Obesity as a chronic disease: BMI, waist size and what doctors look at
Medically Reviewed by Dr. Pedro Yelamo Vera, MD
Licensed Physician No. 484814280 · Official Medical College of Bizkaia (Spain)Last reviewed: 12 October 2026
Obesity BMI cut-offs explained: WHO categories, the limits of BMI, waist-to-height ratio and the new clinical obesity definition, from a GP in Spain.
You don't have to figure this out alone
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Obesity is now seen as a chronic disease, not a matter of willpower. But how do doctors decide who has it? Most people know about BMI. Fewer know its limits, or why your waist size can tell a doctor more. This guide explains the numbers, what they mean and what a GP looks at beyond them.
- The WHO calls obesity a chronic disease. For most adults, a BMI of 30 or more is the cut-off.
- BMI is a quick screen. It does not tell muscle from fat or show where fat sits.
- Waist-to-height ratio adds useful detail. Aim for a waist under half your height.
- A 2025 Lancet Commission split obesity into clinical and preclinical types, based on organ health.
- A good assessment looks at your whole health, not just one number.
Dr. Pedro Yelamo Vera is a GP licensed in Spain (Colegiado No. 484814280).
For a medical emergency, call 112 or go to the nearest local emergency department. Do not wait for a video consultation or an online reply.
Why obesity is called a chronic disease
The World Health Organization describes obesity as a chronic disease. It is common, long-lasting and tends to come back. Genes, hormones, sleep, stress, medicines, income and the food around us all play a part. Blame does not help. It also does not work.
Seeing obesity as a disease changes the goal. The aim is better health over time, not a quick fix. That is also why weight often returns when treatment stops. Read more in our guide on stopping weight-loss medication.
BMI: the first check
Body mass index (BMI) is your weight in kilograms divided by your height in metres, squared. It is quick and free. Doctors use it as a first screen.
| Category (adults) | BMI (kg/m²) | Lower cut-offs used by NICE for some groups* |
|---|---|---|
| Underweight | Below 18.5 | — |
| Healthy weight | 18.5 to 24.9 | — |
| Overweight | 25 to 29.9 | 23 to 27.4 |
| Obesity class 1 | 30 to 34.9 | 27.5 or more |
| Obesity class 2 | 35 to 39.9 | Each class starts 2.5 points lower |
| Obesity class 3 | 40 or more | Each class starts 2.5 points lower |
*NICE, the UK health guidance body, uses lower cut-offs for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background. Health risks such as type 2 diabetes start at a lower BMI in these groups.
These are labels for groups, not a verdict on you. They help a doctor decide what to check next.
The limits of BMI
BMI only uses weight and height. That leaves gaps.
- Muscle. Muscle is heavy. A strong, fit person can have a high BMI with little body fat.
- Where fat sits. Fat around the belly carries more risk than fat on the hips. BMI cannot see this. A person with a healthy BMI can still carry risky belly fat.
- Age. NICE advises care when reading BMI in people aged 65 and over. Health, strength and daily function matter more. A slightly higher BMI may even protect older adults.
- Family background. As shown above, risk rises at a lower BMI in some ethnic groups.
- Health. BMI says nothing about blood pressure, blood sugar, joints or mood.
Waist size and waist-to-height ratio
Your waist size gives a better idea of belly fat. NICE now suggests the waist-to-height ratio. You divide your waist by your height, using the same units. It works for men and women and for all ethnic groups.
| Waist-to-height ratio | What it means (NICE) |
|---|---|
| 0.4 to 0.49 | Healthy. No raised risk. |
| 0.5 to 0.59 | Raised belly fat. Higher health risk. |
| 0.6 or more | High belly fat. Further raised risk. |
A simple goal is a waist less than half your height. For example, a person 170 cm tall would aim for a waist under 85 cm. NICE uses this measure for adults with a BMI under 35. Above that, the BMI already shows high risk.
How to measure your waist at home
- Stand up straight and breathe out gently.
- Find the point midway between your lowest rib and the top of your hip bone.
- Wrap a tape measure around this point. Keep it flat and snug, but not tight.
- Write down the number. Measure at the same time of day if you repeat it.
Clinical and preclinical obesity: the 2025 Lancet Commission
In January 2025, a commission of 58 experts published new ideas in The Lancet Diabetes & Endocrinology. It included people living with obesity. Over 70 groups around the world backed it. Its main points were:
- BMI alone is not enough to diagnose obesity in one person. It is best used to screen. Extra body fat should be confirmed with a waist measure, a direct body fat scan, or similar. In people with a very high BMI, above 40, extra fat can be assumed.
- Clinical obesity is a chronic illness. Excess fat is already harming how organs or the body work. Signs include breathlessness, joint pain that limits walking, sleep apnoea, high blood sugar, or fatty liver with scarring. It also covers clear limits on daily tasks, like dressing or bathing.
- Preclinical obesity means extra body fat while organs still work normally. The risk of future illness is higher, though.
The commission said people with clinical obesity should get timely, evidence-based treatment. People with preclinical obesity should get advice, regular checks and, where needed, steps to lower risk. This framework is still new. Not every guideline uses it yet, but it shapes how many doctors think.
Respectful, stigma-free care
Weight stigma is common, even in health care. Experts agree it harms health and keeps people away from care. Good care should feel safe and respectful. In practice, that means:
- Using "a person with obesity", not labels that define you by your weight.
- Asking before talking about weight.
- Looking for causes and barriers, not blame.
- Focusing on health goals that matter to you.
If you have had an eating disorder, please tell your doctor. Your plan should take this into account. Food restriction is never the aim.
Conditions linked to obesity
Obesity can affect many parts of the body. A doctor will often check for:
- Type 2 diabetes and prediabetes. See HbA1c and prediabetes results explained.
- High blood pressure and high cholesterol.
- Fatty liver. See fatty liver and weight loss.
- Polycystic ovary syndrome (PCOS). See PCOS, weight and insulin resistance.
- Sleep apnoea, which causes loud snoring and daytime tiredness.
- Knee and hip pain from joint wear.
- Heart disease, some cancers, low mood and anxiety.
Get a full picture of your health
Book a video visit with Dr. Pedro Yelamo Vera to review your measures, tests and goals, for patients in Spain.
What the Weight Optimization consultation looks at
The Weight Optimization consultation is a 30-minute video visit for patients in Spain. It is a medical review of weight and metabolic health. Dr. Yelamo looks at:
- Your weight history and what has or has not worked before.
- Your BMI and waist-to-height ratio, using measures you take at home.
- Symptoms that may point to clinical obesity, such as breathlessness, joint pain or poor sleep.
- Linked conditions and family history.
- Your current medicines, as some can cause weight gain.
- Recent blood tests, or which tests may help. See blood tests before weight-loss treatment.
- Your sleep, stress, work, food habits and activity.
- Your goals, and the support that suits you.
Options may include habit changes, referral, or medicines. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines. Any prescription is a private electronic prescription under Spanish law. GLP-1 medicines are not used in pregnancy or breastfeeding. Under-18s need care led by a specialist team.
When you need in-person care
A video visit covers a lot, but not everything. You may need to be seen in person if:
- You need a physical exam, for example of the heart, lungs, skin or joints.
- The doctor suspects a hormone cause, such as thyroid or adrenal problems, that needs tests or a specialist.
- You may need weight-loss surgery. This needs a hospital specialist team.
- You are pregnant, or the patient is a child or teenager.
- You have symptoms such as chest pain, severe breathlessness or fainting. For these, call 112.
Frequently asked questions
What BMI counts as obesity?
For most adults, a BMI of 30 or more is the WHO cut-off for obesity. For people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, NICE uses a lower cut-off of 27.5.
Is BMI accurate for everyone?
No. BMI does not tell fat from muscle, and it does not show where fat sits. It is a useful first check, but doctors add other measures, such as waist size.
How do I work out my waist-to-height ratio?
Measure your waist and your height in the same units, then divide waist by height. NICE says a result of 0.5 or more points to higher health risk, and 0.6 or more to a further raised risk.
What is the difference between clinical and preclinical obesity?
In the 2025 Lancet Commission, clinical obesity means excess body fat is already harming how organs work or limiting daily life. Preclinical obesity means excess body fat with organs still working normally, but with a higher future risk.
Does a high BMI mean I need medication?
No. BMI alone does not decide treatment. Where clinically appropriate, following a genuine evaluation. Treatment and prescriptions are issued solely at the doctor's discretion based on evidence-based clinical guidelines.
Can obesity be assessed by video?
Much of it can: your history, symptoms, home measurements and blood tests. Some people need an in-person exam, and the doctor will tell you if so.
Sources
- World Health Organization. Obesity and overweight fact sheet
- NICE guideline NG246. Identifying and assessing overweight, obesity and central adiposity
- Rubino F et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol, 2025 (PubMed)
- Rubino F et al. Joint international consensus statement for ending stigma of obesity. Nat Med, 2020 (PubMed)
- World Health Organization. Guideline on the use of GLP-1 medicines in treating obesity, December 2025
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Medical Citations & Sources
This content is written and reviewed by a licensed physician with reference to the following official health authorities.
