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The causes of obesity - genes, behaviour and the environment

Find out how genes, the evnironment and behaviour can all lead to obesity. Simply put, obesity is the result of taking in more calories through your diet than you are burning through physical activity.

The reasons for this calorie imbalance vary from person to person. It can be caused by a combination of genes, environment and behaviour. And some drugs and diseases can also contribute to weight gain.

How calorie balance is controlled

In ten years, an average person eats about 10 million calories but their weight typically only changes by about 10 pounds either way. So in most cases, our weight remains stable over time. This is because our bodies control our calorie balance very precisely; much like a thermostat keeps the temperature of a room steady.

Our genes and our environment both affect the setting of this ‘weight thermostat’ and can cause a calorie imbalance. For example, our genes can affect the amounts and types of food that we prefer to eat. And our environment affects the amounts and types of food that are available to eat.

So most of the time, our body weight is controlled by instinctive drives. This is why it is incorrect to say that obesity is simply due to a lack of willpower. This is an overly simple way of looking at a very complex condition. And it can make obese people feel guilty or stigmatised.

Sedentary pasttimes are contributing to rising obesity rates. The environment

Obesity is becoming more and more common. For example, the numbers of obese people in the UK have tripled in the last 25 years. This is due to changes in lifestyle, particularly changes in the foods that are available and the amount of physical activity that people take.

Food tastes better, is available in more varieties, and is cheaper, especially processed foods. Portion sizes are getting bigger, usually at very little extra cost. For convenience, more people are eating pre-packaged food, fast food and soft drinks, which are often high in calories, fat, salt and sugar. And these types of food are heavily advertised, especially to children.

We are also living more inactive lifestyles. The use of cars and public transport over walking and cycling is increasing. Many jobs now involve sitting at a desk for several hours. And inactive pastimes, such as watching television and surfing the internet, are becoming more popular.

Cancer Research UK believes that we need a comprehensive strategy to halt the rise in obesity in the UK population, particularly in children and young adults. We need to have an environment that encourages healthy eating and physical activity. You can read about our work to help bring about such an environment in our Policy section.

Our genes can affect our risk of being obese.Genes

Whatever the environment, some people stay thin and some become obese. Research shows that obesity tends to run in families. Studies with twins and adopted children have shown that genes, rather than shared lifestyles, play a key role in this.

Obesity-related genes could affect how we metabolise food or store fat. They could also affect our behaviour, making us inclined towards lifestyle choices that increase our risk of being obese:

  • Some genes may control appetite, making us less able to sense when we are full.
  • Some genes may make us more responsive to the taste, smell or sight of food.
  • Some genes may affect our sense of taste, giving us preferences for high fat foods, or putting us off healthy foods.
  • Some genes may make us less likely to engage in physical activity.

People with obesity-related genes are not destined to be obese. But they will have a higher risk of obesity. In the modern environment, they may need to work harder than others to maintain a healthy body weight by making long-term, sustained lifestyle changes.

Behaviour changes can help you to reach a healthy body weight. Behaviour

We cannot alter our genetic make-up and it is very difficult to control our environment. But we can learn how to control the lifestyle choices we make.

Because our calorie balance is controlled by instinctive drives, short-term ‘quick fix’ solutions (including most diet programmes) eventually fail. Instead, we must make long-term lifestyle decisions including healthy eating and regular physical activity to reduce our risk of being obese.

This in turn will reduce our risk of cancer, heart disease, diabetes and several other major diseases. Even relatively small losses, such as 10% of our total weight, can have positive effects on our health.

To find out more about how you can make small lifestyle changes to maintain a healthy body weight, go to our Ten Top Tips section.

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Updated: 25 September 2009