All About BMI

What’s your least favorite part of a clinic visit?

I’d wager most people would pick stepping on the scale to be weighed, along with the accompanying BMI (body mass index) classification that’s relayed to you.

What is BMI, why has it become the standard for classifying weight status, and how much information does it actually tell you? There’s actually a lot to unpack.

BMI is really just a ratio of weight to height. It’s calculated by taking weight (in kilograms) divided by height (meters squared). The result is then classified into a category, either underweight (less than 18.5), “normal” weight (18.5-24.9), overweight (25-29.9) or obese (30 and up).

Here’s a list of all the things that BMI doesn’t tell you (at least not definitively):

  • Exactly how much body fat you have. BMI measures your total weight, so differences in muscle mass, water weight, and more aren’t accounted for.

  • Where you carry body fat. Having more fat around your chest, stomach, and abdomen (think “apple” body shape) tends to indicate greater risk of metabolic disease than carrying weight in your hips, butt, and thighs (think “pear” body shape). Waist circumference may be an additional measure to help account for this.

  • The type of body fat you carry. The sumo wrestler paradox illustrates this quite well. Differences in visceral versus subcutaneous fat are important to health, and not captured by BMI.

Each of these three factors is more important in determining health status than simply a ratio of weight to height. So why is your BMI number the only one you see?

The short answer is, because it’s easy to calculate. Measuring body fat percentage is difficult without specialized training and equipment, and a visit to the doctor’s office just doesn’t leave enough time for these measurements. When BMI was originally conceived, it wasn’t meant to be used as an indicator for individual health; rather, it was used for population studies and showing overall trends in health as they relate to weight within a large group of people. Alas, BMI is now firmly entrenched in our culture. If it’s not going anywhere, I’d like to help you understand how to keep a proper perspective on your BMI number.

BMI is one piece of information that you get from a doctor’s visit. You also have your blood pressure taken, you may have blood lipids and blood glucose measured, and much more. All of these pieces together – and don’t forget about mental health – contribute to your individual health and wellness. While all of these pieces matter, it’s easy to leave your annual check-up only thinking about your BMI – undoubtedly a side effect of our culture’s obsession with our weight.

Does BMI help predict how healthy you’ll be and how long you’ll live? Yes and no. And yes. And no. The reason this picture is unclear is because, again, BMI is just one measure and does not reveal all aspects of individual health. For example, in many populations muscle strength is a much better predictor of health and longevity than BMI.

It’s also important to understand that an underweight BMI – or, for some people, even the low end of the “normal” range – carries significant risks as well, for a variety of reasons. This is often lost in the ocean of discussion around high BMI numbers.

My hope is that with improved knowledge and understanding of what BMI is – and isn’t – we can all remove some of the shame, guilt, and fear that comes with discussing this number with a healthcare provider. None of these feelings are helpful for bettering your health, and certainly aren’t productive in a conversation about weight.

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