Obesity is generally caused by eating too much and moving too little.
If you consume high amounts of energy, particularly fat and sugars, but don't burn off the energy through exercise and physical activity, much of the surplus energy will be 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 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 take away hamburger, fries and a milkshake can total 1,500 calories and that's just one meal. For more information, read our guide to understanding calories.
Another problem is that many people aren't physically active, so lots of the calories they consume end up being stored in their body as fat.
Obesity doesn't happen overnight. It develops gradually over time, as a result of poor diet and lifestyle choices, such as:
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 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 don't 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 that adults do at least 150 minutes (two-and-a-half hours) of moderate-intensity aerobic activity, such as cycling or fast walking, every week. This doesn't need to be done all in one go, but can be broken down into smaller periods. For example, you could exercise for 30 minutes a day for five 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.
In some cases, underlying medical conditions may contribute to weight gain. These include:
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.
The term 'obese' describes a person who's very overweight, with a lot of body fat. Ways to lose weight safely include eating a healthy, reduced-calorie diet and exercising regularly.
Obesity is generally caused by eating too much and moving too little. Underlying health conditions and taking certain medications can also contribute to weight gain.
Read about how body mass index (BMI) is used to determine whether a person is overweight or obese. Lifestyle factors, such as diet and physical activity levels, are also important.