Obesity

How obesity is measured

BMI

BMI is weight in kilograms divided by height in meters squared (or weight in pounds times 703, divided by height in inches squared).

For people of Asian descent, health risks rise at lower BMIs, and some guidelines use 23 to 27.5 as cut-offs.

Beyond BMI

BMI does not measure fat directly; muscular people can have a high BMI. Doctors also consider:

• waist circumference: higher risk above 102 cm (40 in) in men and 88 cm (35 in) in women
• waist-to-height ratio: risk rises when waist is more than half of height
• health conditions linked to excess weight
• body fat distribution: fat around the abdomen (visceral fat) is more harmful than fat on hips and thighs

In children

Obesity in children is defined as a BMI at or above the 95th percentile for age and sex on growth charts.

Causes of obesity

Body weight is regulated by the brain, hormones and gut signals that control hunger and fullness. Many factors push this system toward storing more fat:

• genetics: 40% to 70% of differences in weight between people are inherited
• food environment: cheap, calorie-dense, highly processed foods and large portions
• physical inactivity and long periods of sitting
• short or poor-quality sleep
• chronic stress and depression
• medicines: some antidepressants, antipsychotics, steroids, insulin, sulfonylureas, some anti-seizure medicines and beta-blockers
• medical conditions: hypothyroidism, Cushing syndrome, polycystic ovary syndrome, and rare genetic syndromes such as Prader-Willi
• life stages: pregnancy, menopause, quitting smoking
• social factors: income, food access and safe places to be active

When people lose weight, the body responds by increasing hunger and reducing energy use, which makes regaining weight common. This biology is why obesity is considered a chronic disease.

Health risks

Diagnosis and assessment

A doctor will measure weight, height, BMI, waist and blood pressure, and ask about weight history, diet, activity, sleep, mood, medicines and family history. Tests often include:

• blood sugar (HbA1c) and cholesterol
• liver and kidney function
• thyroid function if symptoms suggest it
• sleep study if sleep apnea is suspected

Treatment

Treatment is individualized and long-term. Even modest weight loss brings health benefits.

Nutrition

• a calorie deficit is necessary for weight loss; many dietary patterns work, including Mediterranean, lower-carbohydrate and plant-based approaches
• emphasize vegetables, fruit, whole grains, legumes, lean proteins and healthy fats
• reduce sugary drinks, ultra-processed foods and alcohol
• the best diet is one you can sustain; a registered dietitian can help

Physical activity

• aim for at least 150 to 300 minutes of moderate activity a week, plus strength training twice a week
• activity is especially important for maintaining weight loss and protecting muscle

Behavioral support

Intensive programs with regular contact, goal setting, self-monitoring and problem-solving produce more weight loss than advice alone. Treating sleep problems, stress and depression helps.

Medicines

Anti-obesity medicines are recommended for adults with a BMI of 30 or more, or 27 or more with weight-related conditions, alongside lifestyle changes:

• GLP-1 and dual GIP/GLP-1 receptor agonists: semaglutide and tirzepatide, weekly injections that reduce appetite; average weight loss of around 15% to 20% in trials; semaglutide also reduces heart attack and stroke risk in people with heart disease
• liraglutide daily injection
• phentermine-topiramate, naltrexone-bupropion and orlistat tablets
• setmelanotide for specific rare genetic obesity

Common side effects of GLP-1 medicines are nausea, vomiting, diarrhea and constipation. Weight usually returns if the medicine is stopped, so treatment is often long-term.

Metabolic and bariatric surgery

Surgery is considered for BMI of 35 or more, or 30 to 34.9 with metabolic disease such as type 2 diabetes. Options include:

• sleeve gastrectomy: removes most of the stomach
• Roux-en-Y gastric bypass: creates a small stomach pouch connected to the small intestine

Surgery typically produces 25% to 30% total weight loss, and often remission of type 2 diabetes. It requires lifelong vitamin supplements and follow-up.

Children and teenagers

Family-based lifestyle treatment is the foundation. The American Academy of Pediatrics recommends that adolescents 12 and older with obesity be offered medicines, and those with severe obesity be referred for evaluation for surgery.

Weight stigma

People with obesity often face bias, including in healthcare, which can delay care and worsen health. Using respectful language, such as "person with obesity", and focusing on health rather than appearance improve care.

Prevention

• regular physical activity and limited screen time
• whole-food diets and fewer sugary drinks
• adequate sleep
• breastfeeding and healthy feeding routines in infancy
• community changes that make healthy food and safe activity accessible

Frequently asked questions

What BMI is considered obese?

In adults, a BMI of 30 or higher is classed as obesity, with 40 or higher classed as severe obesity. A BMI of 25 to 29.9 is overweight. Lower thresholds are used for some Asian populations because health risks begin at lower BMIs.

What causes obesity?

Obesity results from many factors: genes, an environment with abundant calorie-dense food, physical inactivity, poor sleep, stress, certain medicines and some medical conditions. The body's weight-regulating systems also resist weight loss, making it a chronic condition rather than a simple matter of willpower.

How much weight do I need to lose to improve my health?

Losing 5% to 10% of your body weight can lower blood pressure, blood sugar and cholesterol, reduce sleep apnea and joint pain, and cut the risk of developing type 2 diabetes. For a 100 kg (220 lb) person, that is 5 to 10 kg (11 to 22 lb).

Do weight-loss injections work?

GLP-1-based medicines such as semaglutide and tirzepatide produce substantial weight loss in most people, often 15% or more, when combined with lifestyle changes. Side effects are mainly digestive. Weight tends to return if they are stopped, so they are usually used long-term.

Is bariatric surgery safe?

Modern bariatric surgery, usually done by keyhole techniques, has a low risk of serious complications in experienced centers. It leads to large, lasting weight loss and often improves or resolves diabetes, high blood pressure and sleep apnea. Lifelong vitamin supplements and follow-up are needed.