Is BMI a legitimate indicator of obesity?

Last updated on September 24, 2021.

Calculator | Standard range | Implications | Alternatives

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:

  • overweight (BMI: 23.0 – 25.0): 20.1%
  • obese (BMI ⪰ 25.0): 29.9%
  • overweight or obese (BMI ⪰ 23.0): 50.0%

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.

BMI calculator


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:

Standard BMI range

BMI standard for Asian adults

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:

Classification

BMI range

Underweight

< 18.5

Normal range

18.5 - 22.9

Overweight

⪰ 23.0

At risk

23.0 - 24.9

Obese I

25.0 - 29.9

Obese II

⪰ 30.0

BMI standard for children

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:

Classification

BMI percentile

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

Implications of BMI

Based on BMI classification, overweight individuals are under risk of developing different diseases.

Health risk of overweight and obesity

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:

  • Sleep apnea
  • Dyslipidemia (Excessive cholesterol can lead to the accumulation of plaque on the inner walls of blood vessels, which narrows the blood vessels and increase the risk of coronary heart disease and stroke)
  • Gallbladder disease
  • Type 2 Diabetes Mellitus
  • Metabolic syndromes

Obese individuals have 2-3 times higher risk of:

  • Coronary Heart Disease
  • Hypertension (High blood pressure is the cause of many diseases, possibly leading to heart disease, stroke, etc.)
  • Osteoarthritis
  • Hyperuricemia
  • Gout

Obese individuals have 1-2 times higher risk of:

  • Increased risk of anesthesia
  • Back pain
  • Polycystic ovary syndrome (PCOS)
  • Impaired fertility
  • Cancer (Menopausal women are at risk of breast cancer, endometrial cancer, colon cancer
  • Reproductive hormone abnormalities

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,

  • 3.4 million adults die each year due to overweight and obesity
  • 3.8% of global disability-adjusted life year (DALYs) (it is a measure of overall disease burden, expressed as the number of years lost due to ill health, disability or early death) in 2010 were caused by overweight and obesity
  • obese individuals spend 30% higher healthcare costs than non-obese individuals 

Obesity paradox

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:

  • Overweight individuals have greater likelihood of receiving optimal medical treatment
  • Increased body fat may impose a cardioprotective metabolic effect
  • Higher metabolic reserves bring certain health benefits

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Alternatives to BMI

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

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:

  • Underwater weighing — gold-standard with the highest accuracy
  • Plethysmography (Bod Pod) — an instrument that measures changes in volume in different parts of the body
  • Dual energy x-ray absorptiometry (DXA) — x-ray imaging

More convenient and accessible:

  • Bioelectrical impedance (BIA) — estimates body fat content by determining the resistance of the body as an electric current flows through
  • Skinfolds — a common measurement made by pulling a fold of skin in particular sites to estimate body fat content

According to American College of Sport Medicine, the standard BF% for men and women of different age groups are as follows:

Waist circumference

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.

  1. Remove items around your waist.
  2. Keep breathing naturally, and put the measuring tape around the waist when exhaling.
  3. The measurement position should be in the middle of the bottom edge of the last rib and the top of the pelvic bone.
  4. Avoid squeezing the abdominal tissues and record the measurement in centimeters (cm).

Central obesity is indicated by a waist circumference equal to or exceeding the following standards for men and women respectively.

Men

Women

Waist circumference

⪰ 90 cm

⪰ 80 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.

FAQs

Is obesity inborn?

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.

How is BMI used?

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.

How is BMI calculated?

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:

 

This article was independently written by Healthy Matters and is not sponsored. 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.