Body Mass Index (BMI), calculated based on your height and weight, is an internationally recognized objective indicator to measure overweight and obesity in adults. BMI classifies an individual's risk of overweight or obesity based on weight with respect to height from a medical point of view. According to the Centre for Health Protection, the prevalence of overweight and obesity in Hong Kong amongst adults are:
The increasing prevalence of overweight and obesity should not be overlooked because obesity increases the risk of many chronic diseases, such as hypertension, heart disease, diabetes, cerebrovascular disease, cancers, etc.
Body mass index (BMI) is calculated by the following equation:
For instance, if a person's weight and height are 60kg and 165cm respectively, then his/her BMI is calculated as follow:
WHO proposed specific BMI standards for different regions around the world. Based on the WHO Western Pacfic Region report conducted in 2000, the classification of weight by BMI for adults in the Asia-Pacfic region (including China, Hong Kong and Taiwan) is as follows:
|Normal range||18.5 - 22.9|
|At risk||23.0 - 24.9|
|Obese I||25.0 - 29.9|
|Obese II||⪰ 30.0|
Because the height and weight of children and adolescents vary greatly, the above BMI standard for Asian adults is not applicable to those under the age of 18, so the WHO has set up another BMI standard to measure whether children and adolescents are obese or overweight. According to Centers for Disease Control and Prevention, the following graphs show the BMI percentiles of boys (in blue) and girls (in red) respectively:
The following table shows the classification of weight by BMI for children and adolescents aged 5-19:
|Underweight||< 5th percentile for children of the same age and sex|
|Healthy weight||5th - < 85th percentile for children of the same age and sex|
|Overweight||⪰ 85th - < 95th percentile of the same age and sex|
|Obese||⪰ 95th percentile for children of the same age and sex|
Based on BMI classification, overweight individuals are under risk of developing different diseases.
Overweight and obesity are defined as the abnormal or excessive fat accumulation that may impair health. According to a WHO report published in 2000 titled Obesity: preventing and managing the global epidemic, common obesity health risks are:
Obese individuals have 3 times higher risk of:
Obese individuals have 2-3 times higher risk of:
Obese individuals have 1-2 times higher risk of：
Obesity is not only a risk factor of many diseases, it is also a major public health issue. According to an analysis of the Global Burden of Disease Study,
The Journal of the American Medical Association in 2013 summarized the all-cause mortality risk for overweight and obesity relative to normal BMI. It is found that obese class I and class II are significantly correlated with higher mortality risk. But what is more interesting is that overweight is associated with significantly lower all-cause mortality than the normal BMI. Some researchers therefore proposed several explanations to account for this surprising finding:
BMI is a handy indicator to provide a rough idea of the risk level of morbidity and mortality caused by overweight and obesity at a population level. However, as BMI does not provide information about an individual's body composition (i.e. muscle to fat ratio) or body shape, it should not be used as the only predictor of overweight and obesity. For instance, athletes have more muscles, which has a heavier mass than fat, thus a higher BMI than people of the same age and gender. Whereas for elderly or disabled people, they usually have low muscle mass. Therefore, a formal body fat measurement and thorough body check is needed to confirm the risk of overweight and obesity.
Body fat percentage (BF%) refers to the ratio of body fat to body weight, which can accurately measure the body fat content. It is a more specific measure of fat mass compared to BMI. There are several methods to measure BF%, namely: More expensive and inaccessible:
More convenient and accessible:
According to American College of Sport Medicine, the standard BF% for men and women of different age groups are as follows:
Although BMI is widely used to predict obesity risk, it fails to effectively reflect the body's fat distribution. Whereas central obesity, the accumulation of excess fat in the abdominal area, is a good estimation of body fat, particularly internal fat deposits. Studies pointed out that central obesity, similar to overall obesity, is a risk factor to assess an individual's risk of developing cardiovascular diseases and other health issues. Waist circumference is a measurement of central obesity. It is measured at the narrowest part of the torso. Measuring waist circumference may be tricky, but the following procedures will guide you through the steps for an easy and accurate measurement.
Central obesity is indicated by a waist circumference equal to or exceeding the following standards for men and women respectively.
|Waist circumference||⪰ 90 cm|
Whether you are planning to lose weight or just want to check your health risks, your BMI results can serve as a good starting point for you to understand your body condition and manage your health better. Talk to your doctor for more advice on weight management.
In fact, genetics is only one of the many causes of obesity, and most obese people gain weight because of lifestyle and environmental factors, such as poor eating habits and lack of exercise. The happy news is that you can choose your way of living, so being born with obesity genes does not necessarily mean that you are predestined to be obese.
BMI is used as a screening tool, but it is not precise enough to diagnose an individual’s risk of overweight and obesity. A set of comprehensive assessments, including body fat measurement, diet tracking, physical activity and family history should be conducted by healthcare practitioners to make an effective diagnosis.
Body mass index (BMI) is calculated by the following equation:
weight (kg) / height x height (m2)
For instance, if a person’s weight and height are 60kg and 165cm respectively, then his/her BMI is calculated as follow:
60 (kg) / 1.65 x 1.65 (m2) = 22.0kg/m2
This article was independently written by Healthy Matters. It is informative only and not intended to be a substitute for professional medical advice, diagnosis or treatment. It should not be relied upon for specific medical advice.
Healthy Matters is Hong Kong’s leading health resource. Our mission is to help you make better health decisions and take control of your health.
Our team of experts is committed to producing reliable health content that is accurate, engaging and relevant, to cover your health & wellness journey from prevention to treatment.
Whether you are looking for trusted information on health conditions, wellbeing or looking for the right doctor or service in Hong Kong, we’re here to help!
Your health matters. Begin your health journey with Healthy Matters today!