Body Mass Index – Poor Measurement of Fat Content

Written by Pliro
Feb 12, 2018 Last updated: Feb 12, 2018

What Is Body Mass Index?

Body Mass Index or BMI is the weight to height ratio in simple words. Physicians often use it to estimate the fat content of an individual. In simpler words, it is the weight in kilograms divided by the square of height in meters. The results put individuals into different categories:

  • Underweight: BMI <20
  • Healthy: BMI 20-25
  • Overweight: BMI 26-30
  • Obese: BMI > 30

BMI Accuracy

With passing time and modern research, people are starting to doubt the accuracy of BMI to assess the body fitness. One of the major reasons is that the BMI completely fails to measure lean muscle mass which is a lot more in weight as compared to fat. This means that BMI cannot differentiate between fat and muscle mass. Hence it will likely categorize athletes having more muscle mass and less fat content under the category of unhealthy, overweight individuals.  Similarly, those with massive fat content can be mistakenly classified as healthy. BMI also fails to distinguish bone weight from fat weight. Thus it can tag subjects with heavy bone structure as Overweight or Obese.

Recent Research

A study published in the journal PLOS One documented such inconsistencies and questioned the reliability of BMI to classify weight status of 1,400 men and women. The study mentioned that about fifty percent of female participants who were not categorized as obese according to their BMI were actually obese. Among the men, in contrast, about a quarter of obese men were classified as healthy. Further, a quarter of the men who were categorized as obese by BMI were not obese based on their body fat percentage. 

Other Methods

It is possible to use some alternative methods which are simpler and more efficient in evaluating the fat content and fitness level.

Body Shape

On the top comes the body shape i.e. the fat distribution in the body. Fat around the abdominal region being the most important factor. A subject having an Apple shaped body (more fat around the abdomen) is at a higher risk of cardiovascular diseases and mortality. People having a Pear shaped body (more fat around the buttocks and thighs) are at a lower risk.
According to the thresholds given by experts, waist circumference above 88cm for women and 102cm for men denotes abdominal obesity and increases the risk of cardiovascular diseases, diabetes and mortality.

Waist to Hip Ratio

Another method to assess the fat content and related risk is Waist to Hip Ratio. This method uses two measurements of body to measure the risk present and which cannot be calculated by BMI alone. Waist to hip ratio is Waist Measurement divided by Hip Measurement and as per threshold for women it should be around 0.85 and for men it should be 1. Anything more than these points towards increased health risks.

Body Fat Percentage

One of the most reliable methods in this regard is the Body Fat Percentage. It measures the fat content and muscle mass separately. There are different formulas for this modality. Some of them use a combination of weight, height, wrist circumference, waist circumference and hip circumference. Others add forearm circumference and neck measurements as well. Different Body Fat Percentage Calculators are available on websites making bedside calculations a lot easier for individuals.

In conclusion, it is easy to calculate BMI and it may help the doctor in making a rough estimation of the fitness level. However one should not depend on the BMI for accuracy. Hence it is not ideal to rely on BMI for the measurement of health risks.