Obesity is a term used to describe somebody who is very overweight, with a lot of body fat.

It's a common problem, estimated to affect around one in every four adults and around one in every five children aged 10 to 11 in the UK.

Defining obesity

There are many ways in which a person's health in relation to their weight can be classified, but the most widely used method is body mass index (BMI).

BMI is a measure of whether you're a healthy weight for your height. You can use the BMI healthy weight calculator to work out your score.

For most adults:

• a BMI of 25 to 29.9 means you are considered overweight
• a BMI of 30 to 39.9 means you are considered obese
• a BMI of 40 or above means you are considered severely obese

BMI is not used to definitively diagnose obesity – as people who are very muscular sometimes have a high BMI, without excess fat – but for most people, it can be a useful indication of whether they may be overweight.

A better measure of excess fat is waist circumference, and can be used as an additional measure in people who are overweight (with a BMI of 25 to 29.9) or moderately obese (with a BMI of 30 to 34.9).

Generally, men with a waist circumference of 94cm or more and women with a waist circumference of 80cm or more are more likely to develop obesity-related health problems.

Read more about diagnosing obesity.

Risks of obesity

Taking steps to tackle obesity is important because, in addition to causing obvious physical changes, it can lead to a number of serious and potentially life-threatening conditions, such as:

• type 2 diabetes 
• coronary heart disease 
• some types of cancer, such as breast cancer and bowel cancer
• stroke

Obesity can also affect your quality of life and lead to psychological problems, such as low self-esteem or depression.

Read more about the complications of obesity.

Causes of obesity

Obesity is generally caused by consuming more calories – particularly those in fatty and sugary foods – than you burn off through physical activity. The excess energy is then stored by the body as fat.

Obesity is an increasingly common problem, because many modern lifestyles often promote eating excessive amounts of cheap, high-calorie food and spending a lot of time sitting at desks, on sofas or in cars.

There are also some underlying health conditions that can occasionally contribute to weight gain, such as an underactive thyroid gland (hypothyroidism), although conditions such as this don’t usually cause weight problems if they are effectively controlled with medication.

Read more about the causes of obesity.

Treating obesity

The best way to treat obesity is to eat a healthy, reduced-calorie diet and to exercise regularly. To do this you should:

• eat a balanced, calorie-controlled diet as recommended by your GP or weight loss management health professional (such as a dietitian)
• join a local weight loss group
• take up activities such as fast walking, jogging, swimming or tennis for 150-300 minutes a week
• eat slowly and avoid situations where you know you could be tempted to overeat

You may also benefit from psychological support from a trained healthcare professional, to help change the way you think about food and eating.

If lifestyle changes alone don't help you lose weight, a medication called orlistat may be recommended. If taken correctly, this medication works by reducing the amount of the fat you absorb during digestion. Your GP will know whether orlistat is suitable for you.

In rare cases, weight loss surgery may be recommended.

Read more about how obesity is treated.

Outlook

There is no "quick fix" for obesity. Weight loss programmes take time and commitment, but they work best when people are able to complete the programmes fully and are offered advice about maintaining the weight loss achieved.

Regularly monitoring your weight, setting realistic goals and involving your friends and family with your attempts to lose weight can also help.

Remember that even losing what seems like a small amount of weight (such as 3% or more of your original body weight), and maintaining this for life, can significantly reduce your risk of obesity-related complications like diabetes and heart disease.