About Body Mass Index (BMI)
Body Mass Index, known commonly as BMI, is essentially a person’s weight in kilograms divided by the square of their height in meters.
This number is used to classify individuals into different groups – optimum weight, underweight, overweight, or obese.
BMI has been used as an indicator to determine an individual’s body fat levels.
However, in recent studies conducted, it has been proven time and again that BMI is a poor representation of an individual’s body fat percentage.
BMI can be used as a screening test rather than a diagnostic test.
It can be used to screen the body fat levels and overall health of the individual.
BMI does not furnish information such as the mass of fat in different regions of the body and does not differentiate between body lean mass and body fat mass.
Also, it does not take into consideration factors such as gender, age, and race.
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How to calculate your BMI?
To calculate the BMI using the metric system, we need to divide the weight of the individual (in kilograms) by the square of their height (in meters), i.e.
WEIGHT(Kg) / [HEIGHT(meters)]^2
Since height is usually measured in centimeters, the formula can be written as:
[WEIGHT(Kg) / HEIGHT (cm)/ HEIGHT(cm)] x 10,000
To calculate the BMI in the English system, the formula is:
WEIGHT(lb) / [HEIGHT(in)]^2 x 703
Before the BMI calculation, the weight needs to be converted into decimal values in case it is given in terms of ounces.
BMI categorization
In order to calculate an individual’s BMI, his/her weight(in kgs) must be divided by the square of his/her height(in meters).
Based on the above-mentioned calculation, individuals are categorized as:
| BMI | Category |
|---|---|
| <18.5 | Underweight |
| 18.5 to <25 | Normal |
| 25 to <30 | Overweight |
| 30 or higher | Obese |
Based on BMI, obese individuals are further classified as:
| BMI | Category |
|---|---|
| 30 to <35 | Mild obesity |
| 35 to <40 | Moderate obesity |
| 40 or higher | Extreme or severe obesity |
Is BMI the right way to determine whether you are healthy?
Absolutely not! It is a screening tool to determine a healthy ratio between an individual’s weight and height.
The calculation does not take into consideration many factors such as muscle mass, fat mass in different regions of the body, gender, race, and age of the individual.
Due to this, the BMI of an individual is usually either an overestimation or underestimation of the true body fat.
One of the prime reasons for this is that BMI doesn’t differentiate between muscle and fat.
A tall individual who is lean but has heavy muscle mass can appear to be obese as per the BMI calculation.
But, this is incorrect because the actual amount of fat, which acts as a risk factor for many diseases, is less.
Another problem with BMI calculation is that it doesn’t differentiate between the fat present in different areas of the body.
For example, fat in the belly is harmful, but fat under the skin is harmless.
Few lean individuals have excess belly fat and still may fall under the ‘normal’ range of BMI.
This is incorrect because belly fat increases the risk of an individual to develop many health problems.
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BMI and obesity
As we saw earlier, BMI is used to categorize individuals into underweight, normal, overweight and obese categories.
However, it is not the right way to determine obesity and the amount of body fat.
Yes, it can be used as a screening test, but definitely not a diagnostic test to determine one's obesity.
The reason for this is that the calculation of BMI does not describe what is ‘abnormal fat accumulation’ and does not differentiate between fat mass and muscle mass.
Inaccurate measurement of body fat using only BMI as a tool is potentially dangerous as it can lead to poor treatment, over-treatment, and even social stigma to the individual.
What are the best ways to measure body fat?
The BMI calculation method neither gives accurate body fat percentage nor its distribution.
However, in many cases, individuals need to determine the exact amount of fat in their bodies.
Some of the more accurate ways to measure body fat include:
Body Fat Scales
- Calipers
- DEXA (Dual-Energy X-ray- Absorptiometry) Scan
- Hydrostatic Weighing
- Bod Pod
- Bioelectrical Impedance Analysis (BIA)
- BioImpedance Spectroscopy (BIS)
- 3D Body Scanners
Before deciding on a particular body measurement scale, it is important to answer the following question:
- What is the purpose of assessing the body fat percentage?
- How frequent is the body fat measurement going to be?
- How accurate should the body fat measurement be? Will a regular BMI calculation solve the purpose?
FTO gene and BMI
FTO or Fat mass and obesity-associated gene, as the name says, have been linked to obesity, increased body fat, and an increased BMI.
A particular SNP rs9939609 has been linked to an increase in total body fat levels.
According to a study conducted, the presence of the AA allele of the FTO gene has shown to contribute to obesity and increased BMI, irrespective of how the adipose tissue distribution is.
Factors influencing an individual's BMI like insulin sensitivity and plasma cholesterol levels are also associated with the SNP rs9939609.
The FTO gene has been observed to be associated with calorie intake.
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SEC16B and BMI
Over 90 genes affect an individual's BMI.
One such gene is the SEC16B gene.
The exact function of the gene remains unknown.
It is said to play an important role in packing and distributing fatty acids, proteins, and cholesterol within a cell and through multiple cells.
How this function helps in BMI is yet to be studied.
But, it was found that people having a C allele of the gene are at a risk for high BMI.
