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 (see box, to the left).

Today's way of life is less physically active than it used to be. People travel on buses and cars, rather than walking, and many people work in offices, where they are sitting still for most of the day. This means that the calories they eat are not getting burnt off as energy. Instead, the extra calories are stored as fat.

Over time, eating excess calories leads to weight gain. Without lifestyle changes to increase the amount of physical activity done on a daily basis, or reduce the amount of calories consumed, people can become obese.

How common is obesity?

In 2008, the latest year with available figures, nearly a quarter of adults (over 16 years of age) in England were obese (had a BMI over 30). Just under a third of women, 32%, were overweight (a BMI of 25-30), and 42% of men were overweight.

Amongst children (2-15 years of age), one in six boys and one in seven girls in England were obese in 2008. The number of overweight children was also around one in seven. 

The number of overweight and obese people is likely to increase. The Foresight report, a scientific report used to guide government policy, has predicted that by 2025, nearly half of men and over a third of women will be obese. 

Outlook

Obesity can cause a number of health problems, such as type 2 diabetes (a condition caused by too much glucose in the blood), and heart disease (when the heart's blood supply is blocked).

Being overweight or obese can also shorten life expectancy (how long a person should live). In obese adults over 40 years of age, obesity can shorten life expectancy by 6-7 years. 

Obesity is treated by losing weight, which can be achieved through a healthy, calorie-controlled diet and increased exercise. The lifestyle changes necessary for long-term weight loss can be challenging to achieve, but there is a wide range of support available. Further advice can be found in the Live Well section, and there are a number of support groups (see useful links, right).

Surgery can be used to treat people who are severely obese and have tried other methods of weight loss with no success. There are a variety of techniques, although these do carry risks and may not be suitable for everyone.