The definition of obesity for adults is a body mass index (BMI) of over 30. You can work out your BMI by dividing your weight in kilograms by the square of your height in metres. Or you can use an online calculator.
Obesity usually means you are eating and drinking more calories than your body can use. The extra calories are stored as fat.
Around one in four adults and one in five children aged 10-11 in the UK is obese.
Obesity is not a disease. However, the effects of obesity can be severe as you are more likely to have serious health problems, such as diabetes, gout, cancer and heart disease. Obesity can also increase your risk of developing arthritis. If your BMI is over 40, you are morbidly obese, which means your obesity may shorten your life.
You can reduce the health risks caused by obesity by losing weight.
Obesity symptoms include breathlessness and being very overweight with a lot of body fat.
Some people have a high BMI but are not obese. That is because they have more muscle or don’t have much fat.
If you are worried about your weight, ask your GP for advice.
Obesity and BMI
The relationship between your BMI and your weight indicates whether your weight is healthy or not. In general, if your BMI is:
If your BMI is over 25, additionally measuring your waist size can indicate whether the excess fat you are carrying will cause health problems. In general, men with a waist size of 94 cm or more and women with a waist size of 80 cm or more are at greater risk of developing health problems due to obesity.
Other body measures, considered alongside your BMI, can also help determine whether your weight may harm your health. This includes your waist-to-hip ratio (WHR), your waist-to-height ratio (WtHR), and how much fat you have and how it is distributed.
Your GP will be able to explain what is a healthy weight and measurement for you. They may calculate your BMI and take your waist measurement, as well as ask you about your general health and lifestyle. This may include asking you about:
They may also ask you how motivated you feel to lose weight and how you feel about your weight eg whether your weight makes you feel depressed.
Your GP may then refer you for tests to check for health conditions including type 2 diabetes and an underactive thyroid.
The Edmonton scale
If you are diagnosed as obese, your doctor may further assess you using the Edmonton scale. This scale relates your weight to your health and has five stages:
If your doctor does not use the Edmonton scale, they will still explain how your weight is affecting your health.
Obesity is a growing problem due to modern lifestyles ie easy access to cheap, high-calorie foods and spending more time sitting. Obesity develops as a result of eating and drinking too many calories without exercising enough to burn them off. It is more likely if your diet is high in fat and sugar eg if your diet includes a lot of alcohol, high-sugar drinks, takeaways or ready-made meals.
Excess calories and poor diet
An average, physically active man needs to consume around 2,500 calories each day to maintain a healthy weight, while an average, physically active woman needs around 2,000 calories each day.
Ready-made meals and takeaways can easily account for the majority of these calories eg one meal that includes a large takeaway hamburger, fries and a milkshake can contain around 1,500 calories. Other ways to easily eat too many calories include:
Obesity can run in families. This may be genetic but also due to unhealthy eating habits that you learnt as a child and continued to follow as an adult.
Leading a sedentary lifestyle
If you eat too many calories than you need and then do not burn them off through physical exercise, those excess calories will be stored as fat.
The UK's Department of Health and Social Care recommends that adults perform at least two and a half hours of moderate aerobic exercise (exercise that increases your heart and breathing rate) every week. You can split this up into shorter stretches of daily exercise such as brisk walking, running or cycling.
If you are overweight or obese, you may need to perform more than two and half hours of exercise every week to lose excess weight. However, make sure you do not overexert yourself — gradually increase the amount and intensity of exercise you do each week.
Physical activity will affect your hormones and your hormones affect how you process food. Physical activity has been shown to help stabilise insulin levels — unstable insulin levels are linked to weight gain.
Not sleeping enough
Poor sleep increases the risk of weight gain in both children and adults. This is because sleep deprivation increases levels of the hormone ghrelin and decreases levels of the hormone leptin. Ghrelin stimulates your appetite while leptin suppresses it.
Genetics and obesity
Most people can lose weight as obesity is usually caused by lifestyle factors eg poor eating habits and a lack of physical activity. However, there are very rare genetic conditions that can cause obesity eg Prader-Willi syndrome.
If you have a healthy lifestyle but still find it difficult to lose weight, it might be caused by a medical condition eg Cushing’s syndrome where your body produces excessively high levels of steroid hormones or an underactive thyroid (hypothyroidism) where your body does not produce enough hormones.
However, if these conditions are treated, they are unlikely to cause weight problems.
Medications and weight gain
Certain medications can make it easier to gain weight. These include:
Day-to-day problems caused by obesity
The day-to-day risks of obesity are caused by the extra strain put on your bones, joints and organs. They include:
Serious health conditions caused by obesity
Obesity causes high blood pressure, high cholesterol and atherosclerosis (fatty deposits that narrow your arteries). These conditions all increase your risk of coronary heart disease and stroke.
Obesity also increases your risk of certain diseases, including:
If you are obese you are also at greater risk of developing:
Obesity can also decrease your fertility. If you are pregnant and obese, you are at greater risk of complications during your pregnancy, including gestational diabetes and pre-eclampsia (a dangerous increase in your blood pressure).
Overall, obesity reduces your life expectancy by, on average, three to 10 years, depending on its severity. Being overweight or obese is estimated to contribute to at least one in every 13 deaths in Europe.
Even if you are unable to reach a healthy weight, losing any amount of excess weight and maintaining that weight loss will reduce your risk of obesity-related health conditions.
Your GP will suggest you follow a healthy, balanced reduced-calorie diet. They may refer you to a dietitian to help you plan your meals. This will involve avoiding high-fat, high-sugar and high-salt foods — salt can increase your blood pressure, which is particularly dangerous if you are obese.
You will also need to check how many calories each of your meals contain so you do not exceed your daily limit — this includes calories in both food and drink. Make sure you eat slowly as this will help you feel full by the end of your meal.
Take extra care when eating out at restaurant meals or having take-out meals, as they can contain a high amount of calories.
Losing excess weight can get harder because once you have lost around 10 kg, your body's metabolism will slow down. This means you will burn fewer calories during physical exercise. It is therefore important to prevent weight gain and seek help from healthcare professionals and support groups when you are trying to lose weight.
Physical activity will help you lose excess weight and maintain that weight loss. It also reduces your risk of several other serious health conditions, including type 2 diabetes.
Your GP may suggest you join a local weight loss group, exercise class or gym to help you stay motivated.
You can also try moderate-intensity activities, such as brisk walking, jogging, swimming or tennis for at least two and half hours and up to five hours every week. Alternatively, you can try one hour and 15 minutes of vigorous-intensity activity every week or a combination of moderate and vigorous activity.
Other useful strategies
If it is very challenging for you to lose weight, your GP may prescribe the medication orlistat to help.
Orlistat reduces how much fat you absorb by about a third when digesting your food. The fat which is not absorbed will pass out of your body via your stools. This approach may help you avoid gaining further weight but may not help you lose weight.
You will only be able to go ahead with the surgery if you are:
Is obesity an eating disorder?
Obesity is not an eating disorder, although there may be psychological aspects to why an individual has developed unhealthy eating habits that have caused excess weight gain.
What weight is classed as obese?
There is no specific weight that is classed as obese. Obesity depends on both your weight and height. You can get an indication of whether or not you are obese by calculating your body mass index (BMI) — this is your weight in kilograms divided by the square of your height in metres. Having a BMI greater than 25 means you are overweight and a BMI greater than 30 means you are obese.
How does obesity cause cancer?
Obesity causes your body to develop excess body fat. This fat sends out signals to the rest of your body eg by releasing fat or chemicals into your blood. This affects other cells and can cause them to divide more, which may lead to cancer. Excess fat can cause increases in the levels of insulin, growth factors, inflammatory chemicals and oestrogen in your body, all of which may trigger cells to divide more.
How does obesity cause diabetes?
The exact mechanism behind how obesity causes diabetes is not fully understood. However, research suggests that in obese people, fat cells are more likely to release chemicals that cause inflammation. This can make your body less sensitive to insulin, which causes type 2 diabetes. Obesity may also change your body's metabolism, causing your fat cells to release fat into your blood, which causes cells to become less sensitive to insulin.
What body fat percentage is obese?
A body fat percentage that is 32% or more is considered obese.
Can obesity cause low oxygen levels?
Obesity puts you at greater risk of heart disease and heart failure, which can reduce oxygen levels in your blood. If your obesity causes sleep apnoea, this can also cause your oxygen levels to drop when you sleep.