BMI, or body mass index, is a measurement used to identify weight in comparison to height. It essentially sorts people into one of six distinct categories, ranging from severely underweight to morbidly obese. The only category that is deemed optimal for a person's health is "normal." However, BMI doesn't take into account any other factors but weight and height, making it somewhat antiquated for standard use.
Equation
The BMI (or Quetelet) equation is a fairly simple one. It factors in a person's height in inches and weight in pounds to determine body mass index. Regardless of sex and body type, the same equation is always used: BMI = (weight x 703) / (height x height). No other factors are used to establish whether or not someone is too heavy or too thin for his or her frame.
Significance
Though this system of measurement has some validity, there comes a problem in regard to obesity. The Mayo Clinic defines obesity as excessive body fat. Yet, body mass index only uses weight (not fat) as a determining factor in its equation and diagnosis, according to Texas A&M University of Corpus Christi. Without taking into consideration muscle mass, lean body mass or body fat percentage, it's difficult to conclude whether or not someone is truly obese.
Types
As mentioned before, there are six distinct categories in regard to body mass index. Anyone with a BMI of 16.4 and below is regarded as severely underweight. Those people with a BMI of 16.5 to 18.4 are considered underweight. Normal BMI is anything between 18.5 and 24.9. Overweight falls from 25.0 to 29.9. Obesity is deemed to be at 30.0 to 39.9. All other values above these numbers are labeled as morbidly obese.
Effects
Higher BMI levels, especially those above 30, take a greater toll on the body than other values. According to the Mayo Clinic, obesity and morbid obesity can increase your risk of heart failure, heart attack, high blood pressure, high cholesterol, stroke, sleep apnea, diabetes, depression, liver problems and cancer.
Theories/Speculation
It may be necessary to turn to other methods of measurement to properly diagnose and treat obesity. By using skinfold thickness measurements, underwater weighing and deuterium dilution, which is a measurement of total body water, medical professionals may gain a better understanding of a person's body fat percentages, which is truly a means of determining obesity. According to the Mayo Clinic, it's possible for someone to have a "normal" body weight and excessive fat at the same time. It's a condition known as normal-weight obesity, and it can cause some of the same consequences as obesity.
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