The World Health Organization (WHO) defines obesity as a BMI of 30 or greater in adults and classifies it as a chronic, complex disease driven by environmental, genetic, and behavioral factors. It is one of the fastest-growing health challenges worldwide, and the WHO treats it as a leading preventable risk factor for heart disease, diabetes, and certain cancers.
How the WHO Defines Obesity in Adults
The WHO uses body mass index, a ratio of weight to height, as its primary screening tool. While BMI doesn’t directly measure body fat, it provides a consistent threshold that can be applied across populations. The standard adult categories are:
- Underweight: BMI below 18.5
- Healthy weight: 18.5 to 24.9
- Overweight: 25 to 29.9
- Class 1 obesity: 30 to 34.9
- Class 2 obesity: 35 to 39.9
- Class 3 (severe) obesity: 40 or greater
These thresholds are the same ones used by the CDC, most national health systems, and clinical guidelines around the world. The three-class system matters because health risks increase with each tier. Someone with a BMI of 31 faces a different risk profile than someone at 42, and treatment recommendations differ accordingly.
Definitions for Children and Adolescents
Because children are still growing, the WHO doesn’t use fixed BMI cutoffs for them. Instead, it compares a child’s measurements against growth reference charts and uses standard deviations from the median to flag problems. The thresholds differ by age group.
For children under 5, overweight is defined as a weight-for-height more than 2 standard deviations above the WHO growth standard median. Obesity is more than 3 standard deviations above. For children and adolescents aged 5 to 19, the measure shifts to BMI-for-age: overweight starts at more than 1 standard deviation above the median, and obesity at more than 2. In practical terms, this means a child who is significantly heavier than the vast majority of healthy children the same age and height would meet the obesity threshold.
Health Risks Linked to Obesity
The WHO identifies obesity as one of four key metabolic risk factors for noncommunicable diseases, alongside high blood pressure, elevated blood sugar, and abnormal cholesterol. Together, these metabolic changes account for the bulk of premature deaths from chronic illness globally.
The conditions most strongly tied to obesity include cardiovascular diseases (heart attacks and stroke), type 2 diabetes, certain cancers, and chronic respiratory conditions like asthma. Excess body fat also raises the risk of joint problems, sleep apnea, fatty liver disease, and mental health conditions including depression. Cardiovascular disease remains the single largest cause of premature death among noncommunicable diseases, and obesity is a major contributor to the high blood pressure, blood sugar, and cholesterol levels that drive it.
What Drives the Obesity Epidemic
The WHO frames obesity as far more than an individual willpower problem. The organization points to what researchers call an “obesogenic environment,” where the food supply, urban design, and economic pressures make it difficult for people to maintain a healthy weight even when they want to.
Cheap, energy-dense processed foods have become the most accessible option in many communities, while whole foods like fruits, vegetables, and lean proteins cost more and take more time to prepare. At the same time, physical activity has been engineered out of daily life through car-dependent cities, sedentary jobs, and screen-based leisure. Marketing of high-sugar, high-fat foods to children compounds the problem from an early age. The WHO treats these systemic factors as the primary explanation for why obesity rates have risen so sharply across virtually every country, not a sudden global collapse in personal discipline.
WHO Guidelines on Diet and Activity
The WHO’s dietary guidance focuses on a few high-impact targets. For sugar, it recommends keeping free sugars (added sugars and those naturally present in honey, syrups, and fruit juices) below 10% of total daily calories, with an ideal target of under 5%. For someone eating about 2,000 calories a day, 10% translates to roughly 50 grams, or about 12 teaspoons of sugar. The 5% target cuts that in half.
On physical activity, the WHO’s 2020 guidelines recommend that adults get 150 to 300 minutes of moderate-intensity aerobic activity per week, or 75 to 150 minutes of vigorous-intensity activity, or some combination of both. Moderate intensity means activities like brisk walking or cycling at a conversational pace. Vigorous intensity means running, swimming laps, or anything that makes sustained conversation difficult. These ranges replaced an older minimum-only target, reflecting evidence that benefits continue to accrue well beyond the 150-minute floor.
Obesity in Children: Prevention and Management
The WHO recommends that all children under 5 who visit a primary healthcare facility have both their weight and height measured to assess nutritional status against the WHO growth standards. When a child is identified as overweight, the guidance calls for counseling parents and caregivers on nutrition and physical activity. This includes promoting and supporting exclusive breastfeeding for the first 6 months and continued breastfeeding until 24 months or beyond.
For children who meet the obesity threshold, the WHO recommends a more thorough assessment and the development of a tailored management plan. This can happen at the primary care level if the provider is adequately trained, or through referral to a specialist clinic. Notably, the WHO advises against routinely providing supplementary foods to children who are moderately wasted or stunted, recognizing that in many countries, undernutrition and overnutrition exist side by side in the same communities.
Global Targets for Reducing Childhood Obesity
The WHO’s original nutrition target, set relative to 2012 prevalence data, aimed for no increase in the rate of overweight among children under 5. In 2025, WHO member states at the 78th World Health Assembly passed a more ambitious resolution: reduce and maintain the prevalence of childhood overweight to below 5% by 2030. This target acknowledges that simply halting the rise is no longer sufficient, and that active reduction is needed. The WHO has published a set of recommended policy interventions to reach that goal, including taxes on sugary drinks, restrictions on marketing unhealthy food to children, clearer food labeling, and investment in school-based nutrition and physical activity programs.