Obesity: what it is, how it is measured, causes and treatment

The NHS describes overweight and obesity as having more body fat than your body needs, and warns that this can lead to serious health problems. NHS inform adds that obesity is a chronic condition, and that it is a complex condition with more than one cause. Both stress that there are lifestyle changes and treatments that can help.

NICE's guidance for the public asks for obesity to be discussed sensitively, with non-judgemental language, so that weight does not stop people getting help for other health conditions. It is a health condition, not a personal failing, and the reasons behind it are often a mix of biology, environment and circumstances.

The NHS and NHS inform both describe body mass index (BMI) as the usual starting point. It compares weight with height. The NHS says the BMI calculator cannot tell the difference between muscle and fat, so someone with a lot of muscle could be placed in the overweight or obese category without having much body fat.

For people with a BMI under 35, the NHS says a waist to height ratio can help show whether you are carrying too much fat around your tummy, and that you should try to keep your waist size to less than half your height.

NHS inform says diagnosing obesity in children and young people under 18 is slightly different: BMI measurements alone are not suitable and it is difficult to measure at home, so a healthcare professional may want to do other tests and check overall health. The NHS says it can be difficult to tell if a child is overweight because they are still growing, and that BMI for children is given as a centile (percentile) using their age, height and weight.

The NHS says children become overweight when they get more energy from food and drink than they use through activity, and that overweight children are more likely to become overweight adults. NHS inform says treatment for children includes making their diet healthier and making sure they get more exercise, with a GP able to advise on calories and refer to a local family healthy lifestyle programme. A specialist may be involved if there is an obesity-related complication or an underlying condition causing it.

• Get active every day, with an activity they enjoy, such as sports, playground activities or walking to school.

• Aim for a healthy, balanced diet that includes fruit and vegetables, starchy foods, protein and dairy or dairy alternatives.

• Set realistic targets and involve your child in deciding what changes to make, using rewards such as stickers or high fives.

• Help them get enough sleep with a regular bedtime routine and no digital screens in the bedroom at night.

• Do not give adult-sized portions, and avoid drinks high in sugar or fat, such as fizzy drinks.

Obesity is a measurement of body fat rather than a set of symptoms, but the NHS says living with obesity can cause day-to-day problems. It also says living with obesity is likely to lower your life expectancy by between 2 and 10 years.

• Breathlessness.

• Difficulty moving around.

• Feeling very tired (fatigue).

• Joint and back pain.

• Snoring.

• Mental health problems, such as feeling very low (depression).

NHS inform says the most important ways of treating obesity are to eat a healthy, reduced-calorie diet and to exercise regularly. The NHS gives the practical version: eat and drink fewer calories than your body uses, and do more physical activity. It adds that it can be hard to lose weight, and that you may need extra support or treatment to lose weight and keep it off.

A GP can give advice on losing weight safely and refer you for treatment. The options form a ladder, and NICE says the NHS can offer advice and support with eating and activity, medicines and surgery, and mental health and other support.

The NHS says you put on weight when your body turns extra calories from food and drink into fat, often because of not doing enough physical activity or eating and drinking more than your body needs. NHS inform makes the point that obesity is more complex than that, describing an environment that makes healthy food choices and physical activity hard, and listing the cost of healthy food, the cost of exercise, time pressures and where we live.

The NHS lists things that can make it harder to stay a healthy weight. Speak to a GP for advice if you have a condition that makes it harder.

• An eating disorder or other mental health condition, such as depression.

• A health condition or disability that makes it difficult to be physically active.

• A learning disability.

• A health condition that causes weight gain, such as polyendocrine metabolic ovarian syndrome (PMOS), an underactive thyroid gland or Cushing's syndrome.

• Certain medicines, including some steroids, antidepressants and beta blockers.

• Genes that make you more likely to gain weight.

• Being over 55, or menopausal or perimenopausal.

The NHS lists the serious health conditions that overweight and obesity increase the risk of. NHS inform adds obstructive sleep apnoea, polycystic ovary syndrome (PCOS) and infertility, depression and low self-esteem, and says people with obesity may be more likely to also have eating disorders such as binge eating disorder.

The NHS stresses that you can reduce your risk of many serious health conditions by losing weight, and that even a small amount, 5 to 10% of your body weight, can improve your health.

• Type 2 diabetes.

• Heart disease and stroke.

• High blood pressure.

• Some cancers, such as breast cancer, womb cancer and bowel cancer.

• Osteoarthritis.

• Fatty liver disease.

The NHS says to see a GP if your BMI is in the obese or overweight category and any of the following apply. It also says self-referral to some local weight loss services is possible, by checking your GP surgery website, asking the reception staff or checking your integrated care board website.

• You want help to lose weight.

• Your waist size is more than half your height.

• You have pre-diabetes, diabetes, high blood pressure, joint pain, high cholesterol, sleep apnoea or cardiovascular disease.

• You have a family history of obesity.

• You have, or think you may have, an eating disorder, such as binge eating.

On the NHS, a GP assesses weight and related conditions and can refer you to a weight management programme, a specialist weight management service, medicines or surgery. NHS inform says your GP may recommend your local health board's weight management programme, and that medicines proven to be safe and effective and used by the NHS include semaglutide, liraglutide, tirzepatide and orlistat.

Some people choose to see a private doctor or a commercial weight management service. The tests, medicines and surgery described here are the same, but access, cost and follow-up differ, so ask what monitoring is included and share your records with your GP. The NHS says that if you take weight loss medicines you should be offered support to help you stay a healthy weight after you stop.