Obesity does not just happen overnight, it develops gradually from poor diet and lifestyle choices and, to some extent, from your genes (the units of genetic material inherited from your parents).

Lifestyle choices

Lifestyle choices are an important factor in influencing your weight. Eating more calories than you need may be down to unhealthy food choices. For example, unhealthy food choices could be:

• eating processed or fast food that is high in fat,
• not eating fruit, vegetables and unrefined carbohydrates, such as wholemeal bread and brown rice,
• drinking too much alcohol - alcohol contains a lot of calories, and heavy drinkers are often overweight, and
• eating out a lot - as you may 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, and
• comfort eating - if you feel depressed or have low self-esteem you may comfort eat to make yourself feel better.

Unhealthy eating habits tend to run in families, as you learn bad eating habits from your parents. Childhood obesity can be a strong indicator of weight-related health problems in later life, showing that learned unhealthy lifestyle choices continue into adulthood.

Lack of physical activity

Lack of physical activity is another important factor that is related to obesity. Many people have jobs that involve sitting at a desk for most of the day, and rely heavily on their cars to get around. When it is time to relax, people tend to watch TV, or play computer games, and rarely take any regular exercise.

If you are not active enough, you do not use up the energy provided by the food you eat, and the extra calories are stored as fat instead.

The Department of Health recommends the following amount of exercise:

• adults - at least 30 minutes a day on five or more days of the week, and
• children and young people - at least 60 minutes every day.

See Live Well for more information on how much activity you should be doing.

Genes

Some people tend to stay the same weight for years without much effort, whereas others find they put on weight quickly if they are not careful about what they eat. This could be due, in part, to your genes.

Some genetic conditions can increase your appetite, so you end up eating too much. There are also genes that determine how much fat your body stores. A particular genetic variation could mean that your body is more likely to store fat than somebody else. 

Medical reasons

Medical conditions that can cause weight gain include:

• Cushing's syndrome - a rare disorder that causes an over-production of steroid hormones (chemicals produced by the body),
• an under-active thyroid gland (hypothyroidism) - when your thyroid gland does not produce enough thyroid hormone (called thyroxine, or T4), and
• polycystic ovary syndrome (PCOS) - when women have a large number of cysts in their ovaries.

Certain medicines, including some corticosteroids and antidepressants, can also contribute to weight gain. Weight gain can also be a side effect of taking the combined contraceptive pill, and from quitting smoking.

Glossary

• Obesity: Obesity is when a person has an abnormally high amount of body fat.
• Genes: Genes contain information that you inherit from your parents, such as eye or hair colour. They are carried by chromosomes.
• Thyroid: The thyroid is a jointed piece or cartilage that enclosed the vocal cords and forms the Adam's apple in men.
• Antidepressants: Antidepressant medicine is used to treat depression. For example Fluoxetine, Paroxetine.