Obesity

What is obesity?

Obesity is a chronic, relapsing and multifactorial disease involving excessive fat accumulation that can impair health. It is not a character flaw or simply a cosmetic concern. Genetics, neurobiology, sleep, medicines, health conditions, stress, food access, marketing, income, neighbourhood design and other social and environmental factors can all influence weight, alongside eating and activity patterns.

People living with obesity have a higher average risk of conditions such as type 2 diabetes, cardiovascular disease, sleep apnoea, fatty liver disease, osteoarthritis and some cancers. These are increased risks, not certainties. A respectful clinical assessment should focus on the person’s health, function, circumstances and goals rather than blame or appearance.

WHO reported that one in eight people worldwide were living with obesity in 2022. The condition affects people in every region and requires both individual healthcare and broader measures that make nutritious food, physical activity and treatment more accessible.

Why obesity develops

Body weight is regulated by complex biological systems that influence hunger, fullness and energy use. Some people have a stronger inherited susceptibility. Sleep deprivation, certain medicines, chronic pain, depression and other mental-health conditions, pregnancy-related changes, menopause and conditions such as hypothyroidism or Cushing syndrome may also be relevant. However, one symptom or laboratory result rarely explains a person’s whole weight history.

A clinician should review the pattern and timing of weight change, current medicines, sleep, mental health, eating concerns, mobility, family history and symptoms of possible contributing conditions. Not every symptom in a person living with obesity should be attributed to weight; doing so can delay the correct diagnosis.

Assessment

For adults, body mass index (BMI) is calculated as weight in kilograms divided by height in metres squared (kg/m²). WHO uses 25 to 29.9 kg/m² for overweight and 30 kg/m² or above for obesity in most adults. NICE further describes obesity classes 1, 2 and 3 beginning at BMI values of 30, 35 and 40 kg/m².

BMI is a practical screening measure, not a direct measurement of body fat or a complete diagnosis of health. It needs cautious interpretation in older adults and people with high muscle mass. Cardiometabolic risk may occur at lower BMI thresholds in some ethnic groups. In adults with BMI below 35 kg/m², waist-to-height ratio can add information about central fat distribution; a clinician can explain how to measure and interpret it appropriately.

Children and adolescents must be assessed with age- and sex-specific growth references rather than adult thresholds. Growth, development, family context and wellbeing all matter, and weight-focused treatment should avoid stigma and disordered eating.

Health evaluation

Blood tests do not diagnose body size. They may help identify complications or possible contributors—for example, glucose or HbA1c, lipids, liver tests or thyroid testing when clinically indicated. Blood pressure, sleep symptoms, reproductive health, joint pain, mental health and the effect of weight on daily activities may also be reviewed. Testing should be individualized rather than presented as a universal panel.

Principles of treatment

Care should be collaborative, non-judgemental and based on informed choice. Useful goals can include better mobility, sleep, blood pressure, glucose control or quality of life, whether or not a particular number appears on the scale. Weight regain is common because biological adaptations oppose weight loss; it should not be described as personal failure.

Food and nutrition

No single named diet is best for everyone. A sustainable plan can reduce energy intake while remaining nutritionally balanced and suited to culture, finances, preferences, allergies, health conditions and family life. Different approaches—including lower-fat, lower-carbohydrate or other structured patterns—may work when they are safe and supportable. Cutting out all carbohydrate, prescribing a mandatory high-protein breakfast or presenting coconut oil as a weight-loss treatment is not justified.

Practical changes may include more vegetables, fruit, pulses, whole grains and appropriate protein foods; choosing mainly unsaturated fats; and reducing sugar-sweetened drinks and highly energy-dense foods where relevant. People with diabetes, kidney disease, gastrointestinal disease, pregnancy, food insecurity or a history of disordered eating need tailored advice. Very-low-energy diets should be reserved for selected adults in supervised specialist programmes and are not a routine long-term strategy.

Physical activity, sleep and behaviour support

Physical activity improves cardiovascular fitness, strength, mobility, mood and metabolic health even without weight loss. Activity should build gradually from the person’s current ability and may include walking, cycling, swimming, resistance exercise, chair-based activity or other preferred movement. A rigid requirement to attend a gym or immediately perform 300 minutes each week can be unsafe and discouraging.

Support may also address sleep, stress, pain, time constraints, food access, self-monitoring preferences and behaviour patterns. Tracking food or weight helps some people but can be distressing or counterproductive for others. Plans should be reviewed and adjusted rather than imposed.

Medicines

Weight-management medicines may be considered alongside nutrition, activity and behavioural support after individual assessment. Options differ by country and may include orlistat or medicines based on gut hormones, such as liraglutide, semaglutide or tirzepatide, for people who meet local eligibility criteria. Choice depends on health conditions, other medicines, potential adverse effects, pregnancy plans, cost and availability. Monitoring and stopping rules are medicine-specific.

WHO’s 2025 guidance makes a conditional recommendation for long-term GLP-1 therapy in adults living with obesity, excluding pregnant women, within comprehensive care. These medicines are not suitable for everyone and should not be bought from unverified sources. Lorcaserin is not a current US treatment option: the FDA requested its withdrawal in 2020 after a safety trial found a cancer signal.

Metabolic and bariatric surgery

Surgery can be effective for selected people, but it is not defined by one universal BMI rule. Local criteria govern referral. NICE, for example, recommends specialist multidisciplinary assessment for adults with BMI of 40 kg/m² or more, or 35 to 39.9 kg/m² with a significant condition that could improve with weight loss, together with willingness to undertake long-term follow-up. Lower assessment thresholds may apply for some ethnic backgrounds or for recent-onset type 2 diabetes.

Procedures include sleeve gastrectomy, gastric bypass and other carefully selected operations. Choice requires discussion of expected benefits, surgical and anaesthetic risks, pregnancy plans, eating and mental-health needs, nutritional consequences and personal preferences. Follow-up includes dietary support, monitoring for vitamin and mineral deficiencies, appropriate supplementation and at least annual long-term review after specialist discharge.

Reducing health risks and preventing excess weight gain

Prevention is not limited to telling individuals to “eat less and move more.” Helpful measures include access to affordable nutritious food, safe places for activity, adequate sleep, supportive healthcare, review of weight-promoting medicines where alternatives exist, and policies that reduce unhealthy food marketing and inequality.

For an individual, small sustainable changes can improve health. Seek professional support if weight changes rapidly or unexpectedly, eating feels out of control, weight-related stigma is affecting care, or a health condition limits activity or food choices. Treatment should protect dignity, avoid extreme restriction and support long-term health rather than promise a quick cure.