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Body Mass Index Basics

Posted Mar 10 2009 3:38pm

Many of you may be familiar with the concept of Body Mass Index (BMI), however, I occasionally receive questions about its utility and role in determining level of care. So...here are the basics about BMI...

As you may know, body mass index (BMI) is a calculation of the ratio between a person's weight and height. Although it is often used by healthcare professionals as an indicator of total body fat, it does not measure body fat directly. Instead, it has been found to correlate with other direct measures of body fat. It is used in conjunction with a thorough health screening and is often preferred over height/weight tables for determining a healthy weight range in adults.

Bmi-chart

A person’s BMI is calculated using a BMI formula, or with the use of a BMI calculator, such as this one  (Click). Or, you can use this formula here:

BMI =
 
  weight (in pounds)   
————————————        X  703
   (height in inches)²

The National Institutes of Health provides guidelines for a healthy BMI range as follows:

Below 18.5   =    Underweight
18.5–24.9    =     Normal
25.0–29.9    =     Overweight
30.0 and Above   = Obese

These BMI ranges  apply to adults; the BMI measurement applies differently to children. With children, a BMI percentage for an individual child is compared to BMI measurements for children of the same age and gender. Generally, children with a BMI below the fifth percentile for age and gender are considered underweight. Click HERE for a reference to Children's Growth Charts.

Important Limitations of BMI:

BMI is a calculation based on weight and height and cannot provide any additional detail about body composition (e.g., percentage of body fat). BMI measurements do not distinguish between body fat mass and lean tissue mass, nor do they consider one’s frame size. Therefore, BMI calculations may be
over-inflated in an athletic or very fit person due to a higher percentage of muscle mass. Likewise, a BMI calculation may not be as accurate for an elderly person with reduced bone density and muscle mass. Other factors such as ethnicity, pregnancy, adult age, gender, physical activity, and bone structure can produce variations in a BMI measurement (Shepphird, 2009).

Source: Shepphird (2009). 100 Questions and Answers about Anorexia Nervosa. Sudbury, MA: Jones & Bartlett Publishers.

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