Last updated on August 27th, 2018
This article will help explain BMI – Body Mass Index.
When most people step on the scales they are probably checking to see if they have become overweight.
In essence, they are hoping to determine if they have become “too fat”. However, the scales don’t provide a particularly good way to determine if you are “overweight” because people who have very muscular physiques will weight more than the average person of the same height.
The figures used to determine if people are the correct weight are arrived at using a system called the Body Mass Index (BMI).
What Is BMI and How Does Is It Calculated?
BMI is a concept intended to determine the amount of tissue mass (bone, muscle, and fat) people have in their bodies and then calculate a figure that grades their weight as normal, underweight, overweight, normal, or obese.
BMI is normally calculated by dividing body mass by body height, but it can also be worked out by using a table or chart that shows contour lines or colors to display BMI as a function of mass.
Note: BMI is universally expressed in kg/m2, (taken from mass in kilograms and height in meters). However, if the original measurements are taken in pounds and inches, a conversion factor of 703 (kg/m2)/(lb/in2) is necessary:
A Closer Look at the BMI Scoring System
The World Health Organisation (WHO) states there are six BMI categories and they range from “Underweight” to “Morbidly Obese”.
|Underweight||Less than 18.5|
|Normal (healthy weight)||18.5 – 24.9|
|Overweight||25 – 29.9|
|Moderately Obese||30 – 34.9|
|Extremely Obese||35 – 39.9|
|Morbidly Obese||Greater than 40|
As you will see, according to the table, the desired BMI to shoot for is 18.5 to 24.9. However, the figure is obtained solely on a calculation based on height and weight.
How Accurate Is BMI?
The accuracy of BMI is under constant debate, but the figures provided by the calculation can give the average person a rough idea if their body weight is healthy or not.
Unfortunately, the system will probably tell very muscular individuals they are overweight, or even hyper obese, even though, in reality, their bodies are very lean and fit. A simple calculation based on height and weight is not enough.
Someone who stands six-feet-tall and weighs 85 kg may very well be overweight, but they could also be in perfect physical shape, depending on their muscle to fat ratio.
Using another example, someone who has a BMI of 19 and is classed as being healthy, may actually have a very narrow frame and poor muscle mass, but is seen as being healthy by the system because the excess fat they are carrying around has brought their weight up to be in the “healthy” range.
As you can see, there are some real problems with using the BMI system alone to classify individuals as being underweight, healthy, or fat.
Muscle Tissue V Fat Tissue
It’s often said that muscle tissue weighs more than fat tissue, but this is not strictly true. Muscle tissue is denser than fat tissue.
For example, if you gained an extra pound of fat on your upper arms, it would make your arms look bigger than if you gained an extra pound of muscle tissue in your biceps.
This is why a lot of people who frequent the gym, workout hard, and eat healthy diets can weigh more than people of a similar height who live couch-potato lifestyles, make unwise food choices, and are carrying around an unhealthy amount of body fat.
Individual build can vary a lot from one person to the next and the BMI system fails to take this into account. Some people have shorter torsos and longer legs.
Others have longer arms. Some people have broad shoulders, others have shoulders that are more narrow, and it’s often said women who have wide hips find it easier to give birth.
We are not all the same and, if you stripped all the flesh from our skeletons, so there was no other tissue left to confuse things, the individual differences would be clear to see. BMI is not reliable. It can provide a rough indication, but that’s about it. Figures based on height and weight alone are too generalized to be relied upon entirely.
A Note About BMI Scales and Machines
It’s possible to purchase BMI scales cheaply and easily these days and it’s easy to see the attraction because scales of this nature bring another factor into play.
Apart from providing figures based on body height and body weight, such scales also utilize bioelectrical impedance analysis to try and ascertain how much of your weight is fat and how much of it is made up of muscle, bone, and other tissue.
The scales arrive at this figure by passing a small, unnoticeable electrical current from one foot to the other. Fat does not conduct electricity as well as the other tissues in the body, so a higher resistance indicates a higher level of body fat. It’s nice in theory, but scales that use bioelectrical impedance analysis do not work well in reality.
For one thing, the scales are only sending a current up one leg and down the other, the upper body is not brought into play and, for most people, the upper body is likely to be where they have the greatest amount of fat.
There are also larger free-standing machines available in certain gyms that perform a similar calculation, but results are based on the resistance caused by passing a current from one hand to the other and, again, the full body is not brought into play.
So, To Sum up BMI
BMI is a simple system that relies on a calculation based on a persons height and weight. It does not take into account their build or how muscular they may or may not be.
BMI scales and machines could be seen as being a better option, but the system leaves a lot to be desired.
If you are worried about your weight and feel you may be “too fat”, it may be worth visiting your doctor and asking him to do an analysis using skinfold calipers. It’s a painless procedure that involves using special calipers that measure the amount of fat that has built up under the skin in various areas of the body and then using the figures to determine if there’s a problem and, if so, how bad it has become.
However, prevention is always the best medicine, so “now” is always a good time to take a look at your present lifestyle and food choices and make a switch for the better.