Our Over-reliance on BMI
For decades Body Mass Index or BMI, has been at the centre of how we define and discuss obesity. BMI tends to be simple, familiar and easy to calculate, making it a useful tool in both clinical practice and population health. But with our evolving understanding of obesity, it is becoming increasingly clear that BMI alone does not provide the complete picture. In the era of modern weight management, perhaps it is time to ask whether we have become too focused on a number.
BMI is calculated using a patient’s weight and height. What it does not tell us is what that weight is comprised of. It cannot distinguish between muscle and fat, nor can it distinguish where the existing fat is distributed throughout the body. Two people can have exactly the same BMI but have completely different body compositions, metabolic and health risks.
Consider two people with a BMI of 33. One may have relatively high muscle mass, little visceral fat, good cardiovascular fitness and no significant metabolic disease. Another person with exactly the same BMI may have substantial excess body fat, increased visceral fat, type 2 diabetes, hypertension and sleep apnoea. Their BMI is identical, but their health and treatment needs can be vastly different.
This highlights one of the fundamental limitations of BMI – BMI measures size but is an inadequate measure of health (1).
The importance of body composition
This is why body composition is becoming increasingly important in the assessment and management of obesity. Instead of simply asking how much someone weighs, we need to understand what that weight represents.
Body composition allows us to consider the proportion of body weight that is made up of fat mass and lean mass. Where possible, it also sheds light on the overall body distribution of fat. Of particular importance, visceral or abdominal fat is reflective and associated with a range of chronic diseases – including insulin resistance, type 2 diabetes, cardiovascular disease and fatty liver disease (2,3).
Understanding body composition also becomes particularly important during weight loss. Not all weight loss is necessarily beneficial to the same degree (4). A patient may lose 15 kilograms, but the clinical significance of that weight loss depends partly on what has been lost. Reducing excess fat while preserving muscle is a very different outcome from losing a substantial amount of both fat and lean tissue.
Muscle is critical for strength, mobility, physical function and metabolic health. Following weight loss interventions, we should be asking not only, “How much weight has the patient lost?”, but also, “What has happened to their body composition?”
Even though a lower number on the scales is important, the quality of that weight loss also matters. Our objective should be to achieve meaningful reduction in excess fat while preserving lean mass and improving overall health.
This is why major bariatric practices and clinics have incorporated body composition scanners into their routine patient follow up assessment. Here at SGOS, we use the EVOLT system and ensure routine scanning to our patients at different stages of their journey.
Moving from obesity defined by BMI to clinical obesity
Another important evolution in our thinking is the concept of clinical obesity. This is major shift in our overall thinking process which has been a key discussion point across recent IFSO congresses (5).
Obesity should not simply be viewed as a state defined by crossing a particular BMI threshold. It should increasingly be understood in terms of the impact that excess adiposity has on a patient’s health and physical function.
A person may have an elevated BMI but relatively little obesity-related disease. Another person may have a similar BMI but already be experiencing significant metabolic complications, sleep apnoea, cardiovascular disease, fatty liver disease, impaired mobility or other consequences of excess adiposity.
Clinical obesity therefore shifts the conversation from “How much does this person weigh?” to “Is excess adiposity causing disease or impairing the person’s health and function?”
This is an important distinction. The purpose of identifying obesity clinically is not simply to assign a label. It is to recognise when excess adiposity has become a disease that warrants faster active medical and/or surgical management (5).
In this context, BMI becomes one piece of information rather than the diagnosis itself. A comprehensive assessment should consider body composition, fat distribution, metabolic health, obesity-related complications, physical function, lifestyle factors and the patient’s individual overall circumstances. Through the evaluation of all of these aspects, we are able to tailor management according to individual patient needs.
Obesity is a chronic disease, not simply a failure of willpower
This broader approach also changes how we think about treatment. Obesity is a complex chronic disease influenced by biological, metabolic, environmental, behavioural and psychological factors. It is therefore unrealistic to expect that every patient can achieve and maintain meaningful weight loss through diet and lifestyle changes alone.
For some patients – nutrition and physical activity may be sufficient. While in others, active weight loss interventions like medication, endoscopic therapies and/or bariatric surgery may be required.
The important principle is that treatment should be patient-specific and clinically driven.
Measuring success differently
This also means we need to rethink what constitutes successful weight management.
For many years, success has been measured primarily through kilograms lost or reduction in BMI. Even though these measurements remain useful, they should be considered alongside more meaningful clinical outcomes.
As such, our overall assessment should include both qualitative and functional questions like – Has excess body fat decreased? Has muscle been preserved? Has visceral fat reduced? Has blood pressure improved? Has glucose control improved? Has sleep apnoea improved? Is the patient more physically active? Can they move more easily? Do they feel better? Has their quality of life improved?
BMI still has a role — but it should not have the final say
Moving beyond BMI does not mean abandoning BMI.
BMI remains a useful, simple and accessible measure and will continue to have an important role in population health and clinical assessment. The problem arises when we use BMI as the sole measure of obesity, disease severity or treatment success.
The future of weight management should therefore be less about chasing a particular number on the scales and more about improving any individual’s health.
It is time to move beyond BMI to weight to body composition.
From BMI to clinical obesity.
From weight loss to meaningful health improvement.
References
- Richman IB, Inouye SK. “Moving Beyond BMI to Define Obesity.” JAMA Internal Medicine. 2025;185(12):1500–1501.
- Silveira, E.A., Castro, M.C.R., Rezende, A.T.O. et al.Body composition assessment in individuals with class II/III obesity: a narrative review. BMC Nutr 10, 142 (2024). https://doi.org/10.1186/s40795-024-00913-2
- American Diabetes Association Professional Practice Committee. 8. Obesity and Weight Management for the Prevention and Treatment of Type 2 Diabetes: Standards of Care in Diabetes-2025. Diabetes Care. 2025 Jan 1;48(1 Suppl 1):S167-S180. doi: 10.2337/dc25-S008. PMID: 39651976; PMCID: PMC11635032.
- Johnson Stoklossa CA, Forhan M, Padwal RS, Gonzalez MC, Prado CM. Practical Considerations for Body Composition Assessment of Adults with Class II/III Obesity Using Bioelectrical Impedance Analysis or Dual-Energy X-Ray Absorptiometry. Curr Obes Rep. 2016 Dec;5(4):389-396. doi: 10.1007/s13679-016-0228-5. PMID: 27591783.
- Rubino F et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol. 2025 Mar;13(3):221-262. doi: 10.1016/S2213-8587(24)00316-4. Epub 2025 Jan 14. Erratum in: Lancet Diabetes Endocrinol. 2025 Mar;13(3):e6. doi: 10.1016/S2213-8587(25)00006-3. PMID: 39824205; PMCID: PMC11870235.
