BODY MASS INDEX (BMI): What’s Normal and Abnormal

The concept of BODY MASS INDEX, or BMI, was conceived by a Belgian astronomer, Adolph Jacques Quetelet, in the 1830’s. There’s not much more to say about the history of BMI, but it means this measure of determining an individual’s obesity has been in use for almost two centuries. That’s a very long time for a medically-related topic to continue use in everyday practice. Usually some researcher is trying his hardest to find a better way, and to take into consideration the many variations in the human body.

Because BMI is based solely on the individual’s height and weight, skeptics have tried to develop systems that also include bone density, muscle mass, age, gender, and general state of health to get a more complete representation of a patient’s gestalt. But a perfect formula has yet to be adopted.

Life insurance companies have used BMI charts for decades to determine the relative insurability and health risks of policy applicants. Physicians also have utilized this method to give patients an upfront awareness of their state of obesity and compare them to other similar individuals.

One major limitation is that BMI does not directly measure a person’s body fat. That is accomplished by skin-fold measurements using special calipers, dual energy X-ray absorptiometry (similar to bone density measurement), bioelectric impedance, or air or water displacement measures. These all take a little more effort than simply plugging height and weight into a mathematical formula or looking at a BMI chart. But studies show BMI and skin caliper measurements correlate well with various metabolic and disease outcomes. So a high percent body fat and a high BMI both are good predictors of poor health situations.

In most offices the calculation of BMI is unnecessary because results are displayed on a multicolored height and weight chart. The doctor merely has to connect the lines to determine BMI. But if that’s not available and you left your smart phone at home, a calculation can be done. The BMI number one obtains is expressed in kilograms per meter squared (kg/m2), but only the number is commonly used in conversation. Two formulas for calculation are used, one for pounds and inches, the other for kilograms and meters.

In lbs & in: weight in lbs divided by height in inches squared, multiplied by 703=BMI

In kgs & meters: weight in kgs divided by height in meters squared=BMI

As you can easily see, having a Body Mass Index chart or computer calculation app or program simplifies the determination.

The Center for Disease Control in Atlanta has a set of guidelines in common use. These are:

A BMI less than 18.5 means the patient is Underweight.

A BMI of 18.5 to 24.9 means the patient has Ideal Body Weight.

A BMI of 25 to 29.9 means the patient is Overweight.

A BMI over 30 means the patient is Obese.

But several deficiencies are evident in the use of BMI. They are:

  1. It is not a measure of fatness
  2. It ignores waist size
  3. Bone and muscle mass are not factored in
  4. It is “logically wrong.”
  5. It doesn’t factor in one’s physical activity level
  6. Category designation (overweight, obese, etc.) have “sharp boundaries” that “don’t make sense”
  7. It has potential to be rigged (different BMI charts use different norms)
  8. Athletes and non-athletes can have the same BMI
  9. Older people with more fat can have the same BMI as a younger, thinner person
  10. An abnormal BMI in an athlete doesn’t mean fatness.

The single number expressed by the BMI can lead to “confusion and misinformation.” Fifty-four million Americans classed as overweight or obese by BMI were found to be healthy when evaluated by “cardiometabolic measures.” Another twenty-one million had normal BMI’s but were determined to be in poor health.” “People are too complex and differ in bone and muscle mass so a single figure leads to error.”

A mathematician from Oxford University in the UK saw these flaws and proposed altering the calculation formula to weight divided by height to the 2.5 power, not just squared. That lowers my BMI from 31.87 to just over 30. I’m not sure how significant that is, but it eliminates “misleading information from the calculations in really short and really tall people.”

Others feel a “waist circumference to height ratio” is a better predictor of heart disease and diabetes risk than BMI. Waist circumference should be less than half your height. A study of 300,000 patients found this ratio correlated better than BMI in predicting high blood pressure, diabetes, heart attack and stroke.

Most experts now recommend combining BMI with another measure. Some use percent body fat, others use waist to height ratio, but it appears BMI alone, although quick and easy to determine, has flaws that must be addressed by using an additional parameter. I’ve seen a lot of patients whose BMI put them in the obese category who did not look obese. They were big people who had well-developed muscular structure and small waists. They really weren’t obese, so additionally incorporating the waist-height ratio would place them in the correct category.

Dr. G’s Opinion: For year’s I’ve thought the BMI mis-categorized a lot of people so using the waist-height ratio in addition should help to clarify the situation. One productive purpose of BMI, however, is to get people to realize they are overweight and encourage them to lose weight. It should serve as a motivating factor, but in my experience it doesn’t have the desired effect. People are deniers and excuse-makers and aren’t worried until it’s too late. Were I still in practice I would now jointly use BMI with waist-height ratio.



NPR.org 209

“Waist to height ratio better than BMI” Paddock, C. Medicalnewstoday; May 13, 2012.


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