The way obesity is diagnosed and defined will change – here’s why and what it means for treatment

Obesity is there related to many common diseases such as type 2 diabetes, heart disease, fatty liver disease and knee osteoarthritis.

Obesity is currently defined by body mass index, or BMI. This is calculated as weight (in kilograms) divided by the square of height (in meters). In people of European origin, the BMI for obesity is 30 kg/m² and more.

However, the risk to health and well-being does not depend solely on body weight and therefore BMI. We have been part of a global collaboration that has spent the last two years discussing how this should change. Today we publish how we believe obesity should be defined and why.

As we present in Lancethaving a larger body shouldn’t mean you’re diagnosed with “clinical obesity.” Such a diagnosis should depend on the level and location of fat tissue and whether it is accompanied by health problems.

What’s wrong with BMI?

The risk of impoverished health depends on the relative proportion of fat, bone and muscle in a person’s body weight, as well as where the fat is distributed.

For example, athletes with relatively huge muscle mass may have a higher BMI. Even if an athlete has a BMI over 30 kg/m², his or her excess weight is due to excess muscle, not excess body fat.

People carrying excess body fat around the waist are most at risk of obesity-related health problems.

Fat stored deep in the abdomen and around the internal organs can release harmful particles into the blood. These can then cause problems in other parts of the body.

However, BMI alone does not tell us whether a person has health problems related to excess body fat. People with excess body fat do not always have a BMI above 30, which means they are not screened for health problems related to excess body fat. This can happen in a very high person or in someone who tends to accumulate belly fat but is at a “vigorous” weight.

On the other hand, other people who are not athletes but have excess body fat may have a high BMI but have no associated health problems.

BMI is therefore an imperfect tool to support diagnose obesity.

What is the recent definition?

Purpose Lancet Diabetes & Endocrinology Committee on the Definition and Diagnosis of Clinical Obesity
was to develop an approach to this definition and diagnosis. The commission, established in 2022 under the leadership of King’s College London, consists of 56 experts dealing with aspects of obesity, including people with real-life experience.

Committee definition and new diagnostic criteria distracts attention from BMI itself. It includes other measurements, such as waist circumference, to confirm excessive or unhealthy fat distribution.

We define two categories of obesity based on objective signs and symptoms of impoverished health caused by excess body fat.

1. Clinical obesity

A person with clinical obesity has signs and symptoms of persistent organ dysfunction and/or difficulty performing daily activities (such as bathing, going to the toilet, or getting dressed).

There are 18 diagnostic criteria for clinical obesity in adults and 13 in children and adolescents. These include:

• shortness of breath caused by the effects of obesity on the lungs

• heart failure caused by obesity

• increased blood pressure

• fatty liver disease

• abnormalities in the bones and joints that limit movement in children.

2. Preclinical obesity

A person with preclinical obesity has high levels of body fat that do not cause any disease.

People with preclinical obesity do not have any symptoms of impaired tissue or organ function caused by obesity and can perform daily activities without any problems.

However, people with preclinical obesity generally have a higher risk of developing diseases such as heart disease, certain cancers and type 2 diabetes.

What does this mean for the treatment of obesity?

Clinical obesity is a disease that requires access to effective health care.

For people with clinical obesity, health care should focus on treating health problems caused by obesity. Patients should be offered evidence-based treatment options after discussion with their doctor.

Treatment will be switch on treatment of obesity-related complications and may include specific obesity treatments aimed at reducing fat mass, such as:

• support for behavior change around diet, physical activity, sleep and screen operate

• obesity medications that reduce appetite, reduce weight and improve health such as blood glucose (sugar) levels and blood pressure

• metabolic bariatric surgery to treat obesity or alleviate weight-related health complications.

Should preclinical obesity be treated?

For people with preclinical obesity, health care should focus on reducing the risk and preventing obesity-related health problems.

This may require health counseling, including support to change health behaviors and monitoring over time.

Depending on a person’s individual risk – e.g. family history of disease, body fat level and changes over time – he or she may choose one of the above obesity treatment methods.

Distinguishing people who do not have the disease from those who already have an ongoing disease will enable a personalized approach to obesity prevention, management and treatment with a more appropriate and cost-effective allocation of resources.

What will happen next?

These recent criteria for diagnosing clinical obesity will need to be incorporated into national and international clinical practice guidelines and a range of obesity policies.

Once adopted, training health care workers and health care managers and educating the general public will be necessary.

Reframing the narrative around obesity can support eliminate misconceptions that contribute to stigma, including false assumptions about the health of people with larger bodies. A better understanding of the biology and health effects of obesity should also ensure that people with larger bodies are not blamed for their condition.

People who are obese or have larger bodies should expect personalized, evidence-based assessment and advice, free from stigma and guilt.