What Is BMI? The Truth about This Bogus Formula

You've probably encountered the body mass index (BMI) at some point in the past. BMI is often used as a means of deciding what constitutes a "healthy" body weight, and is sometimes even used to help design diet and exercise programs. Doctors use it to measure the health of their patients. Countless websites and fitness apps will even do the calculations for you.

However, despite its widespread use and popularity, the BMI formula is actually pretty flawed. So what is BMI, what are the problems with this widely accepted tool, and are there more useful alternatives? Here are some answers.

History and Details of BMI

Although it's sometimes presented as a marvel of modern science, BMI is actually a pretty old tool that was dreamed up by a Belgian mathematician named Lambert Adolphe Jacques Quetelet in the mid-1800s. Originally, the formula was intended to be a tool that the government could use to assess levels of obesity as a whole in order to make better use of their resources.

Interestingly, part of the reason that BMI has seen such persistent popularity is that it's a simple formula. It looks like this: BMI = (weight in pounds/(height in inches x height in inches)) x 703. The number produced is then compared to a chart that places the subject into one of three weight classes—normal, overweight, or obese.

What's the Problem?

Right away something powerful stands out about BMI: The formula was designed to be a statistical model, and was never meant to be applied to individuals. Even Quetelet warned against trying to use the formula to gauge the health and well-being of a specific person.

It's also important to realize that BMI only takes two factors into consideration: your height and your weight. Height has very little to do with your actual health, but weight does offer some insight into overall well-being. However, as a general measurement, this number does not specify where those pounds are coming from. Body fat, muscle, organs, bones, water, and all sorts of other substances are totaled in when you step on the scale. Not all of those factors carry the same amount of influence over your health. In some cases, like muscle, you actually want to be carrying around a few extra pounds.

Because it offers an incomplete picture, BMI sometimes completely misjudges the health of certain individuals. For example, the actors Brad Pitt, George Clooney, and Matt Damon would all be labeled as overweight according to BMI, and it's not uncommon for bodybuilders to fall into the "obese" category after plugging in their numbers.

A Better Way

If we can't rely on the BMI formula, is there a more reliable, useful measurement? Tests that can accurately measure body fat are much more trustworthy. Although calipers are fairly common and inexpensive, their accuracy depends heavily on the person administering the test.

Another common tool is bioelectrical impedance analysis (BIA), which runs a weak electrical current through your body. Since various tissues conduct electricity differently, these machines measure the resistance applied to the charge by your body and estimate the amount of fat on your frame. However, these measurements can vary widely, depending on water retention, how recently you've eaten, when you last worked out, and a host of other factors.

One of the most accurate ways to measure body fat is through a dual-energy x-ray absorptiometry (DXA) scan and hydrostatic weighting. Keep in mind, however, that these procedures require specialized equipment that's operated by a trained professional, and they can be a little pricey.

If you've asked yourself, "What is BMI?" hopefully you now have some answers. The truth is that there are better, more accurate ways to determine how healthy you are than this formula. If you have any questions or concerns, make sure to talk to your doctor for more information.

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