Genes
An underlying tendency to obesity may be the result of our genes. People who generally have little problem controlling their weight seem to have a precisely tuned appetite.

People who gain weight, on the other hand, may be less sensitive to their body's signals of fullness.

Many genes have been identified that either increase or decrease appetite.

People who generally have little problem controlling their weight seem to have a precisely tuned appetite, while people who struggle to control their weight may be less sensitive to their body's signals of fullness.

Studies of twins who've been raised apart attribute almost two-thirds of the difference in body fatness to genetic factors. However, genetic factors don't make obesity inevitable.

Habits
Eating habits develop over many years, and are strongly influenced by our first tastes as babies and dietary patterns formed in early childhood.

These are then continuously reinforced as we grow up, which makes them difficult to change.

Too often, they lead to eating too many calories. Recognising these unhelpful habits and replacing them with positive behaviour are key steps in successful weight control.

If you always reach for sugary or fatty snacks when you watch TV, for example, distract yourself with another activity or make sure the snacks are healthy alternatives, such as fruit or vegetable sticks.

Food
People who tend to choose foods that are high in fat or contain a lot of energy (calories) in small portions are more likely to gain weight than those who fill their plates with bulky, low-energy foods, such as bread, fruit and vegetables.

Bigger portion sizes also mean more calories (see How to lose weight and A healthy weight-loss diet).

Emotions
Overeating can also be triggered by our emotions. Some people turn to food or alcohol in stressful situations, such as after a family argument or a particularly difficult day at work.

Other vulnerable times may be when you're feeling tired, bored or sad.

Identifying triggers and cues that cause you to overeat can help you to change your behaviour in these situations and avoid unwanted calories.

Write down the times when your emotions lead to eating (see Food diary). This will help you to identify situations when you're particularly vulnerable to excess snacking.

Try the following techniques:

Ask yourself if you must have the food - thinking about what you're doing can help you avoid extra snacks
Replace images of food with other positive thoughts
Distract yourself from eating by doing something else you enjoy

Other causes
Medical conditions
Some medical conditions can cause obesity, but these are rare. Prader-Willi syndrome, for example, is a genetic disorder that can result in obesity because people with the condition don't feel full (satiated) and overeat as a result.

Some brain disorders can also cause obesity. For example, brain tumours can result in obesity if they grow in the part of the brain that affects appetite control. However, these are extremely rare.

Medication
Drugs that treat high blood pressure, inflammatory conditions (steroids) and mood disorders can contribute to weight gain by stimulating appetite or decreasing energy expenditure.

If you're concerned, an alternative medicine may be available, but it's vital you consult your GP before stopping any medication.

In some cases, the weight gain is unrelated to the medication. For example, drugs that help to improve low mood may increase appetite simply because they make you feel better, so you're more likely to feel like eating.

Some drugs encourage the body to retain water. This may lead to weight gain, but as it isn't fat the problem will resolve once the underlying disease has been treated.

Physical inactivity
People who lead a physically active life are less likely to gain weight than those who spend most of their day sitting in front of a computer or TV, or in the car.

Have a TV-free day each week and take up a physical activity.

There's evidence regular physical activity can help to keep the weight off in the long term, too.

Warning signs
Obesity doesn't develop overnight. It takes about 3,500 excess calories to gain just 0.5kg (1lb). Few people gain more than 2lb to 5lb each year. Weight fluctuates from day to day, but you should aim to stay about the same weight from week to week.

If you notice a consistent increase in your weight, try to stabilise it before you develop a serious problem.

Begin by reducing the amount of fat in your diet and incorporate 20 to 30 minutes of activity into your day.

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