BMI… What does it mean?

Body mass index, I’m sure you have heard the term before, but what is it and what does it mean for health? Body mass index = kg/m2. Put simply, it is a formula that assesses your weight compared to your height.  

Because we regularly measure weight and height at doctor visits it is an easy way to characterize your weight and potentially determine your risk of diseases related to excess weight: 

BMI  Weight Classification 

>18.5   Underweight 

18.5-24.9  Normal weight 

25-29.9  Overweight 

>30  Obesity 

If your BMI is over 25 you are considered at higher risk for developing diseases such as heart disease, type 2 diabetes and certain forms of cancer to name a few.  

Unfortunately, it is not that simple and that is why using BMI as a sole predictor of disease risk has come under fire in recent years. The BMI formula was developed almost 200 years ago by a mathematician, Lambert Adolphe Jacques Quetelet (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4890841/). He developed the formula as a quick and easy way for the government to assess obesity in the population and allocate resources accordingly.  

The BMI formula, while quick and easy, does not take into account muscle mass or waist circumference, both of which can contribute to disease risk. Another problem with BMI, is that the weight classifications are based on Caucasian males and don’t take into account different genders or races.  

Waist circumference, or even better, waist-to-hip ratio, may be more accurate ways to assess disease risk. To obtain waist circumference you  use a tape measure to measure the smallest portion of your waist, normally a bit above your belly button.  

To obtain your hip circumference you should measure around the widest part of your hips. Then divide your waist circumference by your hip circumference to get your waist-to-hip ratio. I have the classifications pf disease risk below: 

Health Risk  Female  Male 

Low  0.80 or lower  0.95 or lower 

Moderate  0.81 – 0.85  0.96 – 1.0 

High  0.86 or higher  1.0 or higher 

Why waist-to-hip ratio? It takes into consideration your body shape as well as your sex, which we know are stronger predictors of disease risk than BMI alone. Pear-shaped individuals, or folks that tend to carry their weight in their hips/legs, don’t seem to have increased risk of the diseases associated with obesity, even if they are characterized as overweight or obese by BMI classifications.  

Apple-shaped individuals, or those that carry their weight in the belly, do have a significantly higher risk of heart disease, type 2 diabetes and certain forms of cancer. Therefore, waist-to-hip ratio is a better predictor of disease risk than BMI alone.  

So why do we continue to use BMI? It’s easy, as mentioned at the beginning of the post, we always get height and weight at clinic visits so it is something we can easily calculate. While measuring waist and hip circumference seems fairly easy it does require some training, especially when working with people with wider midsections. So we can’t just add it to the to-do list for the nurse at your annual check-up.  

That being said, it would only take a few extra minutes to add the waist-hip-ratio measurements to clinic visits and it can tell us a lot more than BMI alone. Hopefully, with more push from individuals in the healthcare field to take this waist-to-hip ratio route we will see more of this in the near future.  

So what do you think, are you ready to get out your tape measure and calculate your waist-hip-ratio??