What to count: BMI or body fat?
Screening tools aim at flagging potential health concerns
Most of us have had our standing height and weight in street clothes measured when we go to the doctor’s office. Maybe we have been asked to remove shoes, empty our pockets and stand up tall — maybe we haven’t.
While there are possibly several reasons why this information is recorded, calculating our body mass index (BMI) is the leading one.
Simply, BMI is the ratio of our weight to the squared value of our height. Usually, this is represented in its metric format of kg/m2. Using the English system, BMI is calculated by dividing the scale weight (in pounds) by the square of standing height in inches and then multiplying that value by 703 and rounding to one decimal place.
What does that computed value indicate?
BMI is a screening tool used to identify potential health issues based on the relationship between height and weight.
For children, the BMI is evaluated relative to a sex-specific growth chart for the child’s age. These charts have percentile categories corresponding to the various BMI values across ages ranging from two through 20 years.
If the child’s BMI at a given age falls between the fifth and the 85th percentiles, the BMI is considered to represent a healthy weight for the child’s height. Outside that range is where the potential health issues may arise.
Below the fifth percentile is the underweight category; above the 95th percentile is the obese category, and the child is considered to be overweight should the BMI fall between the 85th and 95th percentiles.
For adults, the categories are similarly named, but are defined by discrete values (BMI for underweight individuals is less than 18.5; normal weight 18.5–24.9; overweight 25.0–29.9; obesity categories begin at 30.0).
The assumption about the BMI is found in statistics linking the likelihood of having or developing a health problem due to having too little (underweight) or too much (overweight and obese) body fat.
However, unless you have one of those electrical scales that can estimate your body fat while you stand on it in bare feet, the scale cannot estimate your body fat percentage. Muscle and bone are more dense than is body fat. Consequently, someone who does a lot of weight lifting and has noticeable muscle definition may be erroneously labeled as being overweight or obese even though the actual body fat percentage is within normal limits.
Conversely, there is a concept called “masked obesity” or “skinny fat.” This situation arises when someone with a normal weight BMI has too much body fat making them truly obese by definition — carrying too much body fat.
Likewise, BMI results should be interpreted with caution for nonwhite racial/ethnic populations since the BMI concept was originally developed using Caucasians.
Research is now indicating that it is the amount of fat stored on the torso and around the waist creates a larger health concern than does BMI.
To perform your own simple check, divide your waist circumference (in inches) by your standing height in inches. A healthy value is one that is lower than 0.5 or 50%. You want your waist measurement to be less than one-half of your height.