Does BMI matter?

What does measuring BMI really tell us?

What is BMI?

BMI is an abbreviation for the term Body Mass Index. It is a calculation that relates an individual’s weight in kilogrammes (kg) to their height in metres (m).  

The formula is:

Body Mass Index = weight (kg)

-----------------------------

height x height (m²)

It was originally developed by a mathematician in the early 19th Century called Adolphe Quetelet to study human proportions in groups of people, rather than individuals.  It was renamed ‘Body Mass Index’ in 1972 by a professor called Ancel Keys. Then, in the 1990s the World Health Organisation (WHO) adopted the calculation as a way of looking at the association between obesity and health in populations.  They set cut off points defining the point at which someone is said to have ‘obesity’ and these cut off points are still in use today.  The terminology has changed slightly since the original definition, but the values remain the same.

Weight Range	Body Mass Index Range (kg/m²)
Underweight	less than 18.5
Healthy weight	18.5 to 24.9
Overweight	25 to 29.9
Obesity Class 1	30 to 34.9
Obesity Class 2	35 to 39.9
Obesity Class 3	40 or more

What does having a high BMI mean?

Overweight and obesity are currently defined by the WHO as an ‘abnormal or excessive fat accumulation that presents a risk to health.’ There is a considerable amount of scientific data showing an association between obesity (as defined by having a BMI of greater than 30kg/m²) and many illnesses and even risk of death.  There are studies showing that a higher BMI has been associated with an increased risk of metabolic conditions such as Type 2 diabetes, heart disease and stroke as well as musculoskeletal conditions such as osteoarthritis and back pain. High BMI has also been associated with an increased risk of some cancers.

Being able to identify people who may be more at risk of these conditions may lead to a reduction of risk. Losing excess fat can reduce the risk of developing type 2 diabetes which is strongly linked to developing heart disease and having a stroke.  There is also evidence that weight loss can reduce the risk of developing some cancers although this evidence is harder to gather for all cancers. 

It is important therefore to be able to accurately diagnose whether an individual is carrying excess fat, to avoid people being underdiagnosed or overdiagnosed.  It is also important to be able to identify whether an individual is carrying an abnormal distribution of fat and in particular what is known as visceral fat and ectopic fat.  Visceral fat is the fat tissue that is stored within the abdominal cavity around the organs, and ectopic fat within the organs themselves such as the liver and the pancreas. Both of these forms of fat collection are associated with a greater risk of metabolic and cardiovascular diseases. Visceral fat and ectopic fat are different to the other form of fat storage which is not thought to increase someone’s risk of complications and is called subcutaneous fat and is stored just under the skin.

So is BMI useful?

BMI is used throughout healthcare as a measure of whether someone has the condition ‘obesity’.  Measurements can be made quickly and inexpensively and can be applied to large groups as a screening tool, and to guide access to certain treatments. 

At an individual level however, it is not a direct measure of how much fat tissue someone has.  It also does not give an indication or a measure of how that fat tissue is distributed throughout the body. 

If you are a muscular athlete or a bodybuilder and have a large amount of muscle mass, BMI may overestimate your level of fat tissue.  Similarly, if you have a low level of muscle mass with slim arms and legs and the fat you do have is around your organs, it may underestimate it and wrongly put you in lower risk group.

Additionally, the values at which an individual’s BMI puts them into a higher risk group may be lower depending on a person’s family background as it is known that certain groups have a higher predisposition to storing fat in a central distribution.

Therefore, BMI gives an indication of whether someone may have too much fat tissue, but used alone is not definitive. This has been the consensus of a commission of clinicians and scientists who have proposed a new way of diagnosing how excess body fat affects the body, and introduced the terms ‘preclinical obesity’ and ‘clinical obesity’ which do not rely on BMI measurements alone.

Calculation of someone’s own BMI can be helpful though as a starting point. Use this BMI calculator to find out your BMI.

Work out your BMI and measure your waist circumference

Other measures, easily carried out, give more information on the amount of visceral fat we carry and are also correlated with risk, independent of someone’s BMI.  These include measurement of waist circumference, waist to hip and waist to height ratio.  The updated NICE guidance now advises that if someone has a BMI of over 35kg/m² other measurements should also be taken if permitted.  There are however also limitations to the usefulness of these measurements in assessing someone’s overall health and to what extent they are affected by carrying excess fat tissue, including the difficulty of obtaining reliable measurements.

What next?

In order to be able to assess the risk to your health of the amount of fat tissue or adiposity you have, it can be helpful to see to a doctor who will carry out an assessment that may include, as well as a measurement of BMI, a detailed history, other measurements and potentially also blood tests. Providing a personalised detailed approach to care is really central to the management of this condition.

Contact us or book a free appointment with our specialist weight loss doctor to talk about how she can help.

References

Rubino, F. et al. Definition and diagnostic criteria of clinical obesity. Lancet Diabetes Endocrinol 0, (2025).

Hardefeldt, P. J., Penninkilampi, R., Edirimanne, S. & Eslick, G. D. Physical Activity and Weight Loss Reduce the Risk of Breast Cancer: A Meta-analysis of 139 Prospective and Retrospective Studies. Clin Breast Cancer 18, e601–e612 (2018).

Komaroff, M. & Papadia, F. S. For Researchers on Obesity: Historical Review of Extra Body Weight Definitions. (2016) doi:10.1155/2016/2460285.

Recommendations | Obesity: identification, assessment and management  | Guidance | NICE.

Adab, P., Pallan, M. & Whincup, P. H. Is BMI the best measure of obesity? BMJ (Online) 360, (2018).

Sweatt, K., Garvey, W. T. & Martins, C. Strengths and Limitations of BMI in the Diagnosis of Obesity: What is the Path Forward? Curr Obes Rep 13, 584–595 (2024).