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Why BMI Is Not Accurate?

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Weight x .45 = A, Height (Inches) x 0.25 = B. BMI = A/B². The problem with BMI is that is doesn't account for your bone density, weight or muscle mass. Increasing muscle could increase your BMI, depending on your build. BMI may not be as accurate as other fat testing methods such as Dexa scans and hydraulic testing, which look for fat than mass.

Body Mass Index (BMI) is a way to measure whether or not a person is underweight, overweight or normal weight. It is a calculation that evaluates body mass compared to height. It is widely used to determine if people fall within a weight range that is healthy.

We are living in a time when people are more obsessed than ever in being thin.

The visual images put forth in the media are that of super skinny women and body builder men, which can make us normal people feel a bit self-conscious. Recently, a few actors and models have come forward to show the contrast between their photoshopped selves and what they look like before alteration. However, it is difficult to remember when we open a magazine that we are looking at an artistic creation, not a true reflection of the real world. A measurement like BMI can bring us back to reality with a more realistic assessment of what it means to be healthy. It can also enlighten us to the reality of weight problems. Having said that, BMI does have its limitations.

BMI is calculated by comparing weight in kilograms to height in centimeters. If you’re like me and live in the U.S., you probably have absolutely no clue whatsoever what your weight is in kilograms or what you measure in meters. I remember hearing as a child about how we were inevitably going to convert to the metric system, but I’m a fully grown adult and I’m still waiting. So before we get started, us Americans need to do a little extra calculating.

*First, we need to multiply our weight in pounds by .45.

So I weigh about 120 pounds (I think, Honestly, I only weight myself once a year when I go to the doctor’s office.) So, my first step would be to multiply 120 x .45 which equals 54 kg.

*Then we Americans have multiple our height in inches by .025.

I’m 5 foot, 5 inches, or 65 inches to be exact. So my calculation is 65x.025 which equals 1.625 m. Ok, now we are working with the metric system. Whew, that’s a lot of math, and we haven’t even gotten to the real equation yet!

The equation to determine BMI is weight divided by height squared.

So the first thing we need to do is determine our height squared. Back to using myself as an example, I’ll multiply my height by itself (1.625 x 1.625) which equals 2.64. And the final step is then to divide my weight (54) by my height squared (2.64). This means my BMI is 20.45.

Summary of the results as per the World Health Organization

BMI-FIgures

BMI-FIgures

Determine Your BMI

But here’s the catch. This evaluation doesn’t account for someone’s build. Is the person small boned, stocky or muscular? Having extra pounds of muscle is very different than having extra pounds of fat. Someone who gains weight by muscle lifting will have in increases in BMI, just as someone does who is sedentary and gaining fat. So BMI can be a helpful indicator of health for many, but it doesn’t tell the entire story for everyone. Therefore, it is important when calculating BMI to really evaluate someone’s build. We can quickly see that a body builder is not obese, but muscular. BMI does not evaluate and that is where BMI falls short

There are other, more accurate ways to evaluate body composition. One of the best ways is to measure body fat directly. Excess weight is usually only problematic when it is composed of fat. There are many ways to measure body fat. There is hydraulic testing which involves measuring fat by submerging a person in water. There are Dexa scans, which can also be used to measure bone density and evaluate risk for osteoporosis. A Dexa scan uses beams of light to determine body fat as well as bone density. These methods can give a very accurate reading, however, they also can cost a pretty penny. Skin Calipers are a cheaper method, often used in gyms to pinch the skin and take fat measurements. The skin calipers literally grab the skin and then measure the skin fold. This method is commonly used due to its convenience along with its low price tag. However, because fat is only measured in a few select areas, it is difficult to get a truly accurate reading of overall body fat this way.

For this reason, BMI remains one of the most commonly used measurements to determine body composition. No, it is not a perfect way to evaluate a person. BMI needs to be taken with a grain of salt, and a person needs to be honest with themselves in evaluating their own muscle mass. This can be difficult as, not only do people overestimate the amount of muscle they have, they also underestimate it. My suggestion when looking at a questionable BMI is to follow-up with skin calipers. BMI doesn’t create the entire picture of whether a person is healthy, but it is a convenient tool to start exploring the topic.

Erica Mesirov
Expert

Erica earned her Master’s Degree in holistic nutrition in 2014. She works with people who struggle with eating right or losing weight and know they want to be healthy, but their attempts have come up empty. By helping people understand their individual body and the imbalances that are causing unhealthy patterns, she gives people the opportunity to feel good about eating again.

Erica Mesirov
Expert

Erica earned her Master’s Degree in holistic nutrition in 2014. She works with people who struggle with eating right or losing weight and know they want to be healthy, but their attempts have come up empty. By helping people understand their individual body and the imbalances that are causing unhealthy patterns, she gives people the opportunity to feel good about eating again.