All You Need to Know About the Body Mass Index (BMI)
The Body Mass Index, most commonly known by its acronym, BMI, is a measurement which attempts to work out whether a person’s weight is healthy. BMI is quite a contentious topic within medicine, as many often debate its usefulness, reliability and accuracy.
Many find it a useful indicator of health status in relation to weight, while others believe it to be too simplistic and that it does not necessarily tell the full story about a person’s weight and healthiness.
In this article, I am going to tell you everything you need to know about BMI – how it came to be, what it measures, how it classifies people, pros and cons, as well as some health risks associated with high/low BMI levels.
The Body Mass Index has its roots in the 19th century in the works of Adolphe Quetelet, a Belgian scientist who was responsible for creating the field of â€œsocial physicsâ€ in 1850. The actual term ‘BMI’, however, was not coined until the early 1970s, in a medical paper written by American physiologist Ancel Keys.
The BMI equation is quite simple, and reads as follows:
BMI = (weight (in kilograms))/(heigh ^2 (in metres))
All you need to do is divide your weight in kilograms by your height squared in metres, and this will give you your BMI measure.
Depending on your result, you will fall under one of various categories. These are:
- Underweight: 16 to 18.5 BMI
- Normal weight: 18.5 to 25 BMI
- Overweight: 25 to 30 BMI
- Obese Class I (low-risk): 30 to 34.9 BMI
- Obese Class II (moderate-risk): 35 to 39.9 BMI
- Obese class III (high-risk): >40 BMI
Here is an example of a typical BMI chart:
There are a number of benefits when it comes to using BMI as a measurement to evaluate a person’s weight-height relationship.
One obvious advantage is that carrying out a BMI test is extremely cheap and fast, which is beneficial for doctors and clinics who want get a basic idea in regards to a person’s weight without using too many resources. In contrast, measurements that attempt to find body fat or muscle levels can be much more expensive and time consuming.
Furthermore, BMI seems to work fairly well for what it is intended to do, which is to measure a population’s obesity rates. Particularly in the modern day, where obesity rates are increasing across the world, BMI is useful for researchers as it allows them to get a good idea of how these rates change over time and across different populations.
BMI is also quite useful in terms of helping doctors gain an idea of what sorts of health risks are associated with being underweight, overweight and obese. Despite this, doctors tend to agree that BMI is most useful when used in conjunction with other weight related measurements, which brings us to BMI’s disadvantages.
Due to BMI being a relatively simple calculation that only takes into account a person’s weight and height, it is clear that there are many other important health aspects that it fails to address, simply in virtue of not being designed to cover all of them.
For example, BMI doesn’t differentiate between lean tissue and fat weight. Imagine that a fairly short but extremely fit and overly muscular guy walks into the doctor’s office to get a BMI test. Because muscle tends to be heavier than fat (I know, it’s more complicated than this), his results would probably place him on the overweight, possibly even obese classification, which would be inaccurate.
Furthermore, in relation to the same issue of differentiation, your BMI level may classify you as having a â€˜normal’ weight; however, your body may still hold excess fat, which may bring you many of the same health risks of obesity-related diseases.
Moreover, BMI also fails to differentiate between subcutaneous and visceral fat, which is quite important. Subcutaneous fat is located under your skin, and is what most people think of when they think of body fat. Visceral fat, however, is what people should be paying most attention to, as it is this â€˜hidden’ type of fat that carries with it the most health risks. The latter is the fat located deep in your abdomen, and which protects your vital organs.
Despite all these shortcomings, however, BMI is not useless. Firstly, it is not, and was never meant to be a comprehensive, holistic test that evaluates a person’s health in any meaningful sense. It is simply an indicator of a person’s height-weight relationship, and this can bring some useful information to light.
Secondly, the best approach when it comes to these types of measurements that are concerned with bodyweight and the risks associated with high (or extremely low) fat levels is to take a more holistic approach. This means using a variety of measurements, such as height-to-waist ratio, waist-to-hip ratio and body composition tests (which measure body fat and lean body mass).
Ultimately, BMI is simply an indicator. It is not the most accurate, but neither is it the most useless; particularly considering how easy and cheap it is to use in public health and other situations where resources are restricted.
Despite many of its shortcomings and limitations, BMI has been used to detect the potential for many obesity-related diseases, such as coronary heart disease (whose incidence is proportional to high BMI levels); increased blood pressure and blood cholesterol levels; diabetes and heart disease; dyslipidemia; type 2 diabetes; strokes; gall-bladder disease; osteoarthritis; respiratory problems and some types of cancer.
These, of course, are not trivial and should be taken seriously.