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 Obesity is excess body fat for a given height and gender. It happens when more calories are taken into the body than are burnt up in a given period of time. Once adulthood is reached, everyone has a fairly steady rate of calorie burn-up, called the metabolic rate. This is higher in people who are regularly physically active. This means that someone who works in a very physical job, such as a building-site labourer, may need as many as 4000–5000 calories per day to keep an even weight. This contrasts with an office worker who uses a car and doesn’t exercise, who may need only 1500 calories per day.

If calories in food energy are greater than the calories used every day, the excess energy is stored by the body as fat. This is important as a protection for the body against times of starvation. In developed countries, starvation is rare except in extreme circumstances, so this insurance against hard times is hardly ever needed. Food is plentiful, and a lot of available food is much higher in calories than the human body was originally designed to cope with. The result is that eating more than the body needs is easy. Obesity has become one of the most serious medical problems of the western world.

Obesity can be measured in different ways:

  • An easy way is just to get on the scales and compare your actual weight with your ideal weight. Any calorie-counting book will give this information.
  • A more scientific way is to calculate your Body Mass Index (BMI).   This is your weight in kilograms divided by the square of your height in metres. In England, people with a body mass index between 25 and 30 are categorised as overweight, and those with an index above 30 are categorised as obese. 
  • Modern gyms and some weighing scales can electronically measure the percentage of your body weight that is fat, and can compare this with what would be ideal for you.

Careful research based on measurements over a period of 14 years in more than a million people has shown that the risk of death from all causes increases steadily as obesity increases in both women and men of all ages.  Men with the highest BMI have three times the risk of dying in a particular year compared with men having a normal BMI.

A report from the World Health Organisation published in April 2001 indicates that obesity and lack of exercise contribute to up to one-third of all cancers of the colon, breast, kidney and stomach. Obesity-related deaths are now second only to those related to smoking. Half of European adults are overweight.

Obesity is not just an adult problem. Recent studies have shown that over the period 1989–98 there was a rapid spread of obesity in England, the United States and elsewhere in the developed world.

Among 3-to-4-year-old English children there was a 60 per cent increase in the prevalence of obesity.  Early childhood obesity is a strong predictor of adult obesity and of serious health risks later in life.