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Is BMI an accurate measure of body fat?

Is BMI an accurate measure of body fat?

Is BMI an accurate measure of body fat?

It can be a devastating or uncomfortable experience for many when doctors tell you that your Body Mass Index (BMI) is too high and you need to lose weight.

But you shouldn't jump into panic mode if your BMI is high. Understanding what the BMI number suggests and other tools to measure body fat more accurately is essential.

History of BMI

BMI was devised by Lambert Adolphe Jacques Quetelet, a Belgian mathematician. It is calculated based on a person's body weight and height. We have to note that there were no calculations, computer or electric devices during Quetelet's time, which could be why he might have opted for a super simple system. But why do health authorities continue to use the BMI formula even today? Maybe nobody wants to rock the boat!

Here is what your BMI numbers mean:

  • Below 18.5 is underweight
  • 18.5 to 24.9 is normal
  • 25 to 29.9 is overweight
  • 30 and over is obese

Limitations of BMI

A BMI score can tell you if you carry too much weight, but it cannot tell if you have excessive body fat because BMI cannot differentiate between excess fat and muscle or bone.

Also, the BMI calculations for adults don't consider age, gender, and muscle mass. Even if your BMI score is low, you could still be having excess fat. Generally, muscle weighs more than fat, so BMI will by default classify muscly, athletic people as fatter than they are.

It could mean that even athletes and muscular adults could be classed as "overweight" or "obese" even though their body fat is low. And to complicate the BMI numbers further, adults who lose muscle as they get older may fall into the "healthy weight" range despite carrying excess fat. The BMI calculator results do not apply to pregnant and people with an eating disorder.

Also, BMI is not the best measure of overall health and body fat, especially for Asian origin people. Asians are more likely to have fat around their waistline, and this abdominal fat is particularly more dangerous. So doctors are advised to use a BMI scale with lower cut off points for Asians.

What is the waist size, and why does it matter?

The BMI does not consider how fat is distributed around the body. Belly fat affects the liver, heart, kidney and other organs more than fat around the bottom or hips.

Waist circumference indicates abdominal fat levels. Measuring waist size is a simple and easy way to check you are not carrying too much fat around the belly area, which increases the risk of type 2 diabetes, heart disease, and stroke. You could still have excess tummy fat, which means you could be at risk of developing these chronic conditions.

Waist size measurements

Ideally, waist circumference should be less than half your height. So it is straightforward to calculate. 

However, irrespective of your height or BMI score, you need to consider losing weight if your waist is:

  • Above 94 cm (37 inches) for men
  • Above 80 cm (31.5 inches) for women

Waist circumference of 102 cm (40 inches) or more for men and 88 cm (34 inches) or more for women is considered very high risk for chronic health problems. You should consult your doctor.

Why is the BMI score still used despite its limitations?

Although BMI is not a perfect health indicator by any means, it has many advantages. It is a simple, easy to measure, inexpensive, and convenient way to measure body fat. It is easy for health authorities to understand obesity in general populations. 

Bottom Line

The BMI score is one of many screening tools, which should not be used as the only measure of someone's health. Having a higher BMI score doesn't always mean your health is in some grave danger.

One of the best ways to increase BMI accuracy is to add waist size to the mix. An account of all other factors, including ethnicity, family history, personal history and detailed body measurements, provides a better indicator for your current and future health.

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