The BMI:  There's More to it Than Weight and Height.

What is Your Body Mass?

Why don't they call the BMI (Body Mass Index) the "Body Weight Index"?  As I recall from my basic physics class, matter is anything that has weight and occupies space.  

Mass is another scientific word that gets to the heart of HOW MUCH matter occupies a given space.  Weight is a relative term that depends on the pull of gravity on a body.

So if I weigh 165 pounds at sea level on earth, I wouldn't weigh anything in outer space under zero gravity conditions.  My weight varied from 165 pounds on earth to zero pounds in space but I still had the same amount of mass.

The BMI was developed to give an estimate of a healthy body physique based on ones height and weight.

It is useful for studying weight trends of populations but not a great tool for use with individuals since it does not measure fat and does not differentiate between fat and muscle.

Calculating Your BMI

There are several versions of the body mass index in chart form and all one needs to know is their height and weight to read across each axis of the chart to see where they are in terms of underweight, normal, overweight or obese.  Anything over an index number of 30 is considered obese.

In the absence of a chart, a simple formula can be used to calculate your body mass index.

BMI = weight in pounds times 703 (a constant) divided by height in inches squared.

If someone weighs 170 pounds and is six feet tall, their BMI from the formula is:  170 X 703/(72)(72) = 119510/5184 = 23.05. 

The index reading of 23 is a very healthy normal reading for this individual.  The table below gives the guidelines for the four BMI ranges.  Where are you on the table?

  • Underweight is Below 18.5
  • Normal is 18.5 - 24.9
  • Overweight is 25.0 - 29.9
  • Obesity is 30.0 and above
  • Below: World Health Organization Body Mass Index Chart

    World Health Org. BMI chart

    There are also many convenient interactive tools on-line where you just plug in your height and weight and the tool quickly responds with the index reading.

    The National Heart, Lung and Blood Institute of the U.S. Department of Health and Human Services has a very good online calculator plus a great deal of additional information on each of the BMI ranges.

    You can access their calculator by clicking on NIHLBI.

    If you want to check it now, go ahead, then hit your "back button" to return here.

    Which Weighs More;
    A Pound of Lead or a Pound of Feathers?

    On an individual basis, a body mass index number can be very misleading.  For example if I were a rock-hard body builder without an ounce of fat on me and weighed 215 pounds and stood at five feet, ten inches, my body mass index would be 30.8. 

    If my body mass index was all that a healthcare provider knew about me, she may logically conclude that I was obese and probably in terrible health.  But when I walked into her examining room she would quickly see that was not the case.

    From the example, it is evident that the BMI has some limitations.  Specifically, it tends to overestimate body fat in athletes and others who have a muscular build such as the body builder in our example. Also it can underestimate body fat in older persons and others who have lost muscle mass.  It's good to be aware of the possible bias in a body mass index number.

    Why the misconception?  The body mass index only considers total body weight and not the fat to muscle ratio.  For equal volumes, muscle weighs about 18% more than fat due to its higher density. 

    Remember our basic physics class?  Density describes how much mass there is per unit volume; that is, if we have one liter of fat and one liter of muscle, the fat will weigh .9 kilogram and the muscle will weigh in at 1.06 kilogram. 

    These figures were from "Google Answers" wherein "raisingmyhand-ga" commented on the issue of muscle density vs. fat density and provided solid references for his input.

    If you happen to live in the U.S. and don't think in terms of kilograms and liters, that would be 1.98 pounds for the fat and 2.34 pounds for the muscle.  Do the math; 1 kilogram = 2.2046 pounds.

    So What!  Who Cares
    About a Fat to Muscle Ratio?

    The "so what" regarding fat and muscle tissue is that muscle burns lots of calories and fat doesn't. 

    With proper exercise and adjustments in diet we can build more muscle tissue and lose fat.  Fat does not transform to muscle; muscle cells and fat cells are entirely different animals and never transform into each other.

    In the example of a body builder without an ounce of fat, that individual would be at risk for some serious health issues. 

    It turns out that there is an ideal fat to muscle ratio and it varies by sex and age.

    For females a 12% fat-to-lean ratio is pretty healthy; for males it is around 5%.  The human body requires a certain amount of fat and if that drops too low, serious problems can arise in the cardiovascular and central nervous systems among others.

    Unfortunately, the national averages are hovering between 22% to 25% for women and 15% to 18% for men.  These percentages are from Elizabeth Quinn, M.S. Sports Science and currently an exercise physiologist and sports consultant.

    Go ahead and monitor your body mass index, it's good to know but in the grand scheme of things, but by itself, it doesn't matter all that much. 

    After all, we know if we are overweight and don't really need a number to tell us that.  It could serve the purpose of telling us just how good or bad it is but there are a lot of other things to consider along with our body mass.

    And here is where the inches come in.  Waist circumference is a good indicator of belly fat which is another marker for risk of developing heart disease and diabetes among others. The risk increases with a waist measurement of over 40 inches in men and over 35 inches in women progressing from:

  • Obesity class I.....BMI 30 - 34.9 High for women; very high for men
  • Obesity class II....BMI 35 - 39.9 Very High for both men and women
  • Obesity class III...BMI 40+ Extremely high for both men and women
  • Some of the other things to consider in conjunction with body mass index are the following RISK FACTORS:

  • high blood pressure (hypertension)
  • high LDL-cholesterol ("bad" cholesterol)
  • low HDL-cholesterol ("good" cholesterol)
  • high triglycerides
  • high blood glucose (sugar)
  • family history of premature heart disease
  • physical inactivity
  • cigarette smoking (just don't do it)
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