Obesity
Clinical reviewer: Jacob Moore, MD · Last checked September 24, 2026
What is obesity (medical definition)?
Obesity is a common, chronic disease marked by an abnormally high, unhealthy amount of body fat, generally defined in adults as a BMI of 30 or higher. The World Health Organization classifies it as a chronic, relapsing disease arising from genetics, neurobiology, eating behaviors, and the broader environment. A 2025 Lancet Commission proposed splitting obesity into clinical and preclinical categories.
How is obesity defined and how common is it?
Obesity is a common, chronic disease marked by an abnormally high, unhealthy amount of body fat [1]. NIDDK defines adult obesity as a BMI of 30 or higher, including severe obesity at a BMI of 40 or higher [2]. Using 2017-2018 NHANES data, NIDDK estimates more than 2 in 5 adults (42.4 percent) have obesity, including about 1 in 11 adults (9.2 percent) with severe obesity [2]. Worldwide, the WHO reported 1 in 8 people were living with obesity in 2022 [3].
How does the medical definition of obesity keep evolving?
The WHO classifies obesity as a chronic, relapsing disease arising from complex interactions between genetics, neurobiology, eating behaviors, access to healthy diet, market forces, and the broader environment [3]. In 2025, a Lancet Commission of 58 experts proposed defining clinical obesity as a chronic, systemic illness from excess adiposity that alters the function of tissues or organs, and preclinical obesity as excess adiposity with preserved organ function [4]. In December 2025, WHO published a guideline on the use of GLP-1 therapies for treating obesity in adults [3].
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This entry is for education and is not medical advice. Talk with a licensed clinician about your own situation before starting, stopping, or changing any treatment.
