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

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 bette

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.

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.

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.

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.

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.