What causes obesity?

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What causes obesity?

In most people obesity is caused by eating too much and moving too little. If you consume high amounts of energy from your diet but do not burn off the energy through exercise and physical activity, the surplus energy will be turned into fat.

Our modern lifestyles are also causing more people to be obese. Changing work patterns, using technology, changes in portion size and the built environment all contribute to making us gain weight.


The energy value of food is measured in units called kilojoules. To maintain a healthy weight it is important to balance how many kilojoules you take in (from food and drink) with how many kilojoules you burn off.

How many kilojoules you need each day depends on your age, gender, body size and activity levels. An accredited practising dietitian can help you work this out.

Lifestyle choices

Obesity does not just happen overnight, it develops gradually from poor diet and lifestyle choices.

For example, unhealthy food choices could be:

  • eating processed or fast food 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
  • eating out a lot – 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
  • 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 can learn bad eating habits from your parents.

Childhood obesity can often lead to weight-related health problems in later life. The lifestyle habits you learn as a child can continue into adulthood.

Lack of exercise and physical activity

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

When they relax, people tend to watch TV, browse the internet or play computer games, and may not regularly exercise.

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

Australian adults are recommended to do at least 2 ½ to 5 hours of moderate-intensity activity (for example, cycling or fast walking) every week. This will help you maintain a healthy weight.

However, if you are overweight or obese and trying to lose weight, you may need to do more exercise – in some cases up to an hour on most days of the week may be recommended.

Medical conditions

For some people, a medical condition can cause you to gain weight. These include:

  • underactive thyroid gland (hypothyroidism)
  • Cushing's syndrome

Controlling these with medicine should limit the amount of weight you put on.

Some medicines used to control asthma, epilepsy, diabetes and depression can also cause weight gain.


Some people claim there is no point in losing weight because 'it runs in my family' or 'it is 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 taking longer to burn up kilojoules (having a slow metabolism) or having a large appetite, can make losing weight more difficult. However, it certainly does not make it impossible.

Many cases where obesity runs in families may be due to environmental factors such as poor eating habits learned during childhood.