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

Calories

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 calories a day. 

This amount of calories may sound high, but it can be easy to reach 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 1 meal. For more information, read our guide to understanding calories.

Another problem is that many people are not physically active, so lots of the calories they consume end up being stored in their 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's 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 have low self-esteem or feel depressed, you may eat to make yourself feel better

Unhealthy eating habits tend to run in families. You may learn bad eating habits from your parents when you're young and continue them into adulthood.

Read about eating less saturated fat and how sugar in our diet affects our health.

Lack of physical activity

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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.

For relaxation, many people tend to watch TV, browse the internet or play computer games, and rarely take regular exercise.

If you're 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 and Social Care recommends that adults do at least 150 minutes of moderate-intensity aerobic activity, such as cycling or fast walking, every week. This does not need to be done all in a single session, but can be broken down into smaller periods. For example, you could exercise for 30 minutes a day for 5 days a week.

If you're 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.

Genetics

Some people claim there's no point 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's 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 does not 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.

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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 sometimes be a side effect of stopping smoking.

Body mass index (BMI) is widely used as a simple and reliable way of finding out whether a person is a healthy weight for their height.

For most adults, having a BMI of 18.5 to 24.9 means you're 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.

While BMI is a useful measurement for most people, it's not accurate for everyone.

For example, the normal BMI scores may not be accurate if you're very muscular because muscle can add extra kilos, resulting in a high BMI when you're not an unhealthy weight. In such cases, your waist circumference may be a better guide.

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

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

Visiting your Doctor

If you're overweight or obese, visit your GP for advice about losing weight safely and to find out whether 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; they'll also ask you whether you smoke and how much alcohol you drink 

• any possible underlying causes for your obesity – for example, if you're taking medication or have a medical condition that may contribute to weight gain 

• how you feel about being overweight – for example, if it makes you feel 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 also carry out tests to determine whether you're at increased risk of developing health complications because of your weight.

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These could include measuring your:

• blood pressure 

• glucose (sugar) and cholesterol levels in a blood sample 

• 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 because it can affect your risk of developing certain conditions. For example, some people of Asian, African or African- 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.

After your assessment, you'll be offered an appointment to discuss the results in more detail, ask any questions that you have, and fully explore the treatment options available to you.

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Other useful strategies

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

• setting realistic weight loss goals – if you're obese, losing just 3% of your original body weight can significantly reduce your risk of developing obesity-related complications 

• eating more slowly and being mindful of what and when you're eating – for example, not being distracted by watching TV

• avoiding situations where you know you may be tempted to overeat

• involving your family and friends with your weight loss efforts – they can help to motivate you

• monitoring your progress – for example, weigh yourself regularly and make a note of your weight in a diary 

Getting psychological support from a trained healthcare professional may also help you change the way you think about food and eating. Techniques such as cognitive behavioural therapy (CBT) can be useful.

Avoiding weight regain

It's important to remember that as you lose weight your body needs less food (calories), so after a few months, weight loss slows and levels off, even if you continue to follow a diet.

If you go back to your previous calorie intake once you've lost weight, it's very likely you'll put the weight back on. Increasing physical activity to up to 60 minutes a day and continuing to watch what you eat may help you keep the weight off.

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Surgery

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

Bariatric surgery is usually only available on the NHS to treat people with severe obesity who fulfil all of the following criteria:

• they have a BMI of 40 or more, or between 35 and 40 and another serious health condition that could be improved with weight loss, such as type 2 diabetes or high blood pressure

• all appropriate non-surgical measures have been tried, but the person hasn't achieved or maintained adequate, clinically beneficial weight loss

• the person is fit enough to have anaesthesia and surgery 

• the person has been receiving, or will receive, intensive management as part of their treatment

• the person commits to the need for long-term follow-up 

Bariatric surgery may also be considered as a possible treatment option for people with a BMI of 30 to 35 who have recently (in the last 10 years) been diagnosed with type 2 diabetes.

In rare cases, surgery may be recommended as the first treatment (instead of lifestyle treatments and medication) if a person's BMI is 50 or above.

Treating obesity in children

Treating obesity in children usually involves improvements to diet and increasing physical activity using behaviour change strategies.

The amount of calories your child should eat each day will depend on their age and height. Your GP should be able to advise you about a recommended daily limit, and they may also be able to refer you to your local family healthy lifestyle programme.

Children over the age of 5 should ideally get at least 60 minutes of vigorous-intensity exercise a day, such as running or playing football or netball. Sedentary activities, such as watching television and playing computer games, should be restricted.

Read more about the physical activity guidelines for children and young people. 

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

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

Bariatric surgery isn't generally recommended for children, but may be considered for young people in exceptional circumstances, and if they've achieved, or nearly achieved, physiological maturity.