What follows is a little closer inspection of what BMI actually reflects. The formula once again is as follows:
Body Mass Index
Metric: = weight in Kilograms / (height in Meters)2
American: = weight in Pounds x 703/ (height in Inches)2
The following has been proposed to describe body mass classes:
Underweight: Less than 18.5
Normal Weight: 18.5 – 24.9
Overweight: 25 – 29.9
Obese: 30 or more
The mathematical model renders a number with the units of Kg/meter2 or mass per square meter. A natural question might be, wouldn’t it make more sense to measure mass per cubic meter? By that, cube the height and arrive at a mass per unit volume? It might seem to make sense but in correlation studies using a gold standard to measure percent body fat, the height squared actually correlates much better with percent body fat than the height cubed.
Again, the BMI is a mathematical model used to estimate CHANGES in % body fat. And population-wide that model does indeed hold true in nearly all cases. Most adults with an increase in BMI have a corresponding increase in % body fat. The running backs, competitive weight lifters and short distance sprinters with very low % body fat (but a high BMI, which gets higher with increases in muscle mass) make up only a very small fraction of the population and would be classified as outliers as they don't fit the mathematical model. The reason for that obviously relates to the greater density of muscle and lean tissue compared to adipose or fat tissue. With muscle weighing more than fat, a body builder adding muscle (building up for a competition) would have an increasing BMI with a decreasing % body fat!
Information BMI does not give:
1. Distribution of Body Fat
A high BMI as a number doesn’t necessarily describe percent body fat and absolutely doesn't define where on the body the fat might be distributed. Centrally, hips, thighs or evenly distributed over the entire body, the value give no information. And distribution as it turns out is probably more important than the absolute value of BMI.
2. Absolute Percent Lean Body Mass
Using the weightlifter with heavy muscle mass, as #1 above, the value of his/her BMI is meaningless as a reflection of percent body fat. And by corollary, gives no information about lean body mass.
3. Absolute Percent Body Fat
Although it’s an assumption that as an individuals BMI increases, the percent body fat increases, the value has no real relationship to percent body fat.
Body Mass Index
Metric: = weight in Kilograms / (height in Meters)2
American: = weight in Pounds x 703/ (height in Inches)2
The following has been proposed to describe body mass classes:
Underweight: Less than 18.5
Normal Weight: 18.5 – 24.9
Overweight: 25 – 29.9
Obese: 30 or more
The mathematical model renders a number with the units of Kg/meter2 or mass per square meter. A natural question might be, wouldn’t it make more sense to measure mass per cubic meter? By that, cube the height and arrive at a mass per unit volume? It might seem to make sense but in correlation studies using a gold standard to measure percent body fat, the height squared actually correlates much better with percent body fat than the height cubed.
Again, the BMI is a mathematical model used to estimate CHANGES in % body fat. And population-wide that model does indeed hold true in nearly all cases. Most adults with an increase in BMI have a corresponding increase in % body fat. The running backs, competitive weight lifters and short distance sprinters with very low % body fat (but a high BMI, which gets higher with increases in muscle mass) make up only a very small fraction of the population and would be classified as outliers as they don't fit the mathematical model. The reason for that obviously relates to the greater density of muscle and lean tissue compared to adipose or fat tissue. With muscle weighing more than fat, a body builder adding muscle (building up for a competition) would have an increasing BMI with a decreasing % body fat!
Information BMI does not give:
1. Distribution of Body Fat
A high BMI as a number doesn’t necessarily describe percent body fat and absolutely doesn't define where on the body the fat might be distributed. Centrally, hips, thighs or evenly distributed over the entire body, the value give no information. And distribution as it turns out is probably more important than the absolute value of BMI.
2. Absolute Percent Lean Body Mass
Using the weightlifter with heavy muscle mass, as #1 above, the value of his/her BMI is meaningless as a reflection of percent body fat. And by corollary, gives no information about lean body mass.
3. Absolute Percent Body Fat
Although it’s an assumption that as an individuals BMI increases, the percent body fat increases, the value has no real relationship to percent body fat.
1 comment:
WG, at least to the community who researches this, the insufficiency of BMI is well-known. The health and life insurance industries however continue to use actuarial tables that do not go beyond BMI... So where it matters we are still stuck. Thanks for this new series on BMI.
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