Beyond Your BMI: Why the New Definitions of Obesity Are Important
New guidelines help redefine obesity as preclinical and clinical for better prevention, diagnosis and treatment of the global disease
"Just as it's cheaper to repair a bridge than to build a new one, it's better to keep a heart healthy rather than have to fix one after the damage is done," says Mark Katz, M.D., a cardiologist in Hudson County, New Jersey who is associated with RWJ Barnabas Health Medical Group and Jersey City Medical Center.
New Definition
One of the ways to keep a heart healthy is to prevent and treat obesity — one of the prime contributors to cardiovascular disease. But preventing it is not so easy.
Katz explains, "I talk to patients about lifestyle and about the importance of healthy eating. But when a father of three looks at me and says, 'Doc, I can get chicken fingers for my kids, my wife and myself at the fast-food place down the street. It's one of the cheapest ways to eat, and we all like it. I know it's not good for us but what can I do?' It's clear that there are no easy answers."
Today, March 4, is World Obesity Day. The mission is to raise awareness and improve understanding of the root causes of obesity and the actions needed to address them. Obesity is a chronic disease characterized by excessive fat accumulation or distribution that presents a risk to health and requires lifelong care.
Virtually every system in the body is affected by obesity, and doctors have long known that obesity is a risk factor for cardiovascular diseases including stroke, some cancers, type 2 diabetes and a number of other conditions including dementia. According to the Centers for Disease Control, more than 100 million American adults have obesity and an estimated 1 billion people are impacted worldwide.
On Jan. 14, 2025, in the prestigious journal The Lancet Diabetes and Endocrinology, an international commission of 56 leading experts released a new set of global clinical guidelines for obesity. Clinicians and researchers are hopeful that this will help to alter the landscape of diagnosis and treatment for this often-life-threatening condition.
The new, more nuanced framework has been endorsed by 76 organizations worldwide, including scientific societies and patient advocacy groups. The evidence-based guidelines put obesity into two distinct categories. Preclinical obesity is when a person has extra body fat but no signs or symptoms of organ dysfunction that are related to obesity and have no limitations on day- to-day activities. While associated with variable levels of health risk, there are no ongoing health complications due to excess body fat.
"In most cases, obesity is a multifactorial disease due to environment, psycho-social factors and genetic variants."
The second condition, called clinical obesity, is defined as a chronic disease with specific signs and symptoms of ongoing organ dysfunction due to obesity alone. Criteria include evidence of reduced organ or tissue function revealed in diagnostic tests. Excess fat, or adipose tissue, causes harm to the body, inducing breathlessness, knee or hip pain, or joint stiffness, among others. Cardiovascular disease, type 2 diabetes, fatty liver disease, metabolic syndrome and dysfunction of the kidneys, upper airway or nervous system may be present. People with clinical obesity should receive timely, evidence-based treatment to improve health and prevent the worsening of a condition that might lead to end-organ damage.
Katz explains, "We still don't fully understand obesity even though it is a problem of enormous scope and with huge implications for health and well-being." A March 2024 report titled Obesity and Overweight by the World Health Organization pointed out that "In most cases, obesity is a multifactorial disease due to environment, psycho-social factors and genetic variants."
Factors Other Than BMI
Until now, the diagnosis of obesity has relied solely on body mass index (BMI), an imprecise measure linking an individual's weight relative to their height. The CDC's website, among many others, shows the breakdown of obesity as it has been categorized:
- Underweight BMI below 18.5
- Healthy Weight BMI 18.5 to 24.9
- Overweight BMI 25 to 29.9
- Obesity BMI above 30
The classes of obesity are in three categories:
- Class 1 Obesity BMI 30 to 34.9
- Class 2 Obesity BMI 35 to 39.9
- Class 3 Severe Obesity BMI 40 or greater
Although it can be a useful rubric for identifying people at increased risk of health consequences, BMI is not a direct measure of fat, does not take into account the distribution of fat around the body (abdominal fat is considered to be more dangerous than other types) and cannot determine when excess body fat is a health problem.
A body builder or a pro athlete can have a high BMI yet have no excess fat, and someone with a lower BMI can have medical conditions related to obesity. "It's important to treat the patient and not the number," Katz says. "And we want to treat it as quickly as possible. Just like money in a savings account, risk is cumulative and compounding."
Aging and BMI
As people age, they naturally lose muscle definition and tend to have more body fat, which may affect BMI. Use of BMI alone has led to some people being diagnosed as obese and getting unnecessary treatment and stigmatization while missing others whose health is impaired. The new diagnostic framework emphasizes the use of other measurements of body fat, such as waist circumference, waist-to-hip ratio, or waist-to-height ratio. It also recommends direct body measurement such as a DEXA scan, which measures not only bone density, thickness and strength but also fat and muscle composition in certain parts of the body.
"Just like money in a savings account, risk is cumulative and compounding."
Ethnic and racial factors need to be considered as well since the current classifications apply primarily to people of European ancestry. Asians are more prone to develop diabetes and high blood pressure at lower BMIs. According to the National Institutes of Health, rising obesity rates have affected predominantly non-Hispanic Black women, Native Americans and Hispanics. Factors to be considered when looking at the problem of obesity include social determinants of health, socioeconomic factors, food prices and the availability of healthy foods such as fresh vegetables.
"The formulation of new descriptions of a clinical entity such as obesity allows us to provide better care for our patients," Katz says. Treatment can range from lifestyle and dietary management, the use of drugs including semaglutide and other GLP-1 drugs and bariatric surgery.
Differentiating the types of obesity will support personalized treatment, ensure cost effective use of resources, help to prevent preclinical obesity from worsening, and lead to public health strategies to reduce the incidence and prevalence of obesity. Because weight-based bias and stigma are obstacles in effective efforts to address obesity, health care professionals and policy makers should receive additional training so they can help to shift societal perceptions.
