What is obesity?

What is obesity? A plain definition

Obesity is a long-term disease in which the body stores more fat than is healthy, or stores it in places where it does harm, such as around the organs in the abdomen. Fat tissue is far from passive. It releases hormones and inflammatory substances, presses on the airways and joints, and changes how the liver, heart and blood vessels work.

Over time this raises the risk of type 2 diabetes, heart disease, several cancers and many other conditions.

In everyday medicine, obesity in adults is screened for with the body mass index (BMI): a BMI of 30 or higher counts as obesity. But BMI is a rough first step. Two people with the same BMI can have very different amounts of fat, and very different health. That is why doctors increasingly look further, at waist size, at blood tests and at how a person's body is actually doing.

One more thing belongs in any honest definition: obesity is not a character flaw. It develops through a mix of genes, biology, psychology, medicines, sleep, stress and the food environment around us. If you are living with obesity, you have probably heard every piece of advice going, and been judged for your body more often than you would like to count.

You have likely tried hard to lose weight, perhaps many times. When it does not last, that is not because you did not want it enough. The body defends its weight, and further down we explain how.

How the main health organisations define obesity

Definitions have shifted in recent years, from "too much body fat" towards "a chronic disease". The table shows the definitions that guide doctors today.

Why we say "people living with obesity"

You will notice that this site says "a person living with obesity" and not "an obese person". This is called person-first language, and it is recommended by the World Obesity Federation and by the international consensus statement on ending weight stigma published in Nature Medicine.

The reason is simple. You would not call someone "a cancerous person". A disease is something a person has; it is not who they are. For the same reason we avoid "morbid obesity" and use "class 3" or "severe obesity" instead. More on this on our page about weight stigma.

Is obesity a disease?

Yes. The World Health Organization, the ICD-11 disease classification, the American Medical Association, the World Obesity Federation and the European Association for the Study of Obesity all classify obesity as a chronic disease. It has its own disease code (5B81), it has known biological mechanisms, it causes harm to organs, and it responds to medical treatment.

Like high blood pressure or type 2 diabetes, it tends to return if treatment stops.

Recognition did not happen overnight. This timeline shows the key moments.

Why some people still question it

Some doctors and researchers still question the disease label, mainly because BMI cannot tell who is actually ill. Their worry is that calling obesity a disease turns millions of healthy people into patients overnight. Others fear that a disease label could make people feel less in control.

The Lancet Commission's answer was to keep the disease label but make it more precise: excess fat that already damages health is clinical obesity, a disease that needs treatment; excess fat without harm so far is preclinical obesity, a risk state that needs attention and follow-up.

In practice, the disease view has one big advantage for patients: it opens the door to proper medical care and, in many countries, reimbursement.

Is obesity a disability or an eating disorder?

Disability: obesity is not automatically a disability in law, but it can count as one when it causes long-term limitations. The Court of Justice of the European Union has ruled in the Kaltoft case that obesity can fall under EU disability protection at work when it hinders a person's full participation in professional life on an equal basis with others.

Rules differ elsewhere: in the United States, protection depends on federal case law and on local laws, and New York City and the state of Michigan explicitly ban weight discrimination. See weight and work for details.

Eating disorder: no. Obesity is classified as a chronic metabolic disease, not as a mental disorder. Many people living with obesity have a completely ordinary relationship with food. Binge eating disorder, a recognised eating disorder, is more common among people with obesity and needs its own treatment; our page on psychological support explains the signs.

How is obesity measured?

Doctors usually measure obesity in steps: they start with BMI, check where fat is stored with a tape measure, and then look at the person's overall health. You can do the first two steps yourself at home.

BMI: formula and the obesity BMI range

Your BMI is your weight in kilograms divided by your height in metres squared. Someone who weighs 92 kg and is 1.75 m tall has a BMI of 92 ÷ (1.75 × 1.75) = 30.0. For adults, these are the categories used by WHO and most national guidelines:

The lower cut-offs come from the UK's National Institute for Health and Care Excellence (NICE guideline NG246). They apply to people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, because at the same BMI these groups tend to develop type 2 diabetes and heart disease earlier. WHO had earlier proposed similar "action points" for Asian populations.

What weight counts as obesity for my height?

For an adult, obesity starts at the weight where BMI reaches 30: for someone 170 cm tall, that is 87 kg (191 lb). The table below turns the BMI limits into kilograms and pounds for other heights. It is a quick reference, not a verdict: muscle, age and body shape all matter.

Waist size and waist-to-height ratio

Where fat sits matters as much as how much there is. Fat deep inside the abdomen, around the liver and intestines, is more harmful than fat on the hips and thighs. A tape measure picks this up where BMI cannot.

The simplest test is the waist-to-height ratio: keep your waist to less than half your height. Measure around your waist, halfway between your lowest rib and the top of your hip bone, and divide by your height. A 170 cm tall person should aim for a waist below 85 cm.

Our page on BMI and waist size shows exactly how to measure, and when these thresholds do not apply.

What BMI cannot tell you

• It cannot tell fat from muscle. A rugby player and a desk worker of the same height and weight have the same BMI.
• It does not show where fat is stored. Abdominal fat carries more risk than fat elsewhere.
• Risk at the same BMI differs by ethnic background, age and sex.
• It says nothing about how you feel or function. Breathlessness, joint pain, blood sugar and blood pressure are not in the number.

That is why the Lancet Commission, EASO and NICE all agree that BMI should be used to screen, not to diagnose. It remains very useful for comparing groups and countries, which is how all the statistics further down this page are calculated.

How obesity is defined in children

Children are still growing, so a fixed BMI number does not work for them. Instead, their BMI is compared with children of the same age and sex on growth charts. WHO defines obesity in 5 to 19-year-olds as a BMI more than two standard deviations above the WHO reference median.

In the United States, the CDC uses the 95th percentile on its growth charts. In October 2026 WHO published its first global guideline on child and adolescent obesity, which puts structured eating, physical activity and behaviour-change programmes first and does not recommend medicines or surgery for children under 10. Read more on obesity in children.

How doctors diagnose obesity, step by step

1. Screening: weight, height and BMI, using lower cut-offs where appropriate.
2. Confirming excess fat: waist circumference or waist-to-height ratio. In specialist settings a DEXA scan can measure body fat directly. With a BMI above 40, excess fat can be assumed.
3. Checking health: blood pressure, blood sugar (HbA1c), cholesterol, liver tests, and questions about sleep, breathing, joints, mood and medicines.
4. Deciding the stage: is excess fat already affecting organs or daily life (clinical obesity), or not yet (preclinical obesity)? This guides how urgent and how intensive treatment should be.

Our doctor visit checklist helps you prepare for this conversation.

Classes and types of obesity

Obesity is grouped in three ways: by severity (class 1, 2 and 3), by where fat is stored, and by cause. When people ask about the "3 types of obesity", they usually mean the three severity classes.

By severity: class 1, 2 and 3

Class 1 is a BMI of 30 to 34.9, class 2 is 35 to 39.9 and class 3 is 40 or higher. Each step up is linked to higher health risks. Class 3 used to be called "morbid obesity". Many medical and patient organisations, such as the Obesity Medicine Association, have moved away from that term because it sounds like a judgement, and "class 3" or "severe obesity" says the same thing without the stigma.

By where fat is stored

Central or abdominal obesity, sometimes described as an "apple shape", means fat is concentrated around the waist and organs. It carries the highest risk for diabetes and heart disease. Peripheral obesity, a "pear shape", means more fat on the hips and thighs, which is less harmful to metabolism but can still strain joints.

By cause

ICD-11 distinguishes the most common form, obesity linked to long-term energy imbalance, from drug-induced obesity (5B81.1), caused by medicines such as some antipsychotics, steroids or insulin. Rare forms are caused by a single gene, such as changes in the MC4R gene or a lack of the hormone leptin, or by genetic syndromes such as Prader-Willi syndrome.

These usually show up in early childhood with intense hunger, and some can now be treated with targeted medicines. See genes and biology and medication and weight gain.

Clinical and preclinical obesity: the new definition

A commission of more than 50 experts, published in The Lancet Diabetes & Endocrinology and endorsed by at least 75 medical organisations, proposed a new way to diagnose obesity. Its core idea: excess body fat is a risk, but it becomes an illness when it starts to damage how the body works.

To confirm excess fat, the commission says BMI should be combined with at least one other measure, such as waist circumference or waist-to-height ratio, or body fat should be measured directly. With a BMI above 40, excess fat can be assumed. Not everyone welcomed the proposal.

EASO warned that calling preclinical obesity a state of preserved health could delay treatment for people who would benefit from it early. Both sides agree on one thing: BMI alone is not enough to judge an individual's health. Read the full explanation on clinical obesity.

Overweight and obesity: what is the difference?

Overweight and obesity are two points on the same scale. In adults, overweight is a BMI of 25 to 29.9 and obesity is a BMI of 30 or higher. Overweight raises health risks modestly; with obesity the risks are higher and more likely to show up as actual health problems.

Overweight is not a diagnosis of disease on its own, although EASO argues that people with a BMI of 25 to 30 who also have abdominal fat and weight-related health problems should be treated as having obesity.

Can you be healthy and live with obesity?

For a while, yes, some people can. Researchers use the term "metabolically healthy obesity" for people with a high BMI but normal blood pressure, blood sugar and cholesterol. The new definition of clinical obesity calls this preclinical obesity. But long-term studies suggest it is often a phase rather than a permanent state: risks build up over the years, especially for joints, the heart and the liver.

The practical message is to take health checks seriously even when you feel fine, and to judge progress by health markers, not only by the scale.

What causes obesity?

Obesity develops when, over months and years, the body takes in more energy than it uses and stores the difference as fat. That sounds simple. What decides why this happens to one person and not another is much more complicated, and much of it is outside our control.

• Genes. Twin studies estimate that 47 to 90 percent of the differences in BMI between people are inherited. Genes affect appetite, how quickly you feel full, and where you store fat.
• The brain's weight control. Hormones such as leptin, ghrelin and GLP-1 tell the brain how much energy is stored and when to stop eating. After weight loss, this system pushes back with more hunger and lower energy use.
• The food environment. In a trial at the US National Institutes of Health, people ate about 500 calories a day more on an ultra-processed diet than on an unprocessed one with the same nutrients on offer, and gained about 0.9 kg in two weeks.
• Medicines. Some antipsychotics, antidepressants, steroids, diabetes medicines and anti-epilepsy drugs can cause weight gain.
• Sleep and stress. Short sleep is linked to about 1.5 times the odds of obesity in adults and nearly 1.9 times in children. Chronic stress and depression also play a role.
• Income and neighbourhood. In England, child obesity in the most deprived areas is about double that in the least deprived areas.
• Life stages. Pregnancy, menopause, stopping smoking and getting older all change how the body handles weight.

Energy balance also works differently from what most people think. The old rule that cutting 3,500 calories always means losing one pound is wrong: research by Kevin Hall and colleagues in The Lancet showed that the body adapts, so weight loss slows down over time.

Each section of our causes page goes deeper: food environment, sleep, stress and mental health, and more.

Is obesity genetic?

Partly. For most people, obesity is "polygenic": hundreds of genes each nudge appetite and fat storage a little, and together they make some people far more likely to gain weight in today's environment. Put simply, genes set the tendency and the world around you decides how strongly it shows.

A small number of people have a single strong gene change that causes severe obesity from early childhood. Having a genetic tendency does not mean weight gain is inevitable, but it does explain why the same lifestyle leads to very different results in different people.

Signs and symptoms of obesity

Obesity itself is measured, not felt. But excess body fat often causes symptoms that are worth discussing with a doctor:

• getting out of breath during everyday activities, such as climbing stairs
• loud snoring, pauses in breathing at night, or feeling tired during the day (possible sleep apnoea)
• pain in the knees, hips or lower back
• sweating more than others, and skin problems in skin folds
• heartburn and reflux
• irregular periods or difficulty getting pregnant
• low mood, anxiety or low self-esteem, often made worse by stigma

A less known but serious condition is obesity hypoventilation syndrome. It mostly affects people with severe obesity: breathing becomes too shallow or too slow, so carbon dioxide builds up in the blood, even during the day. It is usually worst during sleep. Morning headaches, extreme sleepiness and swollen legs can be warning signs.

It is treatable, usually with a breathing machine at night and weight treatment, but it is often missed. If these symptoms sound familiar, ask your doctor about it.

Health risks of obesity

According to the latest WHO estimates, a higher-than-optimal BMI causes 3.7 million deaths a year, mostly from heart disease, diabetes and cancer. These are the main conditions linked to obesity:

Obesity and life expectancy

A major study of about 900,000 adults (Prospective Studies Collaboration, The Lancet) found that a BMI of 30 to 35 shortened median survival by about 2 to 4 years, and a BMI of 40 to 45 by about 8 to 10 years. The loss at the highest BMI is comparable with the effect of smoking.

The good news is that the risk can be lowered. In the Swedish Obese Subjects study, people who had bariatric surgery lived on average about 3 years longer than similar people who received usual care.

How common is obesity? The latest global figures

About 1 in 7 people worldwide live with obesity. The most recent global figures come from WHO's fact sheet of October 2026. Global estimates always lag a little behind, because collecting and checking data from every country takes time.

• 1.1 billion people live with obesity.
• 940 million adults (16 percent) have obesity, and 2.6 billion adults (45 percent) have overweight including obesity.
• 170 million children and adolescents aged 5 to 19 have obesity (8 percent), up from 2 percent in 1990.
• 35 million children under 5 have overweight.

Two patterns stand out. First, obesity is more common in women than in men worldwide, although the gap differs a lot by country. Second, the increase has been steady for more than three decades, without a single year of decline at the global level. UNICEF has reported that, for the first time, more school-age children worldwide had obesity than were underweight.

Obesity rates by country

Obesity rates vary enormously between countries, from less than 3 percent of adults in Vietnam and Ethiopia to more than 70 percent in several Pacific island nations. The chart and table use one consistent set of estimates, made by the NCD Risk Factor Collaboration and also published by WHO, so countries can be compared fairly.

Outside the Pacific, the highest rates are found in parts of the Caribbean, the Middle East and the Americas, with Egypt (43.9 percent), Qatar (42.0 percent), the United States (41.0 percent), Chile (40.5 percent) and Saudi Arabia (40.1 percent) all above 40 percent. In South Africa almost half of women (47.5 percent) live with obesity, compared with 14.5 percent of men.

Obesity in the United States, the UK and Europe

• United States: 40.3 percent of adults have obesity, according to the CDC's latest national survey. That figure has been roughly stable for about a decade, but severe obesity (BMI 40 or higher) is still rising and now affects 9.4 percent of adults.
• United Kingdom: 27.4 percent of adults (latest WHO estimates). In England, 22.2 percent of children aged 10 to 11 have obesity, according to the latest school measurements.
• Europe: rates range from around 11 percent in France to over 20 percent in Germany (latest WHO estimates). In the Netherlands it is 14.7 percent, in Spain 15.3 percent and in Italy 14.0 percent.

Is obesity still rising?

Worldwide, yes, but not everywhere at the same speed. A major analysis in Nature in May 2026, covering 200 countries, found that the rise has slowed or levelled off in most high-income countries. Among children and adolescents it may even have started to fall slightly in France, Italy and Portugal.

In most low and middle-income countries in Africa, Asia, Latin America and the Pacific, the rise is continuing at the same pace or accelerating. The World Obesity Federation projects 1.13 billion adults with obesity by 2030, and WHO estimates the global cost of overweight and obesity at US$3 trillion a year by 2030.

How is obesity treated?

Obesity is treatable, and good care starts with the basics: good food, regular movement at your own level, walking, sleep and support. Good care is long-term and sets goals that matter to the person, such as walking without pain, sleeping better or needing fewer diabetes medicines. Doctors may also discuss medical options. These are the main forms of treatment that exist:

WHO has recently issued its first global guideline on GLP-1 medicines. Its recommendations are conditional, because long-term data, costs and fair access are still uncertain, and it pairs medication with support for diet and physical activity.

Our position on weight-loss medication

Whether medication is right for you is something to discuss with your own doctor; that decision is yours. Obesity Foundation does not recommend or promote weight-loss medication. We focus on good food, appropriate exercise and walking, and our eating plans, workout videos and video meetings (in English) are free.

Something to know before you start any treatment: weight usually returns when treatment stops. A large BMJ analysis of 37 studies found that people who stopped weight-loss medicines regained their weight in about 1.7 years on average. That is not personal failure.

It is what happens when treatment of a chronic disease ends, just as blood pressure rises again when blood pressure tablets are stopped. Surgery, too, is a major decision with lifelong follow-up, made together with a specialist team. Read about bariatric surgery, weight regain and the full treatment overview.

What changes in 2027

Several developments expected in 2027 will affect people living with obesity. Much of the news is about medicines; we list it here as neutral information, not as a recommendation. Some items are confirmed; others could still change.

We update this table as decisions are made. For neutral facts about medicines, including side effects and what happens after stopping, see weight-loss medication. Whatever the news, the basics we focus on stay the same: good food, appropriate exercise and walking.

Weight stigma: the part of obesity people rarely talk about

Weight stigma means being judged, mocked or treated unfairly because of your body size, and it is very common. In a survey of almost 14,000 adults in six countries, 56 to 61 percent of people in weight-management programmes said they had experienced weight stigma, and between about two-thirds and three-quarters of them had experienced it from doctors.

Stigma is not a side issue. It makes people avoid healthcare, raises stress hormones, and is linked to more weight gain, not less. Shame has never helped anyone lose weight. Respect, good information and proper treatment do. Read more about weight stigma and stigma in healthcare.

Myths and facts about obesity

Many common beliefs about obesity are outdated or simply wrong. These are the ones we hear most often, with what the evidence actually says.

What you can do now

If you recognise yourself in this page, or someone you love, you do not have to change everything at once, and you do not have to do it alone. Pick the step that feels most manageable today:

1. Measure two things: your BMI with the calculator above, and your waist-to-height ratio with a tape measure.
2. Book a health check with your doctor or general practitioner, and ask for blood pressure, blood sugar (HbA1c), cholesterol and liver tests.
3. Prepare the conversation: write down symptoms, medicines, what you have tried, and what matters most to you. Our checklist helps. If you leave feeling judged rather than helped, it is fine to ask for a different doctor.
4. Ask what support is available, such as a lifestyle programme, a dietitian or psychological support, and what is reimbursed where you live. Questions about medication or surgery are for your doctor.
5. Go for a walk. Start with a short walk every day, even 10 minutes at a comfortable pace, and add a little each week. It is free and gentle on the joints.
6. Move at your own level. Our free workout programme has three levels with 50 videos each. Level Start is made for people living with obesity or joint pain: partly seated, no jumping, a chair always within reach.
7. Eat well, without crash diets. More vegetables, fruit, whole grains and pulses, enough protein, and fewer sugary drinks and ultra-processed foods. Sign up for a free eating plan that fits your taste, or a free video meeting (in English) about your situation and questions.
8. Protect your sleep, and choose goals beyond the scale: better sleep, less pain, more energy, lower blood sugar.
9. Look after your mental health, and seek support if eating feels out of control or stigma is weighing on you.

Supporting someone else? Our guide to supporting a loved one explains what helps and what does not. And if you have more questions, our find help page and glossary are good next stops.

Short answers to the most common questions are collected in our FAQ.