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What is BMI and how is it used?

BMI is a term that gets used quite a bit in discussions about weight, obesity and bariatric surgery. The term has been the source of a lot of confusion.

BMI is short for “Body Mass Index.” It is simply a number calculated from a person’s height and weight. The more a person weighs in relation to their height, the higher their BMl. The formula used to calculate BMI is weight divided by height squared, and it is usually calculated in kg/meter2. Most times the units are not listed but assumed. For example when you say, “BMI of 35,” it is assumed that you mean “35 kg/meter2.”

Sometimes people say it as a percentage, as in “my BMI is 35 percent,” but that is actually incorrect and can be confused with body fat percentage. BMI says nothing about a person’s actual body fat percentage, which while useful to know, is more difficult to measure. BMI also doesn’t tell us anything about a person’s health, such as whether or not they have diabetes, high blood pressure or high cholesterol.

To clarify the difference between BMI versus body fat, consider a football player. The average height and weight of an NFL player is 6 feet, 2 inches and 245 pounds. This gives him a BMI of 31.5, which would be considered “obese” using Word Health Organization nomenclature – see chart below. However, NFL players are professional athletes with a large amount of muscle and not as much fat, and medically you wouldn’t consider most of them “obese.”

The relation of BMI to health problems also can vary by race. For example, individuals of Asian descent tend to have more central body fat and develop metabolic complications of obesity, such as diabetes, at lower BMIs than Caucasians.

So if BMI isn’t a very good measure of a person’s actual weight status and health, why do we use it? We use BMI for primarily two reasons:

  1. It is easy to measure – all you need is a scale and a tape measure.
  2. It is consistent and everyone uses it – from physicians to researchers to insurance plans.

Since most people aren’t professional athletes or bodybuilders, it’s still a pretty useful measure for estimating the presence and degree of obesity.

When it comes to bariatric (weight loss) surgery, such as a sleeve gastrectomy or gastric bypass, BMI is used as a starting point to determine if a person is a candidate. At a BMI of 30 to 39 with certain medical conditions, especially diabetes, a person might be appropriate for surgery. At a BMI of 40 or above, called “class III” or “severe” obesity, a person may be appropriate for surgery without any other related conditions.

Calculate your BMI here: tylerbariatrics.com/am-i-a-candidate

WHO (World Health Organization) definitions based on BMI

BMI

Nutritional Status

< 18.5

“Underweight”

18.5 - 24.9

“Normal weight”

25 - 29.9

“Pre-obesity” (WHO)

“Overweight” (CDC)

30 - 34.9

“Obesity, Class I”

35 - 39.9

“Obesity, Class II”

40 and above

“Obesity, Class III”

“Extreme Obesity or Severe Obesity” (CDC)

 

Information provided by Hugh P. Babineau, MD, FACS, FASMBS, bariatric surgeon who practices at UT Health East Texas Physicians in Tyler.

 

 

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