Commentary
This essay reflects the evidence, regulatory landscape, and market dynamics as they stood at the time of writing.
What do doctors mean by obesity
Many people use the term obesity, but in medicine it has a specific meaning. Doctors define obesity as a chronic health condition characterised by an excess amount of body fat that increases the risk of ill health. [1]
Importantly, what counts as ‘excess fat’ is not defined by appearance, but by the level of body fat at which health risks begin to increase. [1]
Why excess body fat affects health
In a healthy body, fat tissue performs important functions. It stores energy, helps regulate appetite, and releases chemical signals that allow the body to coordinate energy use, blood sugar control, and metabolism. [2]
However, once fat mass increases beyond a certain point, these signalling systems begin to function less well. The body enters a different physiological state, in which appetite regulation, insulin sensitivity, lipid handling, and inflammatory control are altered. This shift helps explain why excess body fat is associated with an increased risk of conditions such as type 2 diabetes, cardiovascular disease, fatty liver disease, and sleep apnoea. [3]
In this sense, obesity is not simply “more fat”, but a state in which normal regulatory systems are under strain and no longer operate optimally. [3]
Why obesity is a ‘chronic’ condition
Once this altered state is established, the body tends to defend the higher level of fat mass. [3][4]
When someone with obesity tries to lose weight by reducing calorie intake or increasing activity, the body often responds by:
This explains why sustaining weight loss is so biologically difficult and what drives the need for ongoing treatment.
These responses make ongoing weight loss harder and increase the likelihood of weight regain once efforts are relaxed. [4]
This biological resistance to weight loss helps explain why obesity is difficult to reverse once established, and why it is recognised as a condition that usually requires ongoing management, rather than a short-term intervention. [3][4]
This is why medications like Mounjaro target these same biological systems, read about how Mounjaro works for weight loss.
How is obesity measured in practice?
There’s no simple way to directly and perfectly measure whether a person has a harmful amount of body fat. More detailed testing can help, but it is usually too expensive or invasive to be practical routinely.
Instead, clinicians rely on a small number of standardised, minimally invasive measurements to estimate whether a person’s level of body fat is likely to be high enough to increase health risk. [1][5]
The purpose of using these measurements is to apply research findings from the wider population consistently and objectively to individual patients — helping clinicians judge whether someone’s level of body fat has reached a point where it is medically significant and likely to contribute to disease, either now or in the future. [1][5]
Body mass index (BMI)
The most commonly used measure is body mass index (BMI), which is calculated from height and weight. In adults, a BMI of 30 kg/m² or above is typically used as a threshold for obesity, although lower cut-offs may be applied in some ethnic groups because health risks can occur at lower BMI values. [1]
On average, people with a higher BMI have more obesity-related health risks. But what applies on average does not apply to every individual, and there are some common situations in which BMI can be misleading when applied to a single person. [5]
First, BMI does not distinguish between fat and muscle. As a result, a very muscular person may have a high BMI without having excess body fat, and assessing their health risk on the basis of BMI alone would be inappropriate. [5]
Second, it is not only the amount of fat a person carries that matters, but also where it is stored. Fat carried around the abdomen is associated with higher metabolic and cardiovascular risk than fat stored elsewhere in the body, such as the hips, legs, or arms. Someone with a “normal” BMI may still carry a high proportion of abdominal fat and have a higher health risk than their BMI alone would suggest. [5][6]
Looking beyond BMI: waist measurements
Because BMI cannot show where fat is stored, clinicians often look at waist measurements to help refine risk assessment. [1][5]
Measures such as waist circumference and waist-to-height ratio provide a simple way of estimating how much fat is carried around the abdomen and are often used alongside BMI, particularly when BMI alone gives an incomplete picture. [5][6]
Individual risk still varies
Ultimately, the health impact of body fat varies between individuals. Factors such as genetics, age, sex, fitness, metabolic health, and existing medical conditions all influence whether a given amount of body fat is likely to translate into disease for a particular person.
At present, medicine does not have a practical way to measure all of these factors precisely when diagnosing obesity. As a result, definitions and thresholds are necessarily imperfect. Clinicians therefore use standardised measurements such as BMI and waist size as starting
points, and then apply clinical judgement to interpret what those measurements are likely to mean for the individual patient. [1][5]
Why definitions matter
The purpose of defining obesity in medicine is not to apply labels for their own sake, but to identify increased health risk in a consistent and evidence-based way. Clear definitions allow clinicians to draw on research from the wider population and apply it, carefully, to individual patients. [1]
Because the measurements used to define obesity are necessarily imperfect, applying them to individuals will always involve uncertainty. This helps explain why people diagnosed with obesity can have very different health journeys over time. Some may develop complications early, while others remain relatively well for many years. [5]
As medical science advances, it is likely that risk assessment will become more precise. Cheaper, non-invasive tests — including blood markers, imaging, and genetic profiling — may eventually allow clinicians to identify who is at greatest risk with far greater accuracy. In time, obesity may be defined less by external measurements and more by underlying biological markers. [3]
For now, medical definitions of obesity provide a practical framework: imperfect, but useful, and continually refined as evidence improves. [1][3]
This essay reflects the evidence, regulatory landscape, and market dynamics as they stood at the time of writing.
Frequently Asked Questions
These answers provide a general overview. For detailed explanations, evidence summaries, and treatment comparisons, see our in-depth guides in the Knowledge Hub.
What Is Obesity?
Commentary
This essay reflects the evidence, regulatory landscape, and market dynamics as they stood at the time of writing.
What do doctors mean by obesity
Many people use the term obesity, but in medicine it has a specific meaning. Doctors define obesity as a chronic health condition characterised by an excess amount of body fat that increases the risk of ill health. [1]
Importantly, what counts as ‘excess fat’ is not defined by appearance, but by the level of body fat at which health risks begin to increase. [1]
Why excess body fat affects health
In a healthy body, fat tissue performs important functions. It stores energy, helps regulate appetite, and releases chemical signals that allow the body to coordinate energy use, blood sugar control, and metabolism. [2]
However, once fat mass increases beyond a certain point, these signalling systems begin to function less well. The body enters a different physiological state, in which appetite regulation, insulin sensitivity, lipid handling, and inflammatory control are altered. This shift helps explain why excess body fat is associated with an increased risk of conditions such as type 2 diabetes, cardiovascular disease, fatty liver disease, and sleep apnoea. [3]
In this sense, obesity is not simply “more fat”, but a state in which normal regulatory systems are under strain and no longer operate optimally. [3]
Why obesity is a ‘chronic’ condition
Once this altered state is established, the body tends to defend the higher level of fat mass. [3][4]
When someone with obesity tries to lose weight by reducing calorie intake or increasing activity, the body often responds by:
This explains why sustaining weight loss is so biologically difficult and what drives the need for ongoing treatment.
These responses make ongoing weight loss harder and increase the likelihood of weight regain once efforts are relaxed. [4]
This biological resistance to weight loss helps explain why obesity is difficult to reverse once established, and why it is recognised as a condition that usually requires ongoing management, rather than a short-term intervention. [3][4]
This is why medications like Mounjaro target these same biological systems, read about how Mounjaro works for weight loss.
How is obesity measured in practice?
There’s no simple way to directly and perfectly measure whether a person has a harmful amount of body fat. More detailed testing can help, but it is usually too expensive or invasive to be practical routinely.
Instead, clinicians rely on a small number of standardised, minimally invasive measurements to estimate whether a person’s level of body fat is likely to be high enough to increase health risk. [1][5]
The purpose of using these measurements is to apply research findings from the wider population consistently and objectively to individual patients — helping clinicians judge whether someone’s level of body fat has reached a point where it is medically significant and likely to contribute to disease, either now or in the future. [1][5]
Body mass index (BMI)
The most commonly used measure is body mass index (BMI), which is calculated from height and weight. In adults, a BMI of 30 kg/m² or above is typically used as a threshold for obesity, although lower cut-offs may be applied in some ethnic groups because health risks can occur at lower BMI values. [1]
On average, people with a higher BMI have more obesity-related health risks. But what applies on average does not apply to every individual, and there are some common situations in which BMI can be misleading when applied to a single person. [5]
First, BMI does not distinguish between fat and muscle. As a result, a very muscular person may have a high BMI without having excess body fat, and assessing their health risk on the basis of BMI alone would be inappropriate. [5]
Second, it is not only the amount of fat a person carries that matters, but also where it is stored. Fat carried around the abdomen is associated with higher metabolic and cardiovascular risk than fat stored elsewhere in the body, such as the hips, legs, or arms. Someone with a “normal” BMI may still carry a high proportion of abdominal fat and have a higher health risk than their BMI alone would suggest. [5][6]
Looking beyond BMI: waist measurements
Because BMI cannot show where fat is stored, clinicians often look at waist measurements to help refine risk assessment. [1][5]
Measures such as waist circumference and waist-to-height ratio provide a simple way of estimating how much fat is carried around the abdomen and are often used alongside BMI, particularly when BMI alone gives an incomplete picture. [5][6]
Individual risk still varies
Ultimately, the health impact of body fat varies between individuals. Factors such as genetics, age, sex, fitness, metabolic health, and existing medical conditions all influence whether a given amount of body fat is likely to translate into disease for a particular person.
At present, medicine does not have a practical way to measure all of these factors precisely when diagnosing obesity. As a result, definitions and thresholds are necessarily imperfect. Clinicians therefore use standardised measurements such as BMI and waist size as starting
points, and then apply clinical judgement to interpret what those measurements are likely to mean for the individual patient. [1][5]
Why definitions matter
The purpose of defining obesity in medicine is not to apply labels for their own sake, but to identify increased health risk in a consistent and evidence-based way. Clear definitions allow clinicians to draw on research from the wider population and apply it, carefully, to individual patients. [1]
Because the measurements used to define obesity are necessarily imperfect, applying them to individuals will always involve uncertainty. This helps explain why people diagnosed with obesity can have very different health journeys over time. Some may develop complications early, while others remain relatively well for many years. [5]
As medical science advances, it is likely that risk assessment will become more precise. Cheaper, non-invasive tests — including blood markers, imaging, and genetic profiling — may eventually allow clinicians to identify who is at greatest risk with far greater accuracy. In time, obesity may be defined less by external measurements and more by underlying biological markers. [3]
For now, medical definitions of obesity provide a practical framework: imperfect, but useful, and continually refined as evidence improves. [1][3]
Frequently Asked Questions
When doctors describe obesity as a medical condition, they are referring to excess body fat that increases the risk of ill health, not simply higher body weight.
In medicine, obesity is defined by the point at which body fat begins to disrupt how the body regulates appetite, blood sugar, and energy use, increasing the likelihood of conditions such as diabetes, heart disease, and fatty liver disease. Appearance alone is not a reliable guide to this risk.
This is why obesity is understood as a health condition with biological effects, rather than a judgement based on size or shape.
Physical appearance can be misleading because it does not reliably show how much fat is stored inside the body or how harmful that fat is for a particular person.
Some people carry more fat internally, around the organs, which is difficult to judge from appearance alone. In addition, the same amount of body fat can carry different levels of risk depending on genetic background, age, sex, and other factors. For example, some ethnic groups develop metabolic disease at lower levels of body fat than others.
Doctors therefore define obesity by health risk rather than appearance, because the aim is to identify people most likely to be harmed by excess body fat — not to judge how someone looks.
Body fat is not just a storage tissue — it also releases hormones and other chemical signals that help regulate appetite, blood sugar, and how the body uses energy.
As body fat increases, these signals can change. Appetite signals may become stronger, making hunger harder to control.
The body can also become less sensitive to insulin — a hormone that helps move sugar from the blood into cells for energy — causing blood sugar levels to rise more easily.
At the same time, excess body fat can trigger low-grade inflammation, a persistent “stress” response that affects blood vessels and organs. Together, these changes help explain why excess body fat is linked to diabetes, heart disease, and other metabolic conditions.
Doctors classify obesity as a chronic condition because, once established, it tends to persist over time and require ongoing management.
The body adapts to higher fat levels and actively works to defend them. This means that weight loss is not simply a matter of short-term effort, and stopping treatment or support often leads to relapse.
Like other chronic conditions, obesity usually benefits from long-term strategies rather than one-off interventions.
When weight is lost, the body often responds by increasing hunger and reducing the amount of energy it burns, even at rest. These changes can persist long after dieting or increased activity.
This response is biological, not psychological. It helps explain why regaining weight does not reflect a lack of willpower, but rather the body actively defending its previous level of fat.
Understanding this makes clear why long-term support is often needed to maintain weight loss.
Because directly measuring body fat is impractical in routine care, doctors use indirect measurements to estimate health risk.
The most common is body mass index (BMI), which relates weight to height and provides a rough indication of whether body fat is likely to be high enough to affect health. Waist measurements are often used alongside BMI to assess where fat is stored.
These measures are interpreted alongside medical history and other health information, rather than used in isolation.
BMI does not distinguish between fat and muscle, nor does it show where fat is stored.
As a result, muscular individuals may have a high BMI without excess body fat, while others with a “normal” BMI may carry more fat around the abdomen and have higher health risks. BMI thresholds can also differ in accuracy across age groups and ethnic backgrounds.
For this reason, BMI is best used as a starting point rather than a definitive diagnosis
Fat stored around the abdomen — often called visceral fat — surrounds internal organs and is more strongly linked to diabetes, heart disease, and metabolic risk than fat stored elsewhere.
This type of fat releases signals that affect blood sugar control, inflammation, and cardiovascular health. As a result, two people with the same weight or BMI can have very different risk profiles depending on how much visceral fat they carry.
This is why waist measurements are often used alongside weight or BMI.
Yes. Some people with a “normal” BMI may still carry excess body fat in higher-risk locations, such as around the abdomen, or have low muscle mass.
In these cases, BMI alone can underestimate health risk related to body fat. Waist measurements and metabolic markers help identify this risk more accurately.
This is one reason why clinicians avoid relying on weight or BMI alone.
The amount of body fat is only one factor influencing long-term health outcomes.
Fat distribution, genetics, physical fitness, diet, sleep, stress, and existing medical conditions all affect how harmful a given level of body fat is for an individual. As a result, people with similar measurements can experience very different health trajectories.
This variability is why personalised assessment and follow-up are essential.
[1] National Institute for Health and Care Excellence (NICE). Obesity: identification, assessment and management (CG189). Updated 2023.
[2] Scheja L, Heeren J. The endocrine function of adipose tissues in health and cardiometabolic disease. Nature Reviews Endocrinology. 2019.
[3] Lingvay I, Cohen RV, le Roux CW, Sumithran P. Obesity in adults. The Lancet. 2024.
[4] Sumithran P et al. Long-term persistence of hormonal adaptations to weight loss. New England Journal of Medicine. 2011.
[5] Cornier MA et al. Assessing adiposity: a scientific statement from the American Heart Association. Circulation. 2011.
[6] Jayedi A et al. Anthropometric and adiposity indicators and risk of type 2 diabetes. BMJ. 2022.
About the Author
Dr Blunt is a UK-licensed General Practitioner with an Extended Role in Lifestyle Medicine, and a specialist interest in metabolic health, obesity management, and evidence-based medicine. He has completed accredited training in medical weight management, including the national SCOPE obesity programme.
His writing focuses on translating high-quality research into clear, practical explanations to help readers understand complex topics in obesity, medication safety, and long-term health.
GMC: 7527933
Medical Disclaimer
This information is for educational purposes only and should not be considered medical advice. Always consult with a qualified healthcare professional for diagnosis and treatment of any medical condition. All content on this website is for general information only and does not replace personalised medical advice. See full Medical Disclaimer.