Obesity

What it is

Obesity is excess body fat that raises health risk; in adults, BMI 30 kg/m² or higher is the usual screening cutoff. The most widely used screening number is BMI 30 kg/m², while BMI 25 to 29.9 kg/m² is usually called overweight. Obesity is not just a matter of appearance or willpower. It is a chronic, relapsing health condition shaped by biology, environment, diet, physical activity, sleep, medicines, stress, and social factors. WHO notes that obesity in adults has risen sharply worldwide, with the global share of adults living with obesity increasing from 7% in 1990 to 16% in 2022.

BMI is useful for population screening, but it does not directly measure body fat or show where fat is stored. Fat around the abdomen, often reflected by a larger waist circumference, is especially linked with higher risk of type 2 diabetes, heart disease, fatty liver disease, and other complications. Some people with a BMI below 30 may still have high metabolic risk, while some muscular people may have a high BMI without excess body fat.

A simple comparison is:

Category	Adult BMI
Healthy range	18.5 to 24.9 kg/m²
Overweight	25.0 to 29.9 kg/m²
Obesity	30.0 kg/m² or higher

In India and other South Asian populations, clinicians often pay closer attention to waist size and metabolic risk because diabetes and cardiovascular disease can occur at lower BMI levels than in many Western populations.

How it works

Obesity develops when energy intake exceeds energy use over time, but the biology is more complex than a simple calorie equation. The body regulates appetite, fullness, and energy expenditure through hormones and brain pathways. Signals involving insulin, leptin, gut hormones, and reward circuits can make weight gain easier and weight loss harder.

Genetics also matter. Some people are more prone to gaining weight in the same food environment. Sleep deprivation, chronic stress, depression, shift work, and some medicines can increase appetite or reduce energy expenditure. Medicines that may contribute in some people include certain antipsychotics, antidepressants, steroids, insulin, and some seizure medicines.

Fat tissue is biologically active. Excess adipose tissue can promote inflammation, insulin resistance, abnormal blood lipids, and hormonal changes. Visceral fat, which surrounds internal organs, is more strongly linked to metabolic disease than fat stored under the skin.

Evidence and uses

Obesity is associated with a wide range of health problems, and risk generally rises as BMI and waist size increase. WHO and NIDDK list major complications including type 2 diabetes, high blood pressure, heart disease, stroke, fatty liver disease, sleep apnea, osteoarthritis, kidney disease, fertility and pregnancy complications, and several cancers.

A large UK cohort study involving about 2.9 million people found that higher BMI was linked with higher risk of many obesity-related conditions. This kind of large observational evidence cannot prove that BMI alone causes every outcome, but it strongly supports the broader evidence that excess adiposity harms health.

Common obesity-related conditions include:

1. Metabolic disease: type 2 diabetes, metabolic syndrome, dyslipidemia, fatty liver disease.
2. Cardiovascular disease: hypertension, coronary artery disease, stroke, heart failure.
3. Mechanical effects: osteoarthritis, back pain, sleep apnea, reduced mobility.
4. Reproductive and pregnancy effects: infertility, polycystic ovary syndrome, gestational diabetes, preeclampsia.
5. Cancer risk: higher risk of several cancers, including endometrial, postmenopausal breast, kidney, liver, colon, and others.

Treatment usually combines lifestyle support, management of related conditions, and sometimes anti-obesity medicines or bariatric surgery. Lifestyle treatment includes changes in eating pattern, physical activity, sleep, and behavior support. WHO recommends at least 150 minutes of moderate-intensity physical activity per week for adults and 60 minutes per day for children.

For some adults, prescription weight-loss medicines may be appropriate when BMI is high or when obesity-related complications are present. Bariatric or metabolic surgery can be considered for selected patients with severe obesity or obesity with major complications. These treatments can improve diabetes, blood pressure, sleep apnea, and quality of life, but they require medical follow-up.

Diagnosis / how it's measured

Diagnosis starts with history, examination, and measurement. BMI is calculated as weight in kilograms divided by height in meters squared. Clinicians often also measure waist circumference because abdominal fat adds risk beyond BMI alone.

Assessment may include:

Measure	What it helps show
BMI	Screening category based on weight and height
Waist circumference	Central or abdominal fat distribution
Blood pressure	Cardiometabolic risk
Blood tests	Glucose or HbA1c, lipids, liver function, thyroid testing when indicated
Sleep and symptom review	Risk of sleep apnea, depression, eating disorders, medication effects

Children and adolescents are assessed differently. In them, obesity is usually defined using age- and sex-specific BMI-for-age growth charts rather than adult BMI cutoffs.

BMI has limits. It does not distinguish fat from muscle, and it may underestimate risk in some South Asian people. That is why clinicians look at the whole picture, including waist size, family history, blood sugar, blood pressure, and lifestyle factors.

When to see a clinician

See a clinician if your weight is rising steadily, your BMI is in the overweight or obesity range, or you have a large waist circumference along with high blood pressure, high blood sugar, snoring, daytime sleepiness, joint pain, irregular periods, or fatty liver. Medical review is also important if a child is gaining weight rapidly or crossing growth percentiles.

You should also seek care if weight gain began after starting a new medicine, if you have symptoms of an endocrine disorder, or if repeated dieting has led to binge eating, depression, or major distress. Obesity care should be nonjudgmental and focused on health outcomes, not blame.

Limitations and open questions

BMI is a practical screening tool, but it is not a direct measure of body fat or health. It can miss high-risk fat distribution and can misclassify very muscular people. Researchers are still refining how best to combine BMI, waist measures, body composition, and metabolic markers.

There is also ongoing debate about the best long-term treatment strategy for different patients. Newer medicines can produce substantial weight loss, but access, cost, side effects, and long-term maintenance remain important questions. Evidence is strong that obesity raises risk for many diseases, but individual risk varies by age, genetics, ethnicity, fat distribution, fitness, and existing health conditions.

For India, one open issue is how best to apply risk thresholds in a population where metabolic complications often appear at lower BMI. This is why clinicians may act on waist size, diabetes risk, and other markers even when BMI is below the standard obesity cutoff.