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If you're obese, it means you weigh much more than is healthy for you. It happens because you eat more calories than your body uses. The extra calories are stored as fat.

Being obese is more than being just a few pounds overweight. Obesity can cause health problems such as diabetes, high blood pressure, arthritis and heart disease. And it makes some people feel bad about themselves.

See How obesity affects your life.

Losing weight isn't easy. You'll need to change the way you eat and the amount of exercise you take. But if you make these changes, you'll be healthier.

• Obesity is a serious medical condition.
• Losing even a small amount of weight can lower the risk to your health.
• Cutting down on calories, taking more exercise and learning good eating habits can help you lose weight.
• Medical treatments may also help.
• You're more likely to lose weight if you get help from a health professional.
• If you're very obese, surgery can help you lose a lot of weight. But there are risks with surgery so doctors usually recommend it only if your weight is a danger to your health.

When doctors talk about a "healthy" or "ideal" weight, they mean a weight that lowers your risk of getting serious health problems, such as heart disease or high blood pressure. It's not based on how thin you would like to look.¹

Most doctors use the body mass index (BMI for short) to figure out whether you're at a healthy weight. Your BMI is a single number that's worked out from your height and weight. You can work out your own BMI.

This table shows what the different BMI scores mean.^{2 3}

You may have read that there are problems with using someone's BMI to find out if they're overweight. For example, an athlete who's very muscular may have a BMI that suggests they're overweight, even though they're healthy. That's because muscle is heavier than fat. However, for most people, working out their BMI is a quick and simple way of finding whether their weight is healthy. To read more, see How do doctors diagnose obesity?

Food gives you energy. The amount of energy in food is measured in calories (short for kilocalories, or kcal).

Your body needs energy all the time, especially for moving and keeping warm. Different people need different amounts of energy. The amount of energy you need depends on your metabolic rate. And the more active you are, the more energy you use.

Your brain helps you stay a healthy weight. It helps to balance how much you eat with how much energy you burn.⁴ Your brain sends out "hungry" or "full" signals that make you want to eat or stop eating.⁵

To learn more, see Why do I feel hungry or full?

Being obese is not the same as being a few pounds overweight. Doctors say someone is obese if their BMI is 30 or higher (see the table above). For most people, having a BMI greater than 30 means they're at least 30 pounds overweight.⁶

If you're obese, it means you're eating more calories than your body is using up as energy. Your body stores the extra calories as fat.

Doctors aren't sure why some people eat more food than they need. There are probably lots of reasons.

One theory is that early humans probably had to go for long periods without food, so our bodies needed to be able to store fat easily in order to survive. But nowadays we can get food easily. Some scientists think that our bodies haven't had a chance to "catch up'. They still work as if we might have to face a famine.⁷

And for some people who are obese, their "full" and "hungry" signals may not work properly.

To read more, see Why do some people gain weight more easily than others?

Like many people who are overweight, you may feel that you don't eat a lot. You may think that your weight problems must be due to something else, such as a "slow metabolism" or your "glands."⁵

But you can only become obese by eating more calories than your body uses.

However, there are some medical conditions and medications that make it more likely that you'll put on weight. Some conditions make you feel more hungry. Other conditions slow down your metabolic rate. And some of these conditions affect your glands.

If you have one of these conditions, you may find it more difficult to keep your weight down. It's important to get medical help.

The bad news is that you only have to eat slightly more calories than you use for the weight to add up.

• The average American over age 25 takes in only 0.3 percent more energy than they use each day. This about 150 calories, which is the number of calories in a small soda.
• Over time, this extra energy adds up. It's stored as fat.
• By the age of 55, that average American has gained 20 pounds.⁸

If the amount of fat in your body increases gradually, your body doesn't seem to notice that you're putting on weight. Your appetite may stay the same.

There are two things about our Western lifestyle that are linked to more and more people becoming obese:

• Modern fast-food culture
• Lack of physical activity.

Because we're less active than people were in the past, and because fatty and sugary foods are so widely available, it's very easy to gain weight. Some doctors think it would make a difference if unhealthy foods were more clearly labeled. Others think that fast-food adverts aimed at children should be banned. These things might make it easier for people to eat a healthy diet and avoid being overweight. But you'd still need to make sure you didn't eat more calories than you needed.

Anybody can become obese if they eat more calories than they use. But there are certain things that make this more likely. These are called risk factors. Some of these you can control, others you can't. They include having parents who are overweight or obese, getting older, being from a certain ethnic group and having emotional problems.

To learn more, see Risk factors for obesity.

Certain medical conditions and medications can also make it more likely that you'll become obese.

This information is for educational use only, and is not a substitute for prompt professional medical advice. Readers should always consult a physician or other professional for advice and treatment. ©BMJ Publishing Group Limited 2008. All rights reserved.