Assessment: The Body Mass Index Revisited

In my inaugural post detailing my general health statistics, I made a short mention that the Body Mass Index is widely criticized as a flawed measurement and that it should not be the sole determinant of whether you are overweight.  For my own education and others, I’ve decided the explore the issue a little further. 

What is BMI?

The National Institute of Health (NIH) defines the Body Mass Index, or BMI, as a “measure of body fat based on height and weight that applies to adult men and women.”  The Index was derived in the 1830’s from a math formula conceived by Lambert Adolphe Jacques Quetelet, a Belgian astronomer, statistician, and sociologist.  Not a physiologist, nor kinesthetics guru… nope, a stars and numbers guy.

The formula is:  BMI = mass(lbs)/height(inches)2 x 703.  This formula yields a number that corresponds to the following categories: Underweight (Below 18.5), Normal (18.5 – 24.9), Overweight (25.0 – 29.9), and Obesity (30+).

Limitations of BMI

Nick Trefethen of Oxford University states that because the human body doesn’t scale equally as it grows, the current BMI formula only leads to confusion and misinformation. The height of short people is divided by the weight too much, and is divided by too little for tall people.  The result is that short people are being told they are thinner than they actually are, and tall people are fatter than they actually are.  Is this a boon for us miniature folks?  I don’t think so, there’s more…

NIH identifies two further limitations of the BMI:  First, it overestimates body fat in athletes and others who have a muscular build, and second, it underestimates body fat in older persons and others who have lost muscle. In other words, it does not consider the dimension and tissue composition of the individual.  Furthermore, many critics point out that BMI does not differentiate between gender, which is of note because women generally carry more body fat.

One size fits all. 

Based on the conventional BMI formula, two individuals of the same height, but with different dimensions and body density, could have comparable BMIs.  This sends the message that if you are X feet tall, you must be Y weight, no exceptions.  Yet, muscle weighs more than fatty tissue, and studies have shown that a greater proportion of muscle tissue to fat tissue improves your quality of life.

At-Home Alternatives to the BMI

Body Adiposity Index (BAI).  
This measure, created by Richard Bergman of the University of Southern California, calculates body fat using hip circumference and height, without the disadvantage of requiring a scale.  However, studies have concluded that this measurement no more accurate than the BMI, particularly among clinically severe obese women.

Waist Circumference Measurement.
NIH regards waist circumference as a particularly useful measurement for individuals who are categorized as normal or overweight on the BMI scale.  Furthermore, it is an independent prediction of health risks, such as type 2 diabetes, dyslipidemia, hypertension, and cardiovascular disease.  It should be noted that while waist circumference is optimal for predicting risk with the elderly, there are variations in predictability between ethnic and age groups. Generally, you are at high risk if your true waist is measured over 40 inches for men, and 35 inches for women.

Waist-to-Hip Ratio.
Similar to Waist Circumference, this measure is also great for predicting health risks, in addition to body fat. To calculate your ratio, use a tape measure to take a reading from both your natural waist line and the widest part of your hips, then divide the diameter of your waist by your hip measurement. If you measure below 0.95 for men, and 0.80 for women, you are considered low risk.

Body Fat Measuring.
If you have access to calipers, this measure is calculated by taking measurements from the waist, shoulder blade, biceps, and triceps.  Those measurements are added up to a single figure, plotted against a chart that takes your sex, age, and measurement into consideration to calculate a percentage.

The New BMI.
To account for the three-dimensional aspects of humans (errr, everything…), Trefethen came up with a “New BMI.”  A colleague came up with a simple New BMI Calculator here.

Et tu, Little Furr? 

As previously mentioned, I am almost 5 feet 4 inches tall, weigh approximately 150 lbs, with a waist and hips of 32 inches and 42 inches, respectively. So am I actually overweight as my BMI suggests, or a victim of bad math?

BAI:  33.3% – slightly overweight, similar to my BMI.
Waist Circumference:  32 inches > 35 inches – not overweight and not at risk.
Waist-to-Hip:  0.76 – regarded to be within a healthy range.
Body Fat %:  No calipers yet. Will update when they are in my possession and I’ve pinched myself.
New BMI:  This calculator compares my traditional BMI of 25.75 with a New BMI of 26.25.  Slightly more fat (harrumph).  It then states that my appropriate weight range is between 105.7 and 142.8 lbs.

My Take-away:  As you can see, my respective measurements plot me as normal to slightly overweight, but with low health risk.  While these calculators are good for getting a rough idea of where I stand, I recognize their limitations, particularly as I am on the cusp of many of the categories.  My goal moving forward will be to re-measure in three months, with a desire to get off the cusp.  I believe I sit squarely in the weight-loss goal category of most Americans – trying to lose that last chunk of weight.  As I previously stated, my specific goal is peak performance, even as a type-1 diabetic.   

And to try not to “lose that butt”: 

Sir Mix Alot


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