The term ‘obese’ describes a person who’s very overweight, with a lot of body fat.

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

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 chart to work out your score.

For most adults, a BMI of:

• 18.5 to 24.9 means you’re a healthy weight
• 25 to 29.9 means you’re overweight
• 30 to 39.9 means you’re obese
• 40 or above means you’re severely obese

BMI isn’t used to definitively diagnose obesity, because people who are very muscular sometimes have a high BMI without excess fat. But for most people, BMI is a useful indication of whether they’re a healthy weight, overweight or obese.

A better measure of excess fat is waist circumference, which 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 (37in) or more and women with a waist circumference of 80cm (about 31.5in) or more are more likely to develop obesity-related health problems.

Read more about diagnosing obesity

Risks of obesity

It’s very important to take steps to tackle obesity because, as well as 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 depression and low self-esteem (see below for more information about the health problems associated with 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 stored by the body as fat.

Obesity is an increasingly common problem because for many people modern living involves eating excessive amounts of cheap, high-calorie food and spending a lot of time sitting down, 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 these type of conditions don’t usually cause weight problems if they’re 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 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 to 300 minutes (two-and-a-half to five hours) a week
• eat slowly and avoid situations where you know you could be tempted to overeat

You may also benefit from receiving 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 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

Other obesity-related problems

Obesity can cause a number of further problems, including difficulties with daily activities and serious health conditions.

Day-to-day problems related to obesity include:

• breathlessness
• increased sweating
• snoring
• difficulty doing physical activity
• often feeling very tired
• joint and back pain
• low confidence and self-esteem
• feeling isolated

The psychological problems associated with being obese can also affect your relationships with family and friends, and may lead to depression.

Serious health conditions

Being obese can also increase your risk of developing many potentially serious health conditions, including:

• type 2 diabetes – a condition that causes a person’s blood sugar level to become too high
• high blood pressure
• high cholesterol and atherosclerosis (where fatty deposits narrow your arteries), which can lead to coronary heart disease and stroke
• asthma
• metabolic syndrome – a combination of diabetes, high blood pressure and obesity
• several types of cancer, including bowel cancer, breast cancer and womb cancer
• gastro-oesophageal reflux disease (GORD) – where stomach acid leaks out of the stomach and into the oesophagus (gullet)
• gallstones – small stones, usually made of cholesterol, that form in the gallbladder
• reduced fertility
• osteoarthritis – a condition involving pain and stiffness in your joints
• sleep apnoea – a condition that causes interrupted breathing during sleep, which can lead to daytime sleepiness with an increased risk of road traffic accidents, as well as a greater risk of diabetes, high blood pressure and heart disease
• liver disease and kidney disease
• pregnancy complications, such as gestational diabetes or pre-eclampsia (when a woman experiences a potentially dangerous rise in blood pressure during pregnancy)

Obesity reduces life expectancy by an average of 3 to 10 years, depending on how severe it is. It’s estimated that obesity and being overweight contribute to at least 1 in every 13 deaths in Europe.

Outlook

There’s no “quick fix” for obesity. Weight loss programmes take time and commitment, and work best when fully completed. The healthcare professionals involved with your care should provide encouragement and advice about how to maintain 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 developing obesity-related complications like diabetes and heart disease.