Over past articles we have provided you with detailed information and advice on how to achieve physical fitness. We have so far discussed flexibility, cardio, muscular strength and endurance. And now, last but not least, in this issue we are addressing Body Mass Index.
The body mass index (BMI) is a statistical measure which compares a person’s weight and height. Though it does not actually measure the percentage of body fat, it is used to estimate a healthy body weight based on a person’s height. Due to its ease of measurement and calculation, it is the most widely used diagnostic tool to identify weight problems within a population, usually whether individuals are underweight, overweight or obese.
Body mass index is defined as the individual’s body weight divided by the square of his or her height and is based upon the following formulas:
Formula and Calculation
|Kilograms and meters(or centimeters)||Formula: weight (kg) / height (m) 2(squared)With the metric system, the formula for BMI is weight in kilograms divided by height meters squared. Since height is commonly measure in centimeters, divide height in centimeters by 100 to obtain height in meters.
Example: Weight = 68 kg Height = 165 cm (1.65 m)
Calculation: 68 divided by (1.65) 2(squared) = 24.98
|Pounds and inches||Formula : weight (lb) / height (in) 2 (squared)Calculate BMI by dividing weight in pounds (lbs) by height in inches (in) squared and multiplying by a conversion factor of 703.
Example: Weight = 150 lbs Height = 5’5” (65”)
Calculation: (150 divided by (65) 2 (squared) x 703 = 24.96
The standard weight status categories associated with BMI ranges for adults are shown in the following table:
|18.5 – 24.9||Normal|
|25.0 – 29.9||Overweight|
|30.0 and above||Obese|
The BMI has become controversial though because many people, including physicians, have come to rely on its apparent numerical authority for medical diagnosis, but that was never the BMI’s purpose. Common sense needs to prevail when using the measurements as obviously not one size fits all. The BMI does not take into account many variables such as proportions of fat, bone cartilage, water weight, individuals who are naturally endomorphic or ectomorphic (i.e. people who do not have a medium frame) and loss of height through aging.
One basic problem, especially in athletes, is that muscle weight contributes to BMI. Some professional athletes would be considered “overweight” or “obese” according to their BMI, despite them carrying little fat and of course being totally physically fit.
Body composition for athletes is often better calculated using measures of body fat, as determined by such techniques as skin fold measurements or underwater weighing.
The U.S. National Health and Nutrition Examination Survey of 1994 indicates that 59% of American men and 49% of women have BMIs over 25. Morbid obesity – a BMI of 40 or more – was found in 2% of the men and 4% of the women. The newest survey in 2007 indicates a continuation of the increase in BMI; 63% of Americans are overweight, with 26% now in the obese category (a BMI of 30 or more).
BMI is used differently for children. It is calculated the same way as for adults, but then compared to typical values for other children of the same age. Instead of set thresholds for underweight and overweight, then, the BMI percentile allows comparison with children of the same sex and age. Again, this also has dangers and limitations. For if as a nation our children are becoming more obese, then a comparison based upon a population of the same height and weight may be totally unacceptable.