Scientists redefine obesity, the "golden index" you have been using is about to change

BMI has long been the most commonly used indicator to measure whether a person is obese. Today, the latest consensus reached by scientists has shaken the foundation of this gold standard. It is no longer the only indicator for clinical diagnosis of individual obesity, but is only used as an optional indicator of health risks at the population level. Obesity is officially regarded as a chronic disease, and the new stratified diagnostic system helps reduce weight bias and stigma.

Written by | Li Juan

Body mass index (BMI) is perhaps the most popular medical term. It is easy to measure and compare. The result can be obtained by dividing the weight by the square of the height, which is used to measure whether a person is obese. In the weight loss industry, BMI is especially regarded as the "gold standard"; in academia, BMI is also widely used in epidemiological studies related to obesity.

However, in actual application, this gold standard often encounters exceptions, thus limiting its diagnosis of obesity. For example, a muscular man may be classified as obese, while a person with a normal BMI may have too much fat. The reason is that the BMI value cannot reflect the specific distribution and content of body fat (subcutaneous fat, visceral fat, ectopic fat in organs such as the liver, pancreas or heart), nor can it determine whether excessive fat poses a threat to health.

On January 14, 2025, a group of 58 researchers from around the world published a new major report in The Lancet Diabetes and Endocrinology, pointing out that BMI is not the only indicator for clinical diagnosis of individual obesity, and BMI should only be used as an optional indicator of health risks at the population level for epidemiological research or screening purposes. The key to defining individual obesity is whether excess fat has caused organ dysfunction.

More than 75 professional organizations around the world have endorsed this report, including the American Heart Association, the European Union of Internal Medicine, and the World Obesity Federation. It can be said that a global expert consensus has been reached on the "definition and diagnosis of obesity."

Classifying obesity

In the report, the researchers proposed a new hierarchical diagnostic system for "Clinical Obesity" and "Preclinical Obesity".

Among them, "clinical obesity" means that excess fat has caused functional damage to the body, regardless of whether there is metabolic abnormality. For example, fat accumulation may cause musculoskeletal injuries, respiratory problems, and increased cardiac load, thus affecting daily life. The new system regards clinical obesity as a chronic disease, and its diagnostic criteria include confirmation of excess fat (such as BMI combined with waist circumference or direct fat measurement) and the occurrence of organ dysfunction or limited movement.

After diagnosis, patients with clinical obesity should receive timely, evidence-based, comprehensive medical management. In addition to lifestyle interventions, more aggressive treatments such as medication or surgery may be needed. The treatment of clinical obesity focuses on improving or reversing organ dysfunction and restoring the individual's normal physiological function. The hallmark of successful treatment should be improvement of symptoms and restoration of organ function, not just weight loss alone.

"Preclinical obesity" describes individuals whose body fat percentage is excessive but has not yet caused organ damage. This group of people may be in the early stages of obesity development, or only show excessive fat storage but have not affected their health. Therefore, preclinical obesity is not a disease, but it needs to be closely monitored to prevent it from developing into clinical obesity or causing other obesity-related diseases. Especially for high-risk individuals, such as those with family heredity, abnormal body fat distribution or specific disease background. It usually includes taking active lifestyle interventions, such as improving eating habits and increasing physical activity.

The above classification fills the gaps in previous definitions, transforming obesity from a simple "weight problem" to a "disease of organ dysfunction", and clearly distinguishing between obesity as a disease (clinical obesity) and obesity as a risk state (preclinical obesity), which also determines the differences in subsequent intervention treatment plans.

The Lancet Diabetes and Endocrinology Commission proposed a definition and diagnosis and treatment system for obesity | Image source: the lancet

It is worth emphasizing that preclinical obesity is different from metabolically healthy obesity because its definition is based on the normal function of all organs that may be affected by obesity, not just the metabolic system. Obesity not only causes disease by affecting the metabolic regulatory system, but may also cause health problems by changing the function of multiple organ systems. Therefore, even if someone's metabolic function is normal, if they have cardiovascular, musculoskeletal, or respiratory symptoms and signs due to excess fat, the individual still meets the diagnostic criteria for clinical obesity.

So, how do we define people with high blood lipids? The report points out that the metabolic diagnostic criteria for clinical obesity are the simultaneous presence of high blood sugar, low HDL, and high triglycerides. If a person has excessive body fat and only a single metabolic abnormality (such as dyslipidemia), they do not meet the criteria for clinical obesity and should be classified as preclinical obesity.

Diagnostic criteria for clinical obesity, the top picture is for adults, the bottom picture is for minors | Image source: the lancet

The establishment of this hierarchical framework makes the diagnosis of obesity more scientific, so that medical resources can be allocated more reasonably and patients who really need medical intervention can receive precise treatment. At the same time, this system will also help reduce the stigma of obesity. For a long time, obesity has often been considered a simple lifestyle problem, and the new standards emphasize that obesity is a complex disease that requires medical management, rather than a moral or willpower defect.

Is obesity a disease?

There has been a long-standing debate about whether obesity should be considered a chronic disease. Many people disagree with this view, and the main points of view are as follows:

First, BMI is not sufficient as a diagnostic criterion for disease: Although there is a clear relationship between BMI, obesity and disease prevalence at the population level, BMI and fat mass do not provide information about an individual's health status. Some people with excessive BMI (such as athletes) do not necessarily have health problems. Diagnosis based solely on BMI may lead to over-medicalization, but it may also lead to under-diagnosis.

Secondly, obesity is only a "risk factor": the traditional medical view is that diseases should have specific pathophysiological mechanisms and clinical manifestations, and obesity is more like a risk factor for other diseases rather than an independent disease. For example, high blood pressure and high cholesterol are considered risk factors for heart disease, but they themselves are not classified as "diseases."

In addition, obesity is highly heterogeneous: some obese people may be healthy throughout their lives, with no obvious organ damage or metabolic abnormalities, while others may have serious health problems when they are only slightly overweight. Therefore, classifying obesity as a disease may ignore individual differences.

The establishment of a new system for defining and diagnosing obesity has reduced controversy from many perspectives.

First, the new system introduces multi-dimensional diagnostic criteria to replace the single BMI standard. The new report emphasizes that in the diagnosis of obesity, BMI is not a single indicator, but a comprehensive diagnosis must be made in combination with waist circumference, body fat percentage, organ function tests (such as transaminase levels, insulin sensitivity) and other factors.

Secondly, the research evidence that obesity can cause diseases fully shows that obesity is not just a risk factor. Studies have shown that excess fat is not only a risk factor for other chronic diseases, but can also directly damage organs and accelerate the progression of diseases, such as from fatty liver to liver fibrosis, from insulin resistance to type II diabetes, from increased cardiac load to heart failure and arrhythmia, and from restricted breathing to sleep apnea syndrome.

Finally, the new system introduces the concept of "preclinical obesity", which provides a clearer explanation of the high heterogeneity of obesity. The new definition recognizes that not all obese people suffer from diseases. Only when excessive fat causes organ dysfunction is it defined as "clinical obesity", thus avoiding over-medicalization. At the same time, the report also pointed out that due to the high heterogeneity of obesity, future research needs to further characterize and develop obesity staging and scoring systems to assist in prognostic assessment and guide treatment.

Fighting obesity bias and stigma

"Control your mouth and move your legs" - this traditional public health propaganda overemphasizes that obesity is a personal responsibility, while ignoring the biological mechanisms behind obesity, such as abnormal hormone regulation and genetic susceptibility. The Lancet report pointed out that weight-based bias and stigma are not only a manifestation of social injustice, but also a key factor hindering the effective prevention and treatment of obesity.

Nowadays, many health programs and social platform content emphasize the role of willpower in weight loss, and obesity is frequently portrayed as "lack of self-control" and "personal failure." Data show that 19% to 42% of adults with high BMI (especially women) report experiencing weight discrimination, and 40% to 50% of people who lose weight have internalized weight stigma, that is, they internalize the negative social evaluation of "laziness and greed" into their self-cognition, which damages their mental health. This stigma also exacerbates social isolation, reduces the willingness of obese people to exercise, and promotes a vicious cycle of sedentary and emotional eating.

In medical scenarios, weight bias also occurs from time to time. For example, the bias of medical staff may cause obese people to be labeled as "uncooperative with treatment", which may further lead to a series of problems such as underdiagnosis, delayed treatment and communication barriers. For example, due to the lack of appropriate examination tools or equipment mismatch, obese people are less likely to receive routine screening such as cervical smears and breast examinations; some doctors believe that "weight loss is the patient's own responsibility", thus ignoring effective treatments such as surgery. These biases make obese people feel helpless and evasive when facing the medical system, further delaying the time for intervention.

Looking at the new system, it not only differentiates the health status of individuals more accurately, but also helps reduce stigma at the root.

First, the new system points out that obesity is a disease, not caused by "laziness". The new system emphasizes that the occurrence of obesity is affected by factors such as genes, neuroendocrine, and metabolic regulation, and is not simply the result of lifestyle. Therefore, obese people, like diabetes or cancer patients, should be regarded as a group that needs medical intervention, rather than being blamed as "personal failures."

Secondly, the new system improves medical plans and increases fairness in diagnosis and treatment. In the past, many medical insurance and health policies only covered "obese people with complications." The new system emphasizes the controllability of risks in the preclinical obesity stage and the medical attributes of clinical obesity, which increases the sense of urgency for clinical intervention. It ensures that obese people who have not yet developed complications but whose organ functions have been affected can also receive medical intervention in a timely manner.

In addition, the new system reduces the social misconception that "thinness is beautiful". The new obesity diagnosis stratification system allows the public to realize that health is more important than weight. Many individuals who are slightly overweight but healthy will no longer be labeled as "sick", correcting the one-sided perception that "obesity = unhealthy".

Therefore, the latest obesity definition and diagnosis and treatment system can correct the common misunderstandings and prejudices among the public, obese people, medical professionals and policymakers, and effectively avoid misleading decision-making.

New system, new challenges

For the new classification system to be widely adopted, it must be fast, cheap and reliable. But at present, there are certain difficulties. The new system needs to combine BMI with other methods. For example, body fat percentage is generally considered excessive when men have more than 25% body fat and women have more than 30%-38% body fat. However, directly measuring body fat is complicated and expensive. Researchers suggest using other health indicators instead, such as waist circumference, waist-to-hip ratio or waist-to-height ratio, but they are obviously more cumbersome than measuring BMI.

Specifically in my country, the implementation of this framework also faces many challenges.

Professor Wang Youfa, deputy director of the School of Medicine and dean of the Institute of Global Health at Xi'an Jiaotong University, told the author: "The definition and classification of obesity by the Lancet Commission will help achieve graded management and precise intervention of obesity. However, it is expected that practice among obese people in my country will face great difficulties. We urgently need to formulate more specific obesity policies and public health projects and establish a social support system to cope with the challenges brought about by the updated obesity definition standards."

First of all, it is not easy to promote the body fat composition analysis method in my country. Wang Youfa pointed out that the cost of high-precision equipment for body fat diagnosis is high, and my country's primary medical institutions lack technical support and professional training. In addition, the policy in this regard is relatively backward, and these methods have not yet been incorporated into the national health screening and medical insurance reimbursement system.

As for medical diagnostic methods, there is currently a lack of accurate and easy-to-use biomarkers to distinguish clinical obesity from preclinical obesity. "At this stage, we recommend that residents measure their waist circumference, which helps reflect abdominal fat accumulation. We can implement a hierarchical diagnosis and treatment plan, that is, primary hospitals, secondary hospitals, and tertiary hospitals use waist circumference, liver ultrasound, and body composition analyzers as abdominal fat assessment methods." said Luo Yingying, chief physician of the Endocrinology Department of Peking University People's Hospital.

"An increase in waist circumference often reflects the accumulation of visceral fat, especially abdominal obesity, also known as central obesity. When the body fat ratio and waist circumference are similar, the accumulation of visceral fat in Chinese people is more significant than that in white people. In my country, adult males with a waist circumference ≥ 90 cm and adult females with a waist circumference ≥ 85 cm can be judged as having central obesity." said Associate Professor Sun Xiaomin from the Institute of Global Health at Xi'an Jiaotong University.

At present, half of the adults in my country are overweight or obese, of which 34.3% are overweight and 16.4% are obese. It is predicted that by 2030, the rates of overweight and obese adults, school-age children and adolescents, and preschool children in China may reach 65.3%, 31.8%, and 15.6%, respectively, and the number of people involved may rise to 789.95 million, 58.92 million, and 18.19 million. By 2030, the medical expenses attributed to overweight and obesity may reach 418 billion yuan, accounting for about 22% of the total medical expenses in the country.

In 2024, the National Health Commission's weight management guidelines proposed a standard process for adult weight management (see figure below), which is basically consistent with the latest clinical obesity diagnostic framework proposed by the Lancet Commission.

Source: National Health Commission's Weight Management Guidelines (2024 Edition)

In terms of obesity research, many of my country's current obesity prevention and control guidelines are based mainly on international data and lack local research support. Therefore, more extensive long-term research is urgently needed to optimize the management of obesity in adults and minors.

"In our country, there are a lot of people in the 'preclinical obesity' stage. How to prevent them from developing into 'clinical obesity' and what specific intervention measures should be taken? At present, our localized scientific evidence is not sufficient," Professor Liu Xin from the Global Health Institute of Xi'an Jiaotong University told the author.

"Most of the previous research evidence did not strictly distinguish between obesity stages at the beginning of its design. Next, we need well-designed prospective cohort studies and more targeted randomized controlled trials to find more localized solutions for the prevention and control of obesity."

Regarding what adjustments need to be made in future clinical research on obesity, Yang Juhong, chief physician of the Department of Endocrinology at the Affiliated Hospital of Guangdong Medical University, suggested that we can conduct staged analysis according to the concept of the new system to clarify the benefit-risk ratio of different populations; increase long-term tracking of the progression/reversal of obesity; use metabolomics, genomics and other tools to explore the differences in molecular mechanisms of different stages; and observe the effects of staged management in clinical practice through real-world research to make up for the limitations of randomized controlled trials.

In addition, the Lancet report emphasizes that the pathophysiological mechanism of obesity is still not completely clear, and how adipose tissue specifically causes systemic diseases (such as cardiovascular disease and neurodegenerative diseases) still needs further study. Moreover, current obesity treatments are mainly lifestyle interventions, drug therapy, and surgery, but the long-term safety of drug therapy and the long-term effects of surgery on metabolism need to be further clarified.

Acknowledgements

We would like to thank Professor Pan An, a member of the Lancet Diabetes & Endocrinology committee and Professor of the School of Public Health, Tongji Medical College, Huazhong University of Science and Technology, for his guidance on this article.

References

[1] Definition and diagnostic criteria of clinical obesity. Rubino, Francesco et al. The Lancet Diabetes & Endocrinology . https://doi.org/10.1016/ S2213-8587(24)00316-4

[2] Pan XF, Wang L, Pan A. Obesity in China 1. Epidemiology and determinants of obesity in China. Lancet Diabetes Endocrinol 2021; 9: 373–392.

[3] Zeng Q, Li N, Pan XF, et al. Obesity in China 2. Clinical management and treatment of obesity in China. Lancet Diabetes Endocrinol 2021; 9: 393–405.

[4] Wang Y, Zhao L, Gao L, Pan A, Xue H. Obesity in China 3. Health policy and public health implications of obesity in China. Lancet Diabetes Endocrinol. 2021; 9:446-461. Chinese version: “The impact of obesity on public health in China and implications for policy responses”

[5] 2024 edition of the “Guidelines for the Diagnosis and Treatment of Obesity”. General Office of the National Health Commission.

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