Here are some of the theories why some people eat more calories than their body needs.

• It's pretty easy for us to overeat if we don't get strong messages to stop eating. People who are obese may not have the right balance of chemicals in parts of their brain. So their brain can't pass on the right messages about how hungry or full they feel.
• People who are overweight might be eating even when they don't feel hungry. For example, they may eat to comfort themselves when they're sad. It's also easy to eat when we're not hungry because we associate a situation with eating. For example, when we visit family or friends, or go to see a movie.
• A very small number of people who are obese don't produce enough of a chemical called leptin. Leptin is made by the cells in your body that store fat. It tells your brain how much fat you have stored.^{1 2} If you aren't making enough leptin, you may still feel hungry, even though you're putting on weight.
• Some medical conditions and medications can increase your appetite and your weight. This can make it hard to keep your weight down. It's important to get medical help.
• People use calories at different rates. Some people find it easier to gain weight and lose weight than others. This may be due to differences in our genetic and chemical make-up, which we can't yet explain.

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.