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The Body Mass Index (BMI) is a weight-for-height index that classifies underweight, overweight, and obesity in adults. BMI is calculated by dividing weight in kilograms by the square of height in meters (BMI values represent units of kg/m2, and all BMI values throughout this article should be assumed to be measured in this unit). There are different BMI categories, explored in detail below1.
BMI result: Below 18.51
Those who fall into this category are defined as underweight.
Being underweight could be a sign you're not eating enough or may indicate an underlying medical condition. If you're underweight, contact your doctor for further evaluation.
BMI result: Between 18.5 and 24.91
The medical community recommends keeping your weight within this range.
BMI result: between 25 and 29.91
People in this category may be at risk of developing obesity2.
There may also be a risk of developing other health problems or current health problems worsening over time. Based on European and American clinical guidelines for obesity management in adults, the following are the most likely recommendations based on BMI ranges.
BMI result between 25 and 26.9
People who do not have weight-related health problems (for
example, high blood pressure or high cholesterol) are encouraged to
eat healthy and increase their physical activity to prevent further
weight gain.
BMI result between 27 and 29.9
People in this range who also have weight-related health
problems will likely be recommended to lose weight by combining
lifestyle interventions and consider anti-obesity medications.
Committing to health interventions can lead to weight loss, improved health, and experiencing a higher quality of life.
There are three classes of obesity based on BMI ranges2:
| Classification | BMI |
| Class I | 30.0–34.9 |
| Class II | 35.0–39.9 |
| Class III | Above 40 |
BMI ranges are based on the effect excess body fat has on an individual’s health, life expectancy, and the risk of developing weight-related health complications3.
BMI result: 30.0 or higher
People who have a BMI equal
to or above 30 may suffer from obesity1.
Health organizations now recognize obesity as a chronic, but manageable disease that is best dealt with using a multi-disciplinary weight loss treatment approach4.
As BMI increases into the range to be considered obese, so does the risk factor of certain chronic diseases3.
Higher BMIs (greater than or equal to 30) have statistically significant associations with several medical conditions. These conditions include but are not limited to - cardiovascular disease, type 2 diabetes, osteoarthritis, and various cancers3,5.
BMI is not a diagnosis of obesity but instead can be used to screen for health risks3.
People with a BMI equal to or above 30 are highly recommended to consult a doctor trained in obesity management.
There are various scientifically proven treatment options for obesity. Treatment options are dependent on the specific needs of the individual, their current health status, and presence of weight-related health complications2.
Treatments may include a combination of the following options2,5:
*Bariatric surgery is considered for adults with a BMI of 40 or above or adults with a BMI of over 35 who also have weight-related comorbidities6.
Disclaimer: This information is not a substitute for the advice of a healthcare provider. If you have any questions regarding your health, you should contact your general practitioner or another qualified healthcare provider.
Obesity is a chronic disease which requires medical attention. For most populations, being overweight or obese (BMI greater than or equal to 25) is associated with an increased risk of mortality and increased risk of comorbidities7.
Obesity screenings can take BMI threshold levels into account. Obesity can also be an indicator of potential future health issues that may require medical advice.
In general, the higher your BMI in the range to be considered obese, the greater the chance of developing other chronic obesity-related diseases including3,5,6:
If you are concerned about any of these diseases and how they relate to your BMI, consult your doctor for further information and evaluation.
BMI can be misleading in certain cases. Research has shown that BMI can less accurately predict the disease risks for some groups of people:
For instance, in certain populations such as elite athletes or bodybuilders an elevated BMI does not directly correlate with their health status. Their increased muscle mass and therefore weight also increases their BMI.
The table below shows how the average body fat percentages differ according to specific groups and categories:
| Description | Men | Women |
| Essential fat | 2 - 5 % | 10 - 13 % |
| Athletes | 6 - 13 % | 14 - 20 % |
| Fitness | 14 - 17 % | 21 - 24 % |
| Acceptable | 18 - 24 % | 25 - 31 % |
| Obesity | >25 % | >32 % |
At present, there is no individual BMI calculation for women and men. However, whilst gender is not factored into BMI calculations, the physiological differences between genders may imply a difference in the degree of risk at a given BMI8.
Men:
In terms of weight distribution, it has been
reported that men tend to accumulate body fat in the upper body,
including the abdomen8.
Abdominal obesity and higher concentrations of visceral fat in men lead to a higher risk of heart disease and type 2 diabetes3,8.
Women:
Whilst women tend to have a higher percentage of
body fat than men, fat deposits in females tend to be distributed in
the hips and buttocks8.
Due to differences in fat deposition, women may be at lower risk of the comorbidities associated with obesity compared to men at the same or similar BMI8.
Adult BMI calculations do not take age into account. However, research suggests that whilst obesity increases mortality risk at any age, this correlation is much stronger in people below the age of 509.
Accelerated weight gain as a child has been shown to imply further weight gain during adolescence and adulthood. Weight gain as a child is therefore a strong indicator of obesity in adulthood9. A study shows that 40% of children with obesity will become obese adults.
Whilst BMI is interpreted differently for children and adolescents compared to adults, growing evidence suggests that BMI guidelines should be age-specific for adult populations as well10.
BMI interpretation in children and adolescents is both age- and gender-specific. This is because girls and boys develop at different rates, with body fat varying during developmental periods such as puberty4,11.
Obesity in childhood has been shown to be a strong predictor of various obesity-related diseases such as type 2 diabetes, dyslipidemia, and sleep apnea. Obese children are also more likely to suffer from psychological distress. This can include low self-esteem, anxiety disorders, and depressive symptoms4.
If you are a parent concerned about the health of your child with relation to their weight, consult your doctor for guidance on weight management and possible treatment options.
The composition of our bodies naturally changes with age. An increase in body fat is statistically likely to occur over adulthood, whilst total muscle mass also decreases with age12.
Muscle mass and strength are considered important for the maintenance of physical activity.
Studies have shown that when using standard BMI calculations, being slightly overweight is associated with a reduced risk of mortality compared to the ‘normal’ weight range in older populations12.
The standard BMI calculation can also underestimate or overestimate the amount of excess fat carried by elderly persons. Assessments such as waist circumference have therefore been recommended as better options when measuring body fat in the elderly12.
Diagnosing obesity should not be limited to measuring BMI alone. However, BMI can help identify people who would experience health improvements from obesity management5.
Diagnostic testing is often ordered during initial obesity assessments, with the aim of discovering metabolic problems and personalizing treatment options. Screening will typically involve various types of laboratory testing:
In addition to these tests, healthcare providers may take a comprehensive diagnostic approach to understand the underlying causes of obesity. A comprehensive approach aims to discover potential contributing factors to a person’s obesity, and therefore provide an individualized treatment program5.
Consult your doctor if you would like to learn more about comprehensive diagnoses for obesity.
To gain a better understanding of health, other diagnostics and measurements may be taken alongside BMI (for example, waist circumference).
Waist circumference is an indirect measure of abdominal fat, whereas BMI is a representation of total body fat6. Waist circumference has therefore been cited as a more accurate measure of obesity-related health risk, such as comorbidity and mortality13.
Researchers have recommended that waist circumference be used together with BMI to more accurately evaluate an individual’s health risk factors13.
Regardless of BMI, you should consult your doctor if you have concerns about your health.
Weight management programs may be relevant if your waist measurements are or exceed5:
Higher waist circumferences are associated with greater health risk. You may want to consider obesity treatment programs if your waist measurements are as follows5:
Typically, people suffering from obesity present a variety of health conditions collectively known as metabolic syndrome14.
Screening for metabolic syndrome is recommended for the majority of people with higher BMIs2.
This involves looking for the metabolic risk factors associated with obesity, including the following2:
At least three metabolic risk factors must be present to be diagnosed with metabolic syndrome. As such, metabolic syndrome is defined as a cluster of conditions, and can raise the risk of heart disease, type 2 diabetes, and stroke14.
The link between obesity and obesity-related complications is strong but not absolute.
Some people with obesity do not present with metabolic syndrome and are reported to have limited health risks at higher BMIs. This group is defined as metabolically healthy obese individuals15.
These individuals have lower risk of developing diabetes and heart disease compared to other people with obesity who suffer from metabolic syndrome15.
However, clinical guidelines specify that metabolically-healthy obese people cannot be considered ‘medically healthy’. They are at greater risk of mortality, as well as other non-metabolic conditions such as depression, back pain, and sleep apnea15,16.
Despite the absence of metabolic risk factors, a study found that metabolically-healthy obese people were likely to develop metabolic abnormalities within 10 years. This means they are still at risk of progressing to an unhealthy metabolic state17.
If you are obese but do not present with chronic disease symptoms associated with obesity, you may consider consulting your doctor to evaluate your metabolic risk factors.
Adopting a healthier lifestyle can help this risk group to prevent medical complications and avoid further weight gain.
The BMI was conceived by the Belgian mathematician, Lambert Adolphe Jacques Quetelet, in the mid-19th century.
Despite not being a doctor, Quetelet introduced the concept of social averages. He noted the relationship between weight and height in what was first known as the “Quetelet Index.”
Keys et al later popularised the measurement, describing it as the Body Mass Index and using it as a classification in population-based studies.
The BMI has been adopted into modern medical practices, especially
in Western societies with rising obesity rates.
BMI is a simple and objective measurement, which can be easily conducted by a doctor or anyone concerned about their health.
However, beyond the previously discussed limitations, you may also consider that BMI measurements do not take account of the following18:
It is also important to remember that obesity is not a definitive indicator of ill-health, just as being at a ‘normal’ weight does not mean you are healthy.
Your BMI does not define you, but knowing and understanding your BMI can be a powerful tool for taking charge of your own health.
Consult with your doctor in order to discuss your weight and health status and evaluate what actions may be needed.
There is an important relationship between the amount of body fat a person has and the impacts on our health. Studies have demonstrated health risks associated with both extremes of the BMI spectrum2.
Various factors independent of weight can put you at risk of developing chronic diseases (such as ethnicity and genetics). It is important to be aware of these factors, and how they might contribute to your risk if you do suffer from obesity.
BMI levels greater than or equal to 30 are associated with increased mortality and risk of health complications6. Obesity screenings should take such BMI thresholds into account.
There are ‘metabolically healthy’ people that sustain limited health issues at higher BMIs. However, obesity can still increase other health risks for such individuals compared to those at lower BMIs15,16.
For the majority of people with obesity, screening for metabolic syndrome is recommended2.
Understanding your BMI can help you find a healthy weight range and identify the best way to reach or maintain it together with your healthcare team. For most populations, having a BMI over 25 increases health risk factors7.
Your BMI should be used as a guideline and first step towards understanding your weight and height. Adhering to a healthy diet and lifestyle is recommended by healthcare professionals – regardless of your current BMI.
For BMIs equal to or above 25, other actions might be needed in addition to diet and physical exercise. Consulting your doctor is the best way to define the right solutions for you.
Seek medical advice if you have any concerns regarding your weight.
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