world.