What Is Obesity? What It Means & How It’s Measured

Many people want a clear answer to what obesity means. And it can be worrying when you see a number like BMI on a chart or a letter from your GP.

But weight and health are not the same thing for everyone. And the topic is often discussed in a way that feels blaming.

So this guide explains what obesity is, how it is measured, what the four stages are, and when to get support in the UK.

What is the Definition of Obesity?

Obesity is usually defined as having excess body fat that may increase your risk of health problems. NHS guidance usually identifies it using body mass index (BMI). A BMI of 30 or above is classed as obese, and a BMI of 40 or above is often described as severely obese.

It can help to think of obesity as a health condition with many causes, not a personal failure. People’s bodies respond differently to food, stress, sleep, hormones, medication, and their environment.

Two people can eat and move in similar ways but gain weight differently.

Also, it does not automatically mean you are “unhealthy”, and having a lower weight does not guarantee good health. What matters is your overall risk and whether excess body weight is affecting you in ways that need support.

How is Obesity Measured?

BMI is the most common way obesity is assessed. The calculation is: BMI = weight (kg) ÷ height (m)².

The NHS uses these BMI ranges for most adults:

• 18.5 to 24.9: healthy weight
• 25 to 29.9: overweight
• 30 to 39.9: obese
• 40 or above: severely obese

BMI is useful because it is quick and easy. But it has limits. It does not directly measure body fat. So people with high muscle mass may have a high BMI.

BMI and Ethnic Background

Some ethnic groups can develop health risks at a lower BMI. This is because of higher central body fat (around the waist).

For people with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African Caribbean background, the National Institute for Health and Care Excellence (NICE) suggests lower BMI thresholds as a practical guide:

• BMI of 23 to 27.4: overweight
• BMI of 27.5 or above: obese

Waist Measurements Can Add Useful Information

Waist size reflects how much fat you store around the middle. So NICE recommends using waist-to-height ratio alongside BMI in adults with BMI under 35 to help assess health risk. A simple recommendation they make is: try to keep your waist to less than half your height.

If you feel anxious about the numbers, it can help to treat them as starting points for a conversation, not a judgement. A clinician can look at your full picture, including blood pressure, blood tests, symptoms and family history.

What Are the Four Stages of Obesity?

BMI tells you how weight relates to height. But it does not tell you how much your weight is affecting your health. That is where obesity stages can help.

A commonly used clinical tool is the Edmonton Obesity Staging System (EOSS). It stages obesity based on health risks, physical symptoms and mental health. It also takes into account how well you can manage day-to-day activities.

The staging system actually includes Stage 0, so there are five stages. The “four stages” usually refer to Stage 1-4, which outline the increasing impact of excess weight on your health. Below is a simple explanation of Stage 0 to Stage 4.

Stage 0 Obesity

This means you have no sign of obesity-related risk factors and aren’t experiencing physical or psychological symptoms. And it’s not limiting your movement or day-to-day life.

Stage 1 Obesity

What is stage 1 obesity? It usually means early or mild impact on your health. You may have borderline risk factors (for example, slightly raised blood pressure), mild symptoms (such as aches and pains) or mild effects on wellbeing. Day-to-day life is mostly unaffected.

Next steps may include: checking key risk markers with your GP. You may want to think about lifestyle support and taking steps to avoid future problems.

Stage 2 Obesity

Stage 2 usually means there is an ongoing health condition linked with weight, or clearer effects on daily function. Examples can include type 2 diabetes, high blood pressure, sleep apnoea, osteoarthritis or reflux. It may also include more noticeable limits with activity, energy or mobility. 

Next steps may include: a more structured weight management plan and treatment of related conditions. You might seek a referral to specialist services.

Stage 3 Obesity

Stage 3 usually means excess weight is causing significant health problems and is limiting day-to-day activities. This can include end-organ damage (for example, cardiovascular disease, heart failure or diabetes complications), severe joint disease that limits movement, and psychological distress.

Next steps may include: specialist support and assessments for higher-intensity treatments. You might also think about medical, physical and psychological support.

Stage 4 Obesity

What is stage 4 obesity? It usually means it is causing severe, disabling complications. Health problems may be advanced and have a major impact on quality of life and daily independence.

Next steps may include: getting specialist services and long-term treatment for complications. A healthcare professional may recommend bariatric surgery, depending on your need and local NHS pathways.

What Causes Obesity?

Obesity rarely has one single cause. It is usually the result of several factors happening together, often over years.

Common contributors include:

• Biology and appetite regulation: hormones and brain signals that affect hunger and fullness can vary between people.
• Genetics: some people are more likely to gain weight in the same environment.
• Life stage and hormones: pregnancy, menopause and some health conditions can affect weight.
• Medicines: some medicines can increase appetite or cause weight gain.
• Sleep and stress: poor sleep and long-term stress can make appetite harder to manage and increase cravings.
• Environment: easy access to high-calorie food makes weight gain more likely. So do busy routines, cost pressures and less opportunity for movement.
• Illness or injury: pain or reduced mobility can reduce activity and make weight gain easier.

Obesity is increasingly being seen as a chronic disease. If you are living with obesity, it can help to avoid thinking a lack of willpower causes it. Instead, focus on practical supports that reduce friction in daily life.

When to Get Support

If you are unsure whether you need support, a good first step is to look at:

• Your BMI
• Your waist-to-height ratio (if relevant)
• How you feel day to day
• Whether you have symptoms or health conditions that could be linked with weight

Consider Treatment and Support If:

• Your BMI is in the obese range (or lower thresholds apply to you).
• You have weight-related symptoms such as breathlessness, joint pain, reflux, snoring or daytime tiredness.
• You have a condition like high blood pressure, prediabetes, type 2 diabetes, high cholesterol, sleep apnoea or arthritis.
• Your weight has changed quickly without a clear reason.
• You feel stuck, anxious or overwhelmed and want a plan that feels manageable.

If you’d like to explore prescription options, you can talk to us about medicines such as GLP-1 treatments like Mounjaro or Wegovy. Complete our assessment form, or speak to our patient care team on 0141 774 5023.

Key Takeaways

• Obesity Is Usually Defined Using BMI: In the UK, a BMI of 30 or above is classed as obese for most adults. BMI is a useful starting point, but it does not tell the full story on its own.
• Obesity Stages Look At Health Impact, Not Just Weight: The “stages” describe how obesity is affecting health and daily life. From mild early effects (Stage 1) to severe, disabling complications (Stage 4).
• Causes Are Complex and Not About Blame: It is linked with many factors. This may include biology, genetics, sleep, stress, medicines, long-term health conditions and the environment you live in.
• Your Waist Measurement Can Add Important Context: Waist-to-height ratio is important. Waist size can help show risk linked with fat tissue around the abdomen, especially if BMI is under 35 or if lower BMI thresholds apply to you.
• Support Is Available In The UK: Worried about your weight or health risks? Your GP or a pharmacist can help you understand your numbers, check for related conditions and talk through realistic next steps.

References

National Institute for Health and Care Excellence (NICE), 2025. Identifying and assessing overweight, obesity and central adiposity. In: Overweight and obesity management (NG246) [Online]. Available from: https://www.nice.org.uk/guidance/ng246/chapter/Identifying-and-assessing-overweight-obesity-and-central-adiposity.

NHS, 2025. Obesity [Online]. Available from: https://www.nhs.uk/conditions/obesity/.

NHS, 2025. Obesity – causes [Online]. Available from: https://www.nhs.uk/conditions/obesity/causes/.

Edmonton Obesity Staging System -Staging Tool. (n.d.). Available at: https://www.ottawahospital.on.ca/fr/documents/2017/05/edmonton-obesity-staging-system-staging-tool.pdf/.

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