Science Explains How to Calculate your BMI

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Have you ever asked yourself this health-related question? Knowing your BMI (Body Mass Index) can help you determine your total body fat. But what is the body mass index?

BMI is a mathematical formula that measures a person’s weight to his/her height. BMI does not provide a direct measure of a person’s total body weight. However, it indicates a person’s body fat.

According to a study report by the Center for Disease Control and Prevention (CDC), the Body Mass Index formula is used by health professionals to screen for several weight categories. Different weight categories are used to determine whether an individual’s BMI is prone to various common health problems or not.

How to Calculate the BMI

BMI calculation is based on two metrics; height and weight. To calculate Body Mass Index for adults, we use a mathematical formula of; Weight in kilograms/ height in meter squared. Alternatively, we can also use weight in pounds divided by height in inches and then multiplied by a conversion factor of 703.

Body Mass Index (BMI) is widely used as a tool for assessing overweight as well as obesity in the general population.

Health professionals use the BMI method for its inherent simplicity. Once you get your BMI score, you can, therefore, use that data to analyze your weight. And you can compare it to that of the general population.

The following table shows various BMI data ranges and their respective body weight status categories.

BMI (BODY MASS INDEXWEIGHT STATUS
Below 18.5Underweight
18.5 – 24.9Normal
25.0 – 29.9Overweight

Body Mass Index for Children and Teens

Calculating BMI for children under the age bracket of 2-19 years, a different BMI formula is used to put into account age and gender.

The initial procedure involves taking the child’s weight and height records. Once you have the data, you can then proceed to CDC BMI for age growth charts for both boys and girls.

The child’s BMI number is then closely scrutinized in comparison to the BMI of other children who are of the same gender and age bracket. Here, you will get a percentile cluster showing you where your child belongs.

For instance, if a child’s BMI is below the 5th percentile, it means that it’s lower than the BMIs of 95 % of other children of similar age and gender. In such a case, the child is considered underweight.

Check the following table, which indicates various BMI percentile range and their weight status.

PERCENTILE RANGEWEIGHT STATUS
Less than the 5th percentileUnderweight
5th percentile to less than the 85th percentileNormal weight
85th to less than the 95th percentileOverweight
Equal to or greater than the 9th percentileObese

In adults, BMI is still the measurement of choice used by many health professionals for determining weight-related health risks.

However, for more accurate results, BMI data should be used together with other disease risk measurements. Measuring waist circumference and cross-examining the data with a person’s BMI, can help health professional make a more informed decision.

For children, the BMI-for-age percentile range is a dependable pointer of body fat. However, it should be used for screening purposes only. When making a diagnosis, health professionals must also conduct other assessments. These include evaluating the physical activity, family history, and diet to get more accurate data.

What Does BMI Data Indicate?

According to the National Heart, Lung, and Blood Institute, a person’s BMI can be used to gauge the risk of getting diseases that are associated with increased body fat.

The higher the person’s BMI, the higher the risk of getting heart disease, high blood pressure, type 2 diabetes, respiratory problems, and certain types of cancer.

Besides, the Center for Disease Control and Prevention (CDC) emphasizes that BMI results should not be used exclusively as a diagnostic tool. Even if a person gets high BMI results, it might not automatically translate to being at a higher risk of getting weight-related diseases.

Proper assessment must be done by a qualified physician and use the BMI results in conjunction with other assessment results.

What are the limitations of BMI?

Due to the fact that BMI relies on a person’s height and weight only, it does not provide any reliable data that indicates body fat percentage or distribution. There are crucial elements that can affect the overall correlation between BMI and the amount of body fat. They include factors such as muscle mass, ethnicity, bone mass, age, and sex.

For instance, persons with more muscle mass, and they exercise more often are more likely to have a higher BMI compared to those individuals with less muscle mass who exercise fewer times. In such a situation, doctors might not classify a person with a higher BMI as less healthy.

BMI results are also not a good indicator of individuals who are at risk of unhealthy fat distribution in the body. Such an indicator can play a significant role in determining a person’s disease risk potential.

Other Considerations

Individuals who carry excessive weight in the abdominal area than other lower areas are at higher risk of getting heart disease, high blood pressure, and certain types of cancers compared to those who have a balanced fat distribution.

That’s why it’s paramount for health professionals to use both BMI data together with waist measurements to get a more reliable indicator of a person’s weight status as well as any potential risk of chronic diseases.

Also, elderly persons who are weak might fall in the normal weight category while in actual sense they may have little muscle and bone mass and a high body fat percentage.

Another drawback of calculating BMI is the fact that BMI calculation was initially based on Caucasian body types. It’s for this reason that BMI results may differ for individuals of different ethnicity.

A study result conducted on the ethnic difference in BMI indicated that indeed, there is a variance. When researchers compared White Europeans and Asians of the same BMI, Asians appeared had a four percent higher total body fat comparatively.

Maybe this could be a contributing factor to the high cases of abdominal obesity in South Asia.

History of BMI (Body Mass Index)

BMI concept is no new as you may have thought. Funny enough, the history of BMI dates back into the 1840s where a man called Adolphe Quetelet a Belgian statistician came up with a mathematical formula he called the Quetelet Index of Obesity.

The Quetelet Index of Obesity calculated obesity of a person using a simple mathematical formula abbreviated as w/h2. To get the obesity index, he would divide a person’s weight (in kilograms or pounds) by the square of his/her height (in inches or centimeters).

Quetelet famously once stated, “If a man increased equally in all dimensions, his weight at different ages would be as the cube of his height.”

Here’s what Quetelet implies. During the initial years of our life, we have a more substantial breadth compared to our height. However, we grow taller in proportion to our height over time.

However, it’s worth noting that when Quetelet developed the Quetelet Index of Obesity, we were not interested in studying obesity. His primary interest was to apply probability calculus to human physical features. That’s how Quetelet came up with the index to measure relative weight.

In the late 1940s, doctors saw a surge in overweight sicknesses and mortality after World War II. Science validated The Quetelet Index. Then, doctors began trusting it as a useful index to measure overall relative body weight.

Later, science renamed it to the current Body Mass Index (BMI). Then, the World Health Organization adopted it in 1995. Today, it still serves as a simple tool to quickly estimate the level of obesity in human beings.

What Did Doctors Use Before The Adoption Of BMI Concept?

BMI It was not until the early 1980s that the medical community adopted the BMI as the international standard for measuring obesity. The concept of BMI became popular to the broader public in the 1990s. It was after the United States of America government launched an initiative dumbed the Clinton Health Plan.

The Clinton Health Plan aimed to encourage healthy eating and exercise among American citizens. Before the introduction of BMI as the recognized international standard for measuring obesity in the 1980s, health professionals used the weight-for-height table.

There were two separate weight-for-height tables for both men and women. And each one stipulated the ranges of body weights with corresponding height inches. One of the significant drawbacks of this kind of table was that they depended on bodyweight only. The did not focus on body composition.

The US National Institute of Health in 1998, decided to lower the overweight threshold for Body Mass Index from the initial 27.8 to 25 to align it to the recommended international guidelines. By adjusting the “overweight” category to the said margin, over 30 Million Americans were classified as overweight although initially they were considered to be of “healthy” category.

BMI

Final Thoughts on the Evolution of BMI as a Standard

Over recent years, scientists have continued to devise more advanced but straightforward and accurate methods to measure BMI. Among the latest development that has tried to give more precise results is the Skinfold caliper measurement on seven body parts.

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