Overweight and Obesity
A clinical overview of overweight and obesity — how they are defined and assessed.
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Overweight and Obesity: Causes, Health Risks and Treatment in the UK
A clinical overview of overweight and obesity — how they are defined and assessed, what causes them, the associated health risks, and the UK treatment options including lifestyle, medication and bariatric surgery.
Key fact: Obesity is now recognised as a complex chronic condition, not a lifestyle choice. In the UK around 26% of adults are living with obesity and a further 38% are overweight. Losing just 5–10% of body weight produces meaningful improvements in blood pressure, cholesterol, blood sugar, and risk of heart disease, type 2 diabetes and certain cancers.
What Are Overweight and Obesity?
Overweight and obesity are conditions in which excess body fat has accumulated to a level that increases the risk of health problems. In the UK, a body mass index (BMI) of 25 to 29.9 is classed as overweight, and a BMI of 30 or above is classed as obesity.
They are not simply about appearance — they are recognised by NICE, NHS England and the World Health Organization as a complex chronic condition that increases the risk of more than 200 health problems.
Obesity is not caused by lack of willpower. It is driven by an interaction of genetics, hormones, environment, food supply, psychology, sleep, medications, and physical activity. Around 40 to 70% of the variation in body weight between people is explained by genetic factors. This is why treatment increasingly combines medical, behavioural and pharmacological approaches — not just dietary advice.
Language matters. NICE, the Royal College of Physicians and patient organisations recommend person-first language: “a person living with obesity” rather than “an obese person”. The condition does not define the person.
How Are Overweight and Obesity Assessed?
UK clinicians use a combination of measurements to assess weight-related health risk. Body mass index (BMI) is the starting point but is not used in isolation.
Check your BMI: use our free BMI calculator to work out your BMI in seconds and see which category you fall into — including the adjusted thresholds NICE recommends for some ethnic backgrounds.
| BMI (kg/m²) | Category | Notes |
|---|---|---|
| Below 18.5 | Underweight | May indicate undernutrition; needs assessment |
| 18.5–24.9 | Healthy weight | Range associated with lowest health risk |
| 25.0–29.9 | Overweight | Lower thresholds for some ethnic groups |
| 30.0–34.9 | Obesity class I | Eligibility for prescription weight loss treatment may begin here |
| 35.0–39.9 | Obesity class II | Bariatric surgery may be considered with comorbidities |
| 40.0 and above | Obesity class III | Bariatric surgery routinely considered |
Important: BMI is not perfect. Very muscular people may have a high BMI without being overfat. People of South Asian, Chinese, Middle Eastern, Black African and African-Caribbean backgrounds develop weight-related health problems at lower BMIs — NICE recommends lower thresholds (typically 23 for overweight, 27.5 for obesity) for these groups. Older adults, pregnant women and children all need different assessment approaches.
Waist circumference measures abdominal fat, which carries greater health risk than fat elsewhere. NICE risk thresholds:
- Women: over 80cm — raised risk; over 88cm — high risk
- Men: over 94cm — raised risk; over 102cm — high risk
- South Asian men: over 90cm — high risk (lower than European threshold)
A useful additional measure is waist-to-height ratio: aim for waist circumference less than half your height. This works across all ethnic groups and is easier to remember.
What Causes Weight Gain?
Weight gain happens when energy intake exceeds energy expenditure over time. But this simple equation hides a complex picture of underlying drivers — explored in more depth in our guide to the causes and health risks of weight gain:
Genetics
40–70% of body weight variation is genetic. Family history of obesity strongly increases risk. Rare genetic syndromes account for a small minority of severe early-onset obesity.
Hormonal causes
Underactive thyroid, polycystic ovary syndrome (PCOS), Cushing’s syndrome, and growth hormone deficiency can all cause weight gain. Worth screening for in unexplained weight change.
Medications
Some antidepressants (especially mirtazapine, olanzapine), corticosteroids, antipsychotics, insulin, some beta-blockers, and pregabalin can promote weight gain.
Mental health
Depression, anxiety, chronic stress, disordered eating (including binge eating disorder) and trauma all interact with weight regulation.
Sleep
Sleeping less than 6–7 hours a night is associated with higher BMI and increased appetite hormones (ghrelin up, leptin down).
Food environment
Availability of energy-dense ultra-processed food, large portions, and food marketing all contribute to overconsumption at population level.
Physical activity
Modern life involves less daily movement than at any time in human history. Sedentary occupations and screen time both contribute.
Life transitions
Pregnancy, menopause, retirement, stopping smoking, and starting medication are common triggers for weight gain.
What Are the Health Risks of Obesity?
Obesity substantially raises the risk of more than 200 health conditions. The risk rises with increasing BMI and with abdominal fat distribution. Major associations include:
- Type 2 diabetes — the strongest association. Obesity accounts for the majority of type 2 diabetes risk.
- Cardiovascular disease — high blood pressure, heart attack, heart failure, stroke.
- Cancer — obesity is now the second-largest preventable cause of cancer in the UK after smoking. Strong links with breast, bowel, womb, kidney, pancreatic, oesophageal and liver cancer.
- Sleep apnoea — obstructive sleep apnoea is strongly linked with central obesity and neck circumference.
- Non-alcoholic fatty liver disease — can progress to cirrhosis and liver failure.
- Osteoarthritis — particularly affecting knees and hips, both from mechanical load and inflammatory effects of fat tissue.
- Mental health — obesity is associated with depression and anxiety, in both directions.
- Fertility — reduced fertility in both men and women; greater risk in pregnancy.
- Gout, gallstones, kidney disease, dementia risk, asthma severity — all higher.
The good news: losing 5 to 10% of body weight produces meaningful reductions in many of these risks — including blood pressure, cholesterol, blood sugar, fatty liver, sleep apnoea severity, and joint pain.
How Are Overweight and Obesity Treated?
NICE NG246 (2022) sets out a stepped approach to weight management in adults. The approach combines lifestyle measures (everyone), medication (eligibility-based), and surgery (severe obesity or class II with comorbidities). Treatment is now understood as long-term — weight regain is common when interventions stop, so weight management is increasingly viewed as ongoing care, not a finite intervention.
Lifestyle measures
First-line for everyone. Dietary changes, physical activity, behavioural support. May be all that’s needed for some, especially with BMI under 30.
Prescription medication
Considered for adults with BMI 30+ (or 27.5+ with comorbidities; lower for some ethnic groups). Options: tirzepatide, semaglutide, orlistat.
Bariatric surgery
Considered for severe obesity (BMI 40+) or BMI 35–40 with significant comorbidities when other measures have not worked.
Lifestyle Measures
Lifestyle interventions remain the foundation of weight management. Effective programmes combine three elements:
- Dietary changes — aim for a 500–600 kcal daily deficit. The Mediterranean dietary pattern, lower-carbohydrate approaches, and very-low-calorie meal replacement plans all have evidence. The best diet is the one a person can sustain.
- Physical activity — aim for 150 minutes of moderate activity (or 75 minutes vigorous) per week, plus strength activities twice weekly. Activity supports weight maintenance more than weight loss; both matter.
- Behavioural support — goal setting, self-monitoring, problem-solving, and dealing with relapse. Tier 2 NHS weight management services provide this for around 12 weeks.
NHS-funded weight management programmes are available in most areas, often via GP referral. The NHS Digital Weight Management Programme and Tier 2/3 services can be effective starting points. For practical, realistic strategies, see our guide to healthy weight management for busy adults.
Weight Loss Medication
Prescription weight loss medication is recommended by NICE for adults whose BMI meets the threshold and who have not lost enough weight with lifestyle measures alone. For a full introduction, see What Is Weight Loss Medication? and our UK guide to prescription options. The main options are:
| Medication | How it works | Average weight loss |
|---|---|---|
| Tirzepatide (Mounjaro) | Weekly injection. Dual GIP and GLP-1 receptor agonist. Reduces appetite and slows gastric emptying. | ~15–21% over 72 weeks (SURMOUNT trial) |
| Semaglutide (Wegovy injection or Wegovy tablets) | Weekly injection or daily oral tablet. GLP-1 receptor agonist. Reduces appetite. | ~12–15% over 68 weeks (STEP trial) |
| Orlistat (Xenical) | Oral capsule taken with meals. Blocks dietary fat absorption. | ~3% on top of lifestyle (over a year) |
Tirzepatide is currently the most effective licensed option — our guides explain what Mounjaro is, how it works, the UK dose schedule and what to eat while taking it. If you are weighing up semaglutide, see What Are Wegovy Tablets? and Wegovy Tablets vs Mounjaro.
Eligibility: usually BMI 30 or above, or 27.5 or above with at least one weight-related comorbidity (such as type 2 diabetes, high blood pressure, sleep apnoea, or cardiovascular disease). Lower BMI thresholds apply for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background.
Important considerations: GLP-1 medications (tirzepatide, semaglutide) are not suitable for people with a personal or family history of medullary thyroid carcinoma or multiple endocrine neoplasia type 2 (MEN2). They are not licensed in pregnancy and should be stopped before conception. Common side effects (especially when starting or after dose increases) include nausea, vomiting, diarrhoea, constipation, and reflux — usually mild and improving with time (see our full guide to Mounjaro side effects). Serious adverse effects (pancreatitis, gallbladder problems) are uncommon. Stopping treatment typically leads to weight regain unless lifestyle change has been embedded.
Bariatric Surgery
Bariatric (weight loss) surgery is considered for adults with severe obesity when other measures have not worked. NICE recommends consideration for:
- BMI 40 or above (any comorbidity status)
- BMI 35–40 with a significant weight-related health condition that could improve with weight loss (such as type 2 diabetes, sleep apnoea, severe cardiovascular disease)
- Lower thresholds in adults from certain ethnic backgrounds, in line with the lower-BMI approach for obesity
The most common procedures are sleeve gastrectomy and gastric bypass. Both produce around 25–35% sustained weight loss and substantial improvements in obesity-related conditions, particularly type 2 diabetes. Bariatric surgery is a major operation with lifelong follow-up requirements and is provided via NHS specialist centres or accredited private providers.
When Should You See a Doctor About Your Weight?
Consider speaking to a clinician if:
- Your BMI is 30 or above (27.5 or above for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds)
- Your weight is affecting day-to-day activities, mobility, or mental health
- You have a weight-related health condition (type 2 diabetes, high blood pressure, sleep apnoea, fatty liver, joint problems)
- You have unexplained weight gain — particularly if rapid or accompanied by tiredness, low mood, hair changes, or menstrual changes (may indicate thyroid or hormonal problems)
- Lifestyle measures haven’t worked and you want to discuss medical options
- You are considering, or have been advised about, weight loss medication or surgery
An online consultation can be a convenient starting point for assessing eligibility for prescription weight loss medication, with a GPhC-registered pharmacist independent prescriber.
Weight Loss Guides
In-depth guides on specific aspects of weight loss and weight loss medication:
Frequently Asked Questions
What counts as overweight or obesity in the UK?
In the UK, body mass index (BMI) is the standard screening tool. A BMI of 18.5 to 24.9 is considered a healthy weight. A BMI of 25 to 29.9 is classed as overweight. A BMI of 30 or above is classed as obesity, with three subcategories: class I (BMI 30–34.9), class II (BMI 35–39.9), and class III (BMI 40 or above). NICE guidelines recommend lower thresholds for adults of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, who develop weight-related health problems at lower BMIs.
What causes weight gain and obesity?
Weight gain happens when energy intake (food and drink) consistently exceeds energy expenditure (activity and metabolism). However, the underlying drivers are complex and include genetics (around 40–70% of body weight variation is genetic), hormonal changes (thyroid problems, polycystic ovary syndrome, Cushing’s syndrome), medications (some antidepressants, steroids, insulin, antipsychotics), mental health (stress, depression, disordered eating), sleep problems, the food environment, socio-economic factors, and reduced physical activity. Obesity is not simply a matter of willpower — it is recognised as a complex chronic condition.
What are the health risks of obesity?
Obesity substantially increases the risk of type 2 diabetes, high blood pressure, cardiovascular disease, stroke, certain cancers (including breast, bowel, womb, oesophageal and kidney cancer), obstructive sleep apnoea, non-alcoholic fatty liver disease, osteoarthritis (particularly knees and hips), fertility problems, depression, and gout. The risks rise with increasing BMI and with central (abdominal) fat distribution. Losing 5–10% of body weight produces meaningful reductions in many of these risks.
How is weight loss treated medically in the UK?
NICE NG246 (2022) sets out a stepped approach. Lifestyle measures are first-line for everyone — dietary changes (a 500–600 kcal daily deficit), physical activity, and behavioural support. Prescription medication may be considered for adults with a BMI of 30 or more, or 27.5 or more with weight-related comorbidities (lower thresholds for some ethnic groups). Options include tirzepatide (Mounjaro), semaglutide (Wegovy) and orlistat (Xenical). Bariatric surgery is considered for severe obesity (BMI 40 or above, or 35–40 with significant comorbidities) when other measures have not worked.
What is Mounjaro and how does it work?
Mounjaro is the brand name for tirzepatide, a once-weekly injection that activates two gut hormone receptors: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). This dual mechanism reduces appetite, slows stomach emptying, and improves insulin sensitivity. In the SURMOUNT trials, average weight loss was approximately 15–21% of body weight over 72 weeks at therapeutic doses, depending on dose. Mounjaro is licensed in the UK for chronic weight management in adults with obesity, or overweight with at least one weight-related comorbidity. It must be prescribed by a qualified prescriber following clinical assessment.
How much weight can I expect to lose with treatment?
Average weight loss varies by treatment. Intensive lifestyle programmes typically achieve 5–10% weight loss in the first year. Orlistat adds around 3% on top of lifestyle changes. Semaglutide (Wegovy) achieves around 12–15% average weight loss over a year. Tirzepatide (Mounjaro) achieves around 15–21% over 72 weeks at therapeutic doses. Bariatric surgery typically produces 25–35% sustained weight loss. Results vary widely between individuals — some respond strongly, some less so. Weight regain is common when treatment stops, so weight management is increasingly viewed as a long-term condition requiring ongoing care.
Is BMI a fair measure for everyone?
BMI is a useful screening tool but has important limitations. It does not distinguish between muscle and fat, so very muscular people may have a high BMI without being overfat. It does not account for fat distribution, even though abdominal fat carries greater health risk. BMI thresholds were derived from European populations and underestimate risk in people of South Asian, Chinese, Middle Eastern, Black African and African-Caribbean backgrounds — NICE uses lower BMI thresholds for these groups. Older adults, pregnant women, and children also need different assessment approaches. Waist circumference and waist-to-height ratio are useful additional measures.
Is obesity genetic?
Genetics play a substantial role — an estimated 40–70% of the variation in body weight between people is inherited. Genes influence appetite, fullness signalling, fat storage and metabolic rate, which is why obesity often runs in families. However, genes are not destiny: they interact with the environment, and lifestyle changes, medication and surgery remain effective in people with a strong family history of obesity.
Can obesity be reversed?
Obesity can be treated effectively, but it is best understood as a long-term condition to be managed rather than cured. Losing 5–10% of body weight produces meaningful improvements in blood pressure, blood sugar, cholesterol and fatty liver. Intensive lifestyle programmes, medications such as tirzepatide and semaglutide, and bariatric surgery can all produce substantial weight loss, but weight regain is common when treatment stops, so ongoing support matters.
What is class 3 obesity?
Class 3 obesity (previously called morbid obesity) means a BMI of 40 or above. It carries the highest risk of weight-related complications such as type 2 diabetes, heart disease and obstructive sleep apnoea. NICE recommends that bariatric surgery is routinely considered at this level, alongside medication and specialist weight management support.
Speak to a UK Prescriber About Your Weight
If your weight is affecting your health and lifestyle measures haven’t produced the result you need, a short online consultation with our GPhC-registered pharmacist independent prescribers can help you access the right treatment.
View Weight Loss Treatments →Browse: Mounjaro · Wegovy · Wegovy Tablets · Orlistat · Xenical
References
- NICE NG246. Obesity: identification, assessment and management. 2022. nice.org.uk/guidance/ng246
- NICE TA1026. Tirzepatide for managing overweight and obesity. nice.org.uk/guidance/ta1026
- NICE TA875. Semaglutide for managing overweight and obesity. nice.org.uk/guidance/ta875
- NHS. Obesity. nhs.uk/conditions/obesity
- Office for Health Improvement and Disparities. Adult obesity statistics. gov.uk/government/statistics
- SURMOUNT-1 trial. Tirzepatide once weekly for the treatment of obesity. N Engl J Med 2022;387:205-216.
- STEP-1 trial. Once-weekly semaglutide in adults with overweight or obesity. N Engl J Med 2021;384:989-1002.
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Always consult a qualified healthcare professional for diagnosis and treatment. In a medical emergency, call 999.
