Emma writes...
Thoughts, resources and comment from the NHD Editor.
Defining and diagnosing overweight and obesity – what’s the latest?
The World Health Organisation (WHO) defines overweight as a ‘condition of excessive fat deposits’ and obesity as ‘a chronic complex disease defined by excessive fat deposits that can impair health’.(1)
In 2022, globally, 43% of adults aged 18 years and over were overweight and 16% were living with obesity.(1)
Worldwide since 1990, adult obesity has more than doubled, and adolescent obesity has quadrupled.(1)
Body mass index (BMI): a look at the history
Currently, BMI is used to define and categorise obesity, with additional measurements, such as waist circumference, aiding the diagnosis of the condition.(1) BMI is an old measurement! It was defined in 1832 by Adolphe Quetelet, a Belgian statistician, mathematician and astronomer.(2) Quetelet established quantifiable characteristics of the ‘normal man’ via his passion for statistical analysis and bell-shaped curves. This became known as the Quetelet Index.(2)
Further work to utilise BMI as a measure of obesity didn’t come until the 1950s, when Louis Israel Dublin, another statistician and vice president of a life insurance company, noticed that claims from obese policyholders were increasing. Dublin created tables of ‘normal weights’, which did not specify age but did categorise the body frame sizes small, medium and large.(2)
In 1972, physiologist Ancel Keys officially coined the term ‘body mass index’. Keys analysed over 7000 ‘healthy men’ across five countries and concluded that BMI was a preferable and convenient calculation.(3)
The WHO formally acknowledged that there was a global obesity epidemic in 1997. BMI has since been the acknowledged measure of obesity and is now used globally in its diagnosis.(1)
Whilst BMI holds a central role in obesity diagnostic criteria, it doesn’t take into consideration other parameters that are considered vital for accurately assessing and diagnosing the condition. For example, it doesn’t account for abdominal adiposity, or other factors such as age, sex, race, ethnicity or socioeconomic status. All of which have an impact on morbidity and mortality outcomes.
Latest guidelines and definitions
NICE guideline [NG246] Overweight and obesity management (4) was published on 14th January 2025. It covers the prevention and management of overweight, obesity and central adiposity in children, young people and adults, bringing together and updating all of NICE's previous guidelines on overweight and obesity. The guidance does not cover pregnancy. NICE recommends that these general guidelines should be used by health and social care professionals when delivering patient care.
On the same day, The Lancet Diabetes & Endocrinology Commission published a paper which lays out new definitions and diagnostic criteria for clinical obesity. (5) The commission aimed to ‘define clinical obesity and identify objective and pragmatic criteria for its diagnosis’. It also aims to inform the decision-making process for clinicians and policymakers and assists in the identification of priorities for clinical interventions and public health strategies.
Whilst the two publications aim to provide guidance on the identification and diagnosis of overweight, obesity and central adiposity, there are differences between the two, which may cause confusion and will no doubt spark debate amongst those who are working with this population group.
Summary of differences regarding body mass index and excess body fat measurements
NICE GUIDELINE [NG246] 2025
Relies on the use of BMI and waist circumference as a measure of overweight, obesity and central obesity in adults. But it also recommends cautious interpretation and acknowledges that it is not a direct measure of central adiposity.
Acknowledges that not all symptoms should be attributed to overweight and obesity. The presenting clinical condition should be addressed before discussing weight.
Recommends that clinical judgement should be used when interpreting the healthy weight BMI category, as people within this category may have central adiposity.
Waist-to-height ratio is recommended to classify the degree of central adiposity.
Provides specific recommendations for certain ethnicities.
THE LANCET DIABETES & ENDOCRINOLOGY COMMISSION 2025
Acknowledges the limitations of the current definition of obesity:
Defined solely by BMI
Not a direct measure of fat
Doesn’t establish the distribution of body fat
Cannot determine when excess body fat is a health problem
Stipulates that relying on BMI only to establish obesity is inaccurate and can lead to under-diagnosis of many whose health is impaired or over-diagnosis of many who are healthy.
Waist circumference, waist-to-hip ratio and waist-to-height ratio are all indicated as suitable measurements to confirm excess body fat.
Recommends the use of ethnicity-appropriate cut-off points for all anthropometric criteria.
A new diagnostic approach
Whilst the NICE guideline [NG246] follows the traditional approach to identifying and assessing overweight, obesity and central adiposity, by using BMI and waist-to-height ratio, plus the traditional BMI categorisation, The Lancet Diabetes & Endocrinology Commission shares a new diagnostic approach which focuses on different measures of body fat and actual signs and symptoms of ill health.
The criteria acknowledges that BMI is useful but it shouldn’t be used alone to establish if someone is obese. To this end, the commission has introduced two new categories of obesity.
Pre-clinical obesity:
Presence of excess body fat, variable level of health risk, no ongoing illness.
No evidence of reduced organ or tissue function due to obesity.
Can complete day-to-day activities without hindrance.
At higher risk of developing chronic conditions such as type 2 diabetes, CVD, clinical obesity and some cancers.
Clinical obesity:
A chronic disease due to obesity alone.
Signs and symptoms are present – organ dysfunction +/- reduced ability to complete daily activities.
Organ dysfunction can include breathlessness, joint pain and/or stiffness, reduced range of motion in joints, metabolic abnormalities, and dysfunction of other organs including kidneys and nervous and reproductive systems.
There are some differences between this new process and the current recommendations by NICE. These are summarised below In order to diagnose preclinical obesity or clinical obesity The Lancet Diabetes & Endocrinology Commission has provided guidance on the diagnostic process here...
Comparison of diagnostic processes
NICE GUIDELINE [NG246] 2025:
Calculate BMI
Measure waist circumference in people with a BMI below 35 kg/m2 to measure waist to height ratio can be calculated
Use BMI and waist circumference to classify overweight, obesity and central obesity*
After the initial assessment of overweight or obesity, identify any comorbidities and other factors that may affect or be affected by the person's weight
*The recommendations also include guidance on circumstances where interpretation with caution is required or with further specific recommendations regarding ethnicity. Does not recommend the use of bioimpedance as a substitute for BMI as a measure of general adiposity in adults.
THE LANCET DIABETES & ENDOCRINOLOGY COMMISSION 2025
Confirm excess body fat. Either at least one measurement of BMI and body size (waist circumference, waist-to-hip ratio, waist-to-height ratio) or at least 2 measurements of body size, regardless of BMI, or a direct body fat measurement, e.g. a DEXA scan.
No excess body fat is present = No obesity
Excess body fat present and confirmed = Obesity
Confirmation of excess body fat should be followed by further assessment to establish whether an individual with obesity has an illness. This includes: medical history, physical examination, and standard blood test to check whether there are signs/symptoms of organ dysfunction and whether the individual experiences limitations of day-to-day activities.
No signs symptoms and no limitations = pre-clinical obesity
Yes, signs/symptoms of organ dysfunction. Is the organ dysfunction obesity related?
No = pre-clinical obesity
Yes = clinical obesityYes, limitations of daily activities = clinical obesity
There are some significant differences between the two publications, but also many similarities. However, the change in approach from the Lancet Diabetes & Endocrinology Commission reframes obesity with a shift away from the traditional view that a high BMI and excess body fat automatically categorise someone as obese.
Whilst there will be sceptics to this approach, it brings to the fore a new perspective on the condition and the accuracy of diagnosis. The Lancet Diabetes & Endocrinology Commission definitions are not yet incorporated into UK clinical guidelines. Work will need to be done to clarify best practice for health and social care and to negotiate the practical use of the definitions if they are to become part of UK official guidelines.
Emma has been a Registered Dietitian for over 18 years and has experience in adult and paediatric dietetics. She has been the Editor of NHD for nine years, steering the editorial content and supporting the production process.
Emma currently works in industry.
Emma Coates, RD
References
WHO (2024). Obesity and overweight. Obesity and overweight
Pray R and Riskin S (2023). The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review. The History and Faults of the Body Mass Index and Where to Look Next: A Literature Review - PMC
Keys et al (1972). Indices of relative weight and obesity. Indices of relative weight and obesity - ScienceDirect
NICE (2025). NICE guideline [NG246]. Overweight and obesity management. Overview | Overweight and obesity management | Guidance | NICE
Rubino et al (2025). The Lancet Diabetes & Endocrinology Commission - Definition and diagnostic criteria of clinical obesity. Definition and diagnostic criteria of clinical obesity - The Lancet Diabetes & Endocrinology
The Lancet Diabetes & Endocrinology Commission (2025). Diagnosing clinical obesity. Lancet infographic - Clinical Obesity
