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

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:

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.


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.

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Obesity is generally caused by eating too much and moving too little.

Obesity is generally caused by eating too much and moving too little.

If you consume high amounts of energy from your diet, particularly from fat and sugars, but do not burn off the energy through exercise and physical activity, much of the surplus energy is then stored by the body as fat.


The energy value of food is measured in units called calories. The average physically active man needs about 2,500 calories a day to maintain a healthy weight, and the average physically active woman needs about 2,000 a day.

This may sound high, but it can be easy to reach this limit if you eat certain types of food. For example, eating a large takeaway hamburger, fries and a milkshake can total 1,500 calories – and that's just one meal.

Another problem is that many people are not physically active, so lots of the calories they consume end up being stored in the body as fat.

Poor diet

Obesity does not happen overnight. It develops gradually over time, as a result of poor diet and lifestyle choices, such as:

  • eating large amounts of processed or fast food that is high in fat and sugar
  • drinking too much alcohol – alcohol contains a lot of calories, and people who drink heavily are often overweight
  • eating out a lot – you may be tempted to also have a starter or dessert in a restaurant, and the food can be higher in fat and sugar
  • eating larger portions than you need – you may be encouraged to eat too much if your friends or relatives are also eating large portions
  • drinking too many sugary drinks – including soft drinks and fruit juice
  • comfort eating – if you feel depressed or have low self-esteem, you may eat to make yourself feel better

Unhealthy eating habits tend to run in families, as you learn bad eating habits from your parents when you are young and continue them into adulthood.

Lack of physical activity

Lack of physical activity is another important factor related to obesity. Many people have jobs that involve sitting at a desk for most of the day. They also rely on their cars, rather than walking or cycling.

When people relax, they tend to watch TV, browse the internet or play computer games, and rarely take regular exercise.

If you are not active enough, you do not use the energy provided by the food you eat, and the extra energy you consume is stored by the body as fat.

The Department of Health recommends adults do at least 150 minutes of moderate-intensity aerobic activity, such as cycling or fast walking, every week. This doesn’t need to be done in one go, but can be broken into smaller periods. For example, you could exercise for 30 minutes a day for five days.

If you are obese and trying to lose weight, you may need to do more exercise than this. It may help to start off slowly and gradually increase the amount of exercise you do each week.

Read more about the physical activity guidelines for adults.


Some people claim there is no point in trying to lose weight because "it runs in my family" or "it's in my genes".

While there are some rare genetic conditions that can cause obesity, such as Prader-Willi syndrome, there is no reason why most people cannot lose weight.

It may be true that certain genetic traits inherited from your parents – such as having a large appetite – may make losing weight more difficult, but it certainly doesn't make it impossible.

In many cases, obesity is more to do with environmental factors, such as poor eating habits learned during childhood.

Medical reasons

In some cases, underlying medical conditions may contribute to weight gain. These include:

  • an underactive thyroid gland (hypothyroidism) – where your thyroid gland does not produce enough hormones
  • Cushing's syndrome – a rare disorder that causes the over-production of steroid hormones

However, if conditions such as these are properly diagnosed and treated they should pose less of a barrier to weight loss.

Certain medicines, including some corticosteroids, medications for epilepsy and diabetes, and some medications used to treat mental illness – including antidepressants and medicines for schizophrenia – can contribute to weight gain. Weight gain can also sometimes be a side effect of stopping smoking.

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Body mass index (BMI) is widely used as a simple and reliable way of finding out if you are overweight or obese.

Body mass index (BMI) is widely used as a simple and reliable way of finding out if you are overweight or obese.

BMI measures whether you're a healthy weight for your height.

For most adults, having a BMI of 18.5 to 24.9 means you are considered to be a healthy weight. A person with a BMI of 25 to 29.9 is considered to be overweight, and someone with a BMI over 30 is considered to be obese.

However, while BMI is a useful measurement for most people, it is not accurate for everyone.

For example, the normal BMI scores may not be accurate if you are very muscular, because muscle can add extra pounds and may mean you have a high BMI when you are not an unhealthy weight. In such cases, it may be more helpful to use your waist circumference as a better guide (see below).

What is considered a healthy BMI is also influenced by your ethnic background. The scores mentioned above generally apply to people with a white Caucasian background. If you are from an ethnic minority background, the threshold for being considered overweight or obese may be lower.

BMI should not be used to work out if a child is a healthy weight, as their bodies are still developing. If you want to find out whether your child is overweight, speak to your GP.

Visiting your GP

If you are overweight or obese, visit your GP for advice about losing weight safely and to find out if you have an increased risk of health problems.

Your GP may ask about:

  • your lifestyle  particularly your diet and how much physical activity you do, and also whether you smoke and how much alcohol you drink
  • any possible underlying causes for your obesity  for example, if you are taking medication or have a medical condition that may contribute to weight gain
  • how you feel about being overweight – for example, if you are feeling depressed
  • how motivated you are to lose weight
  • your family history  as obesity and other health conditions, such as diabetes, are often more common in families

As well as calculating your BMI, your GP may perform tests to determine if you are at increased risk of health complications because of your weight. These could include:

  • measuring your blood pressure
  • measuring your glucose (sugar) and cholesterol levels in a blood sample
  • measuring your waist circumference (the distance around your waist)

People with very large waists (generally, 94cm or more in men and 80cm or more in women) are more likely to develop obesity-related health problems.

Your GP may also take your ethnicity into account, as these can affect your risk of certain conditions. For example, some people of Asian, African or Afro-Caribbean ethnicity may be at increased risk of high blood pressure (hypertension). Healthy waist measurements can also be different for people from different ethnic backgrounds.

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If you are obese and you want to lose weight, speak to your GP for advice about losing weight safely.

If you are obese, speak to your GP for advice about losing weight safely.

Your GP can advise you on the type of diet you should be following and how you can much exercise you should do.

They can also let you know about other services, such as:

  • local weight loss groups  these could be provided by the NHS or may be commercial services you have to pay for
  • exercise on prescription  where you are referred to a local active health team for a number of sessions under the supervision of a qualified trainer

If you have underlying problems associated with obesity, such as polycystic ovary syndrome (PCOS)high blood pressurediabetes or obstructive sleep apnoea, your GP may recommend further tests or specific treatment. In some cases, they may refer you to a specialist.

Read more about how your GP can help you lose weight.


While there is no single rule that applies to everybody, most obese people are advised to reduce the energy intake from their diet by 600 calories a day.

The best way to achieve this is to swap unhealthy and high-energy food choices – such as fast food, processed food and sugary drinks (including alcohol) – for healthier choices.

A healthy diet should consist of:

  • plenty of fruit and vegetables
  • plenty of potatoes, bread, rice, pasta and other starchy foods (ideally you should choose wholegrain varieties)
  • some milk and dairy foods
  • some meat, fish, eggsbeans and other non-dairy sources of protein
  • just small amounts of food and drinks that are high in fat and sugar

Try to avoid foods containing high levels of salt as these can raise your blood pressure, which can be dangerous in people who are already obese. Read some tips for a lower-salt diet.

You will also need to check calorie information for each type of food and drink you consume, to make sure you do not go over your daily limit.

Some restaurants, cafés and fast food outlets provide calorie information per portion, but providing this information is not compulsory. Be careful when eating out, as some foods can quickly take you over the limit, such as burgers, fried chicken and some curries or Chinese dishes.

Read more about calorie counting.

Diet programmes and fad diets

You should avoid fad diets that recommend unsafe practices, such as fasting (going without food for long periods of time) or cutting out entire food groups. These can make you feel ill and are not sustainable, because they don’t teach you long-term healthy eating habits.

This is not to say that all commercial diet programmes are unsafe. Many are based on sound medical and scientific principles and can work well for some people. A responsible diet programme should:


  • educate you about issues such as portion size, making changes to behaviour and healthy eating 
  • not be overly restrictive in terms of the type of foods you can eat 
  • be based on achieving gradual sustainable weight loss rather than short-term rapid weight loss, which is unlikely to last


On average, people attending a lifestyle weight management programme lose around 3% of their body weight, although this varies considerably.

Read about the pros and cons of different diets.

Very low calorie diets

very low calorie diet (VLCD) is where you consume less than 1,000 calories a day.

These diets can lead to rapid weight loss, but they are not a suitable or safe method for everyone. VLCDs are usually only recommended if you have an obesity-related complication that would benefit from rapid weight loss, such as severe sleep apnoea.

VLCDs should usually not be followed for any longer than 12 weeks at a time, and they should only be adopted under the supervision of a suitably qualified healthcare professional.

Further information

For more information on diet and weight loss read about:


Reducing the amount of calories in your diet will help you lose weight, but if you want to keep off the weight, you have to combine a calorie-controlled diet with regular exercise.

Your GP or weight loss adviser can provide an exercise plan suited to your circumstances, which will probably involve several hours of moderate-intensity physical activity a week.

Moderate-intensity physical activity is any activity that increases your heart and breathing rate, and may make you sweat, but still allows you to hold a normal conversation. Examples include:

  • fast walking
  • jogging
  • swimming
  • tennis
  • using a step-trainer (or similar) at the gym

Choose physical activities that you enjoy, as you are more likely to continue doing them.

You should aim to start gradually. For example, start off by doing 15 to 20 minutes of exercise five times a week and then build on it.

The Department of Health recommends most adults should do at least 150 minutes of moderate-intensity exercise every week. However, if you are obese and trying to lose weight, or have successfully lost weight and want to keep it off, you probably need to do more exercise. In most cases, up to five hours a week is recommended.

Further information

For more information on exercise, see:

Other useful strategies

Evidence has shown that weight loss can be more successful if it involves other strategies, in addition to diet and lifestyle changes. This could include things like:

  • setting realistic weight loss goals  even losing just 3% of your original body weight can significantly reduce your risk of obesity-related complications
  • eating more slowly and being mindful of what and when you are eating (for example, not being distracted by watching TV)
  • avoiding situations where you know you may be tempted to overeat
  • involving your friends and family with your weight loss efforts, as they can motivate you
  • monitoring your progress  for example, weigh yourself regularly and note your weight in a diary

You may also find psychological support from a trained healthcare professional helps you change the way you think about food and eating, through techniques such as cognitive behavioural therapy (CBT).


Many different types of anti-obesity medication have been tested in clinical trials, but only one has proved to be both safe and effective: orlistat.

Orlistat works by preventing around a third of the fat from the food you eat from being digested. This undigested fat is not absorbed into your body and is passed out with your faeces (stools).

This will help you avoid gaining weight, but will not necessarily cause you to lose weight. Therefore, it's still important to stick to your recommended diet and exercise plan.

When medication is used

Orlistat is usually only recommended if you have made a significant effort to lose weight through diet, exercise or changing your lifestyle.

Even then, orlistat is only prescribed if you have:

  • a body mass index (BMI) of 28 or more and other conditions related to weight, such as high blood pressure
  • a BMI of 30 or more

Treatment with orlistat must be combined with a low-fat diet and other weight loss strategies, such as doing more exercise. If you are prescribed orlistat, you will also be offered advice and support about diet, exercise and making lifestyle changes.

Orlistat is not usually recommended for pregnant or breastfeeding women.

Dosage and duration of treatment

One orlistat capsule is taken with each main meal (up to a maximum of three capsules a day). You can take the capsule either before, during or up to one hour after each meal.

If you miss a meal or the meal does not contain any fat, you may not need to take the orlistat capsule. Your GP should explain this to you, or you can check the patient information leaflet that comes with your medication.

Treatment with orlistat should only continue beyond three months if you have lost 5% of your body weight. It usually starts to affect how you digest fat within one to two days. If orlistat has not worked after three months, it is unlikely to be an effective treatment for you.

If you have type 2 diabetes, it may take you longer to lose weight using orlistat, so your target weight loss after three months may therefore be slightly lower.

If orlistat is successful after three months, your prescription may be continued for up to a year. After that, your GP will review your condition and decide whether you should continue with orlistat.

Side effects

Common side effects of orlistat include:

These side effects are much less likely to occur if you stick to a low-fat diet.

Women taking the oral contraceptive pill are advised to use an additional method of contraception, such as a condom, if they experience severe diarrhoea while taking orlistat. This is because the contraceptive pill may not be absorbed by your body if you have diarrhoea, so it may not be effective.


Weight loss surgery, also called bariatric surgery, is sometimes used to treat people who are severely obese.

This type of surgery is usually only available on the NHS to treat people with severe obesity that have not responded to other measures.

Severe obesity is defined as:

  • having a body mass index (BMI) of 40 or above
  • having a BMI of 35 or above and having another serious health condition that could be improved if you lose weight, such as type 2 diabetes, obstructive sleep apnoea or high blood pressure

In rare cases, surgery may be recommended as the first treatment if your BMI is 50 or above.

Read more about weight loss surgery.

Treating obesity in children

Treating obesity in children is based on much the same principles as treating obesity in adults  by combining a calorie-controlled diet with regular exercise.

The amount of calories your child should eat every day will depend on their age and height. Your GP should advise a recommended daily limit.

Children should ideally get at least one hour of moderately-intense exercise a day, such as running, playing football or netball.

You should restrict sedentary activities, such as watching television or playing computer games, to less than two hours a day (14 hours a week).

Referral to a specialist in treating childhood obesity may be recommended if your child develops an obesity-related complication, or there is thought to be an underlying medical condition causing the obesity.

The use of orlistat in children is only recommended in exceptional circumstances – for example, if a child is severely obese and has an obesity-related complication.

Further information:

For more information on diet and exercise in children, see:

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

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

Some of the day-to-day problems that can be caused by obesity include:

  • breathlessness
  • increased sweating
  • snoring
  • difficulty doing physical activity
  • feeling very tired a lot of the time
  • joint and back pain
  • low confidence and self-esteem
  • feeling isolated

You may also find that some of the psychological problems associated with being obese affect your relationships with family members and friends, and may lead to depression.

Further health problems

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

Obesity reduces life expectancy by an average of 3 to 10 years, depending on how severe the problem is. It's estimated that being overweight or obese contributes to at least 1 in every 13 deaths in Europe.

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Husband and wife Brendan and Debbie Byrne lost a combined total of 9st 6lb by binning takeaways in favour of a healthy diet.

Husband and wife Brendan and Debbie Byrne lost a combined total of 9st 6lb by binning takeaways in favour of a healthy diet.

The Byrne's family diet used to comprise Chinese takeaways, kebabs, pizzas and little else. Debbie and Brendan, who live in Essex with their children, knew it wasn't ideal.

"When you both work full-time, it's hard to find time to cook," she says. "Brendan and I had struggled with our weight for years. I could see that the children were gaining weight, too."

By early 2006, Debbie and Brendan, who both work for the police force, each weighed 16st 4lb. On a visit to his GP, Brendan was told that his blood pressure was too high. Meanwhile, Debbie's self-esteem was at an all-time low because of her weight, and she was on medication for depression.

"When we heard about Brendan's blood pressure, we knew we really had to act," she says. "Our eldest daughter, Rhiannon, is 21. She was 12st 5lb, around 21lb overweight."

Their eldest son Liam and youngest child Ciaran, 13, could also have benefited from losing few pounds, says Debbie. "I didn't want the children to grow up facing the same health problems as us."

When a friend recommended local slimming classes, Debbie immediately booked a slot for herself and Brendan. "It was quite daunting at first," says Debbie. "But we soon both found the classes an enormous support."

A consultant provided advice on diet and exercise at the weekly meetings, and group members were encouraged to share their experiences.

"At home, I started cooking with less fat," says Debbie. "I changed from white bread to wholemeal, and we started eating lots more fruit."

A food system devised by the slimming club consisted of a diet full of fresh fruit, vegetables, fish and lean meat. Dieters are also allowed occasional treats, small indulgences for which you plan ahead. By August 2006, Debbie had lost three stone and Brendan four.

"There's flexibility in the diet, and that was important for the children," says Debbie. "They could see the good it was doing us."

Rhiannon, Liam and Ciaran had also started to lose weight. Along with weight loss came an energy boost. The family started running together, and Debbie even joined a gym.

By May 2007, Debbie had reached her target weight of 10st 2lb and had dropped five dress sizes, from a 20 to a 10. Brendan had also reached his target of 12st 10lb. Daughter Rhiannon had lost 21lb, reaching 11st, and Ciaran and Niall had also lost pounds.

"It's so good to know that the children's weight is under control," says Debbie. "Weight loss has also done wonders for my self-esteem, and I'm no longer reliant on antidepressants. Brendan's blood pressure has dropped to a normal level, too.

"Life is so much better at a healthy weight. You feel great, and people see you differently. I only wish I'd done this years ago."

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