Why the BMI chart is wrong

Body mass index (BMI) is a popular technique that is used to indicate whether an individual’s weight is healthy in proportion to their height. This technique is outdated, however, and there are much more accurate ways to calculate an individual's health.

What is BMI?

In 19th century Belgium, Adolphe Quetelet invented BMI. It was not until the mid-20th century that BMI became a popular way to describe individual health though, with insurance companies looking for a way to determine how much to charge clients for life insurance.

BMI, or body mass index, is a calculation that divides an adult’s body weight in kilograms by their height in metres squared. A healthy BMI for the average adult is between 18.5 and 24.9.

A BMI that exceeds 25 is classified as obese. Obesity is associated with a greater risk of developing health-related issues such as:

• Cardiovascular diseases like coronary heart disease

• Type 2 diabetes

• High blood pressure

• Metabolic disease

• Sleep apnea

• Certain types of cancer

• Arthritis

• High cholesterol

A BMI under 18.5 is regarded as underweight. Individuals who are underweight are more likely to develop illnesses like:

• Hypothermia

• Malnutrition

• Osteoporosis

• Fertility issues

• Decreased muscle strength

• A poor immune system

BMI	Range
< 16	Severely underweight
16.1 - 18.5	Underweight
18.5 - 24.9	Healthy weight
25 - 29.9	Overweight
30 - 34.9	Moderately obese
35 - 39.9	Severely obese
> 40	Very severely obese

BMI Chart / Wikimedia Commons

Problems with BMI

Does not distinguish between body fatness, bone density and muscle mass

One of the biggest issues with the BMI chart is that it does not distinguish between body fat, bone density, and muscle mass.

Muscle is far denser than fat, so two individuals may weigh the same but this does not mean they necessarily have the same fat percentage. This issue is particularly prevalent with athletes and bodybuilders.

For example: There are two 6 foot tall men who both weigh 90kg. One is an athlete and one leads a sedentary lifestyle. Both these men would be classified as overweight, with a BMI of 26, but this is clearly an inaccurate description of the athlete as he is in peak physical health.

Does not take into account the differences in sex, age, and ethnicity

Notably, the founder of the BMI was not a physician. Quetelet’s studies included mathematics, astronomy, sociology, and statistics, but not science. When Quetelet founded the BMI, he only used data from individuals of European, Anglo-Saxon descent.

BMI only takes into account sex and age, alongside height and weight, for children and adolescents between the ages of 2 and 18. It does not take into account that:

• Men and women have different body compositions and physiques - women have more body fat than men even with the same BMI

• Older adults and women can be in the healthy BMI range but still have too much body fat

• Black, Asian, and minority ethnic groups are at a greater risk of developing certain diseases, such as type 2 diabetes, with a BMI of over 23

Does not factor how body fat is distributed

BMI does not take into account how fat and weight is distributed on the body and instead regards all weight as fat.

Visceral fat and upper body fat is more harmful for individual health than fat found in other areas of the body. These fats are more commonly associated with obesity-related health problems.

Alternatives to BMI

Fortunately, there are many alternatives to BMI that can determine an individual’s health.

Waist-to-height ratio

The waist-to-height ratio calculator uses an individual's waist circumference and their height to measure the fat distribution on their body.

For both men and women, a healthy waist-to-height ratio is between 0.4 and 0.49. Those with a ratio between 0.5 and 0.59 are at a greater risk of developing diseases related to obesity.

Body Shape Index

The Body Shape Index (ABSI) calculates an individual’s risk of premature mortality, using their sex, age, height, weight, and waist circumference.

ABSI z-score	Mortality risk
< -0.868	Very low
Between -0.868 and -0.272	Low
Between -0.272 and +0.229	Average
Between +0.229 and +0.798	High
> 0.798	Very high