The term obese describes a person who's very overweight, with a lot of body fat.
It's a common problem in the UK that's estimated to affect around 1 in every 4 adults and around 1 in every 5 children aged 10 to 11.
The most widely used method to check if you're a healthy weight is body mass index (BMI).
BMI is a measure of whether you're a healthy weight for your height. You can use the NHS BMI healthy weight calculator to work out your score.
For most adults, a BMI of:
BMI is not used to diagnose obesity because people who are very muscular can have a high BMI without much fat.
But for most people, BMI is a useful indication of whether they're a healthy weight.
A better measure of excess fat is waist size, which can be used as an additional measure in people who are overweight (with a BMI of 25 to 29.9) or moderately obese (with a BMI of 30 to 34.9).
Generally, men with a waist size of 94cm or more and women with a waist size of 80cm or more are more likely to develop obesity-related health problems.
It's very important to take steps to tackle obesity because, as well as causing obvious physical changes, it can lead to a number of serious and potentially life-threatening conditions.
Obesity can also affect your quality of life and lead to psychological problems, such as depression and low self-esteem.
Obesity is generally caused by consuming more calories, particularly those in fatty and sugary foods, than you burn off through physical activity. The excess energy is stored by the body as fat.
Obesity is an increasingly common problem because for many people modern living involves eating excessive amounts of cheap high-calorie food and spending a lot of time sitting down at desks, on sofas or in cars.
There are also some underlying health conditions that can occasionally contribute to weight gain, such as an underactive thyroid gland (hypothyroidism), although these types of conditions do not usually cause weight problems if they're effectively controlled with medicines.
The best way to treat obesity is to eat a healthy reduced-calorie diet and exercise regularly.
To do this, you should:
You may also benefit from receiving psychological support from a trained healthcare professional to help change the way you think about food and eating.
If lifestyle changes alone do not help you lose weight, a medicine called orlistat may be recommended.
If taken correctly, this medicine works by reducing the amount of fat you absorb during digestion. Your GP will know whether orlistat is suitable for you.
In some cases, weight loss surgery may be recommended.
Obesity can cause a number of further problems, including difficulties with daily activities and serious health conditions.
Day-to-day problems related to obesity include:
The psychological problems associated with being obese can also affect your relationships with family and friends, and may lead to depression.
Being obese can also increase your risk of developing many potentially serious health conditions, including:
Obesity reduces life expectancy by an average of 3 to 10 years, depending on how severe it is.
It's estimated that obesity and being overweight contribute to at least 1 in every 13 deaths in Europe.
There's no quick fix for obesity. Weight loss programmes take time and commitment, and work best when fully completed.
The healthcare professionals involved with your care should provide encouragement and advice about how to maintain the weight loss achieved.
Regularly monitoring your weight, setting realistic goals, and involving your friends and family with your attempts to lose weight can also help.
Remember that even losing what seems like a small amount of weight, such as 3% or more of your original body weight, and maintaining this for life, can significantly reduce your risk of developing obesity-related complications like diabetes and heart disease.
Our guide to care and support explains your options and where you can get support.
If it's been a long time since you did any exercise, you should check out the Couch to 5K running plan.
It consists of podcasts delivered over the course of 9 weeks and has been specifically designed for absolute beginners.
To begin with, you start running for short periods of time, and as the plan progresses, gradually increase the amount.
At the end of the 9 weeks, you should be able to run for 30 minutes non-stop, which for most people is around 5 kilometres (3.1 miles).