Introduction

What Does BMI Really Tell You?

Sara Suchy Jan 11, 2013 (updated Nov 20, 2014)
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There is a lot of buzz about Body Mass Index (BMI), but what does the number actually tell you about your health?

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Overweight is healthy?
Overweight is healthy?
A study published in the Journal of the American Medical Association raised more than a few eyebrows with its conclusion that people who are overweight might live longer than people of a healthy weight. This particular finding is worth a closer look.  One reason is because the study looked only at body mass index (BMI) at the time of death.  
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BMI is…
BMI is…
According to the Centers for Disease Control and Prevention (CDC), BMI is a number calculated from a person’s height and weight that is used to determine the weight status of that person. BMI does not measure body fat directly, but provides a general measure of body fat on the average person.  
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How BMI is calculated
How BMI is calculated
The BMI formula assumes that people of a certain height should fall within a certain weight range. The exact formula is: weight (kg) / [height (m)](squared) = BMI….but don’t worry about remembering that, check out our BMI calculator
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Used to define and screen
Used to define and screen
Your BMI is used to define your weight status according to these parameters set by the CDC: Underweight  - Below 18.5 BMI; Normal weight – 18.5 to 24.9 BMI; Overweight – 25 to 29.9 BMI; Obese – 30 and above BMI.
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Why BMI is used
Why BMI is used
BMI is favored because it is easy to standardize.  The only information needed to determine a person’s BMI is their weight and height. This is why the CDC and other groups use BMI in studies and surveys that determine and compare the overall weight and health of large sample sizes - such as an entire country - rather than more expensive and time-consuming calculations of body fat composition. 
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What BMI does not tell you
What BMI does not tell you
BMI is a screening tool and nothing more. A person’s BMI takes only into account weight and height, and does not include factors such as age, gender, race and body composition, all of which are significant in determining if the person in question is healthy or not. 
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How BMI can mislead
How BMI can mislead
Because so many important factors are omitted when determining a person’s BMI, studies that use only BMI to determine overall health can be misleading. For example: at the same BMI, women tend to have more body fat than men and older people usually have more body fat than younger people.  
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Body composition and BMI
Body composition and BMI
Imagine two women: one carries most of her excess fat on her stomach while the other carries it on her hips. But they have the same BMI. Many studies suggest – and the American Heart Association confirms – that fat accumulated around the waist is more unhealthy than fat on the hips. So, the woman with excess fat on her hips is likely healthier. But, you would never know that if you only knew each woman’s BMI. 
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Athletes and BMI
Athletes and BMI
Many professional athletes are considered overweight or obese according to the BMI scale because of their muscle mass. Muscle is denser and weighs more than fat so an athlete’s weight is generally higher than the average person of the same height.  This means an athlete has a higher BMI than the average person, even though the athlete is healthier by every other measure. 
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BMI in studies
BMI in studies
In the JAMA study claiming overweight people live longer, using only BMI as the determining health factor may have skewed the results. The study was a meta-analysis that looked at the BMI of 3 million people at the time of death.  According to the data, more people have a low BMI when they die. Therefore, the results indicate people with a high BMI have a lower risk of death than people with a low BMI.
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A problematic conclusion
A problematic conclusion
The study results fail to consider that many people lose weight at the end of their life, especially if they’ve been battling a disease.  This means that their BMI is lower at the time of death than it had been through their lifetime. In this case, concluding that people with a high BMI live longer than those with a low BMI can be misleading. 
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BMI: A screening tool
BMI: A screening tool
The bottom line is that BMI is a good first step to determining the overall health of a person, but by no means comprehensive. If you do have a high BMI, see it as a cue to take a thorough look at your lifestyle and, with the help of a doctor, paint a clearer picture of your overall health. 
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Sources
Sources
Centers for Disease Control and Prevention, About BMI for Adults (December 2012). The Atlantic, The Problem With All of This ‘Overweight People Live Longer’ News. (January 2013). Journal of the American Medical Association: Association of All-Cause Morbidity With Overweight and Obesity Using Standard Body Mass Index Categories. (January 2013).