The New Lancet Commission Definition of Obesity

Introduction: Obesity has historically been diagnosed using the Body Mass Index (BMI), a simplified tool for a complex disease that does not consider individual metabolic, cardiovascular, functional, or mental health variations. The Lancet Commission’s new obesity classification, was informed by 60 world experts including three Irish experts, Professor Edward Gregg, Professor Carel le Roux, and Vicky Mooney. The Irish Society for Clinical Nutrition and Metabolism (IrSPEN) also endorsed the findings which redefines obesity into Pre-Clinical Obesity and Clinical Obesity. This distinction aims to improve precision in diagnosis and management by shifting the focus from individual weight on the scale to overall health status.

Defining Clinical and Preclinical Obesity

The core differentiation between the two types of obesity lies in the presence or absence of organ dysfunction rather than absolute weight or BMI.

1. Clinical Obesity is defined when excess adiposity actively impairs organ function, leading to metabolic, cardiovascular, or mechanical complications. These include 18 specifically defined obesity related complications such as hypertension, obstructive sleep apnea, metabolic-associated fatty liver disease (MAFLD), and osteoarthritis.
2. Preclinical Obesity describes individuals with excess adipose tissue who do not yet have organ dysfunction but are at increased risk of progressing to clinical obesity. These individuals may exhibit early warning signs such as insulin resistance, visceral fat accumulation, endothelial dysfunction, or chronic inflammation, the precursors to future disease progression.

Introduction

Obesity has historically been diagnosed using the Body Mass Index (BMI), a simplified tool for a complex disease that does not consider individual metabolic, cardiovascular, functional, or mental health variations. The Lancet Commission’s new obesity classification, was informed by 60 world experts including three Irish experts, Professor Edward Gregg, Professor Carel le Roux, and Vicky Mooney. The Irish Society for Clinical Nutrition and Metabolism (IrSPEN) also endorsed the findings which redefines obesity into Pre-Clinical Obesity and Clinical Obesity. This distinction aims to improve precision in diagnosis and management by shifting the focus from individual weight on the scale to overall health status.

Defining Clinical and Preclinical Obesity

The core differentiation between the two types of obesity lies in the presence or absence of organ dysfunction rather than absolute weight or BMI.

1. Clinical Obesity is defined when excess adiposity actively impairs organ function, leading to metabolic, cardiovascular, or mechanical complications. These include 18 specifically defined obesity related complications such as hypertension, obstructive sleep apnea, metabolic-associated fatty liver disease (MAFLD), and osteoarthritis.
2. Preclinical Obesity describes individuals with excess adipose tissue who do not yet have organ dysfunction but are at increased risk of progressing to clinical obesity. These individuals may exhibit early warning signs such as insulin resistance, visceral fat accumulation, endothelial dysfunction, or chronic inflammation, the precursors to future disease progression.

While widely used, BMI fails to distinguish between fat and lean mass, assess adipose tissue distribution, or predict metabolic dysfunction. As a result, individuals with a high BMI with no organ dysfunction may be misclassified, whereas those with significant visceral fat at a lower BMI but with organ dysfunction may be overlooked.

The new framework is a multifaceted diagnostic approach:

• Adiposity is not only measured by BMI but also requires thresholds to be met using waist circumference, wait-to hip ratio, waist-to-height ratio, or imaging-based fat quantification.
• Organ function analysis includes liver function tests, inflammatory markers, and cardiovascular risk stratification.

The Look AHEAD and DiRECT trials demonstrated that structured lifestyle interventions can effectively treat patients with clinical obesity who have type 2 diabetes, emphasizing the importance of timely intervention in those with established organ dysfunction. Pharmacologic advances such as semaglutide and tirzepatide, have demonstrated substantial weight loss alongside improvements in organ function, as evidenced by the STEP and SURMOUNT trials. Medications are currently licenced to treat both patients with clinical obesity and preclinical obesity because the evidence supports the idea of preventing patients with preclinical obesity progressing to clinical obesity as well as patients with clinical obesity reverting to preclinical obesity.

In both clinical obesity and preclinical obesity, treatment is often enhanced by multidisciplinary and combining interventions. These approaches can include lifestyle changes, pharmacotherapy, and bariatric surgery. In preclinical obesity, the urgency of the intervention may be less because the goal is to halt the progression of organ dysfunction by addressing modifiable risk factors early.

On a broader scale, public health strategies require better evidence to prove pre clinical obesity and clinical obesity can be prevented. Current strategies, including taxation on processed foods, enhanced urban infrastructure to promote active lifestyles, and targeted educational initiatives to raise awareness about obesity have thus far not impacted the incidence of obesity, but more evidence of the effectiveness of these strategies are awaited.

The reclassification of obesity provides a roadmap to stratify the disease of obesity. It allows healthcare providers to focus on obesity related complications and not only BMI. The reclassification also underscores the necessity of shifting research focus and clinical focus from weight loss to health gain as optimal markers of success.

This refined classification should be integrated into clinical guidelines worldwide, bridging the gap between obesity diagnosis and precision medicine. The future of obesity care lies in early identification, phenotyping, and personalised interventions, ensuring that treatment is tailored to the patient’s specific needs rather than a one-size-fits-all approach.

In conclusion, the Lancet Commission’s reclassification of obesity marks a shift in obesity medicine, transitioning from weight-based metrics to an organ function approach. By differentiating between clinical and preclinical obesity, this framework enables earlier interventions, supports personalised treatment strategies, and enhances patient outcomes. As its adoption grows within the medical community, it has the potential to redefine global obesity management and inform more effective public health policies.

References

• Gregg EW, et al. (2012). Look AHEAD trial. N Engl J Med. 366:1209-1217.
• Lean ME, et al. (2018). DiRECT trial. Lancet. 391(10120): 541-551.
• Wilding JP, et al. (2021). STEP trials. N Engl J Med. 384:989-1002.
• Jastreboff AM, et al. (2022). SURMOUNT-1 trial. N Engl J Med. 387(3):205-216.

Written by Dr. Faisal I. Almohaileb1, 2 and Professor Carel le Roux2
1Diabetes Complications Research Centre, Conway Institute, University College Dublin
2College of Medicine, Family and Community Medicine Department, Qassim University

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