The limitations of body mass index (BMI)

 166608586A few days ago a friend on Facebook reposted a cartoon of a woman declaring that according to the BMI chart, she was too short. For those who aren’t familiar, Body Mass Index, or BMI, is a simple measurement tool used to calculate height-to-weight ratio.

Some people argue that BMI is too simple a measurement. Is BMI still useful for someone concerned about their weight?

“It can be helpful as one factor to consider in determining chronic disease risk in the general population,” explained Chelsea VanDusen, a clinical dietitian at Houston Methodist Hospital.

She explained that other factors such as age, gender, past medical history, family history and lifestyle factors including diet, smoking and physical activity levels all play a part in helping to determine risk and must all be considered in order to have an accurate picture.

The limitation of BMI as Chelsea explained it is that it’s intended as a “measure of fatness” but doesn’t actually take body fat percentage into account. “It’s therefore skewed in individuals who are highly muscular or those who have muscle wasting,” she said.

BMI doesn’t take body fat percentage into account, making it a skewed metric Click To Tweet

For example, a bodybuilder – whose weight may be higher than normal due to increased muscle mass – may appear obese according to his BMI number. In comparison, an elderly person’s BMI number may appear in the healthy range (<27) even though they’ve had significant muscle loss.

166596238“In fact, elderly people may have increased mortality risk with a BMI of less than 22, whereas that is considered in the ‘healthy range’ for younger adults,” Chelsea said.

Another limitation of BMI is the fact that it does not differ between men and women even though men tend to have larger frames in addition to a more muscular build. So even though a man and a woman may have the same BMI, the woman would likely have a higher percentage of body fat than the male.

BMI also doesn’t take fat distribution into account.  For example, a person’s waist circumference can be used to determine their amount of visceral fat, which is located around vital internal organs and is often associated with increased levels of inflammation and chronic disease risk.

Why do people continue to use BMI as a measurement to determine how healthy someone is?

“While other measures of body fat including waist measurement, skin-fold measurements and bioelectrical impedance are useful, they tend to be more expensive, intrusive and/or difficult to standardize due to human error,” Chelsea explained.  “Therefore, BMI continues to be widely used.”

Regardless of your BMI number, remember – it’s only one piece of the puzzle. And unless you’re planning on growing a few more inches, the best way to reduce it is by eating better and getting some exercise

Want to see what your BMI is? Try the calculator below: