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What is Body Mass Index (BMI)?

Posted: Sep 10 in Health And Wellness, Obesity Medicine by
Dr Lazarus doing a body composition test on a patient

Body Mass Index, or BMI, is a ratio of a person’s weight divided by their height squared in metric. If working with pounds and inches, take pounds divided by inches squared and multiply by 703. Alternatively, use this handy BMI calculator (below) to calculate your BMI.

unit US    
age  
sex    
height ftin
weight lb
height cm
weight kg

BMI: Some History and Definitions

Well, yes and no. First a little history lesson. BMI was invented by a Belgian astrologer named Adolphe Quetelet in 1832 (originally it was termed the “Quetelet Index.” He observed that an individual’s weight seems to increase relative to their height squared. It was forgotten about for centuries, but in the early 1970’s, Ancel Keys, a prominent cardiologist in America (who also recommended low-fat diets – now that’s a whole story on its own) promulgated the use of BMI to measure obesity in America. These days, America has agreed on the following ranges for BMI in adults:

  • 18 – 24.9: Normal
  • 25 – 29.9: Overweight
  • 30 – 34.9: Obesity (Stage 1)
  • 35 – 39.9: Obesity (Stage 2)
  • 40 or more: Obesity (Stage 3)

However, some countries use different cut points. For example, some countries in Asia define obesity as a BMI of 25 or more. The World Health Organization defines obesity as “Excessive or abnormal body fat that affects health. BMI may be used as a crude population measurement.

Does BMI Matter?

Yes and no. To compare one state to another, or one country to another, yes, BMI can provide a lot of information. However, for the individual, it leaves a lot to be desired. The problem with BMI is that it doesn’t tell you how much of your body is fat vs. muscle vs. bone. A well-muscled individual will often have a BMI above 25 as a result of lifting weights. That being the case, I have never seen a person with a BMI over 40 because of having so much muscle.

At very high numbers, BMI correlates with significantly increased health risks including a lowering of the lifespan – for example, for a male age 20-39 with a BMI above 35, the lifespan is reduced by 8.4 years.

BMI is also used to determine the appropriateness of different treatment options. For example, weight loss medications are approved by the FDA if the BMI is above 27 with other weight-related problems, or above 30 without. Similarly, weight loss surgery is indicated if the BMI is above 35 with weight-related medical problems, or above 40 without.

Finally, during this latest COVID-19 scare, our CDC has recognized a BMI of 30 or more as a risk factor for developing severe complications of COVID.

Should Everybody Have a Goal of Achieving a “Normal” BMI?

Absolutely not. Telling a person with Stage 3 obesity that they must have a BMI of 24 is not appropriate. Remember, this is more of a population gauge than an individual goal. It is NOT recommended to use these BMI ranges to set goals with patients.

Instead, if a person is at increased health risk as a result of their elevated BMI, it is recommended to reduce the BMI by 5-10%. Greater reductions are possible (up to 20% with medical treatment, sometimes 30% with surgery), but the first 5-10% provides significant health benefits and is achievable with currently available tools.

I hope one day we have tools to normalize the BMI for all people! But, sometimes a dose of reality can help a person manage expectations depending on the treatment(s) chosen, although I have had the rare patient lose 30 and even 40% of their body weight.

Body Composition vs. BMI

At Clinical Nutrition Center (and most other Obesity Medicine Physician Specialists), we prefer to measure an accurate Body Composition in addition to measuring the BMI. We purchased a medical body composition analyzer made by SECA, and have found the test results very illuminating in helping a person understand their body composition and health risk.

In addition to measuring BMI, it separates the measurement into a fat mass index (FMI) and a fat-free mass index (FFMI). It further measures visceral fat (organ fat – the more deadly type of fat) and muscle distribution.

With this scale, we can see if the BMI is elevated as a result of too much fat or too much muscle. Many normal-BMI individuals still have an elevated fat mass index (FMI) and/or elevated visceral fat (so-called “normal” weight obesity). Or sometimes we will find a person with an elevated BMI, but the fat and visceral fat are normal, and the fat-free mass and muscle weight are high (body-builders, for example).

In addition, while losing weight, we can see what a person lost! Did they lose mostly fat weight? Has the visceral fat come down? And, if exercising, is the muscle weight improving? Also, in many people the muscle distribution is asymmetric – for example, in a person who mostly walks, we may see good lower-body muscle weight, but poor upper-body muscle weight. In that person instead of counseling to “exercise more” i.e. “walk more” we may recommend adding upper-body resistance or yoga.

Measuring My Body Composition

If you want to think beyond BMI, maybe it is time to check your body composition. Just let us know at your next visit and we are happy to perform your Seca mBCA. Leave about 10 minutes. It is non-invasive and doesn’t hurt (well, at least not physically). Review it with your provider or dietitian at your next visit, and use it to set compelling goals.

BMI: Some History and Definitions

Well, yes and no. First a little history lesson. BMI was invented by a Belgian astrologer named Adolphe Quetelet in 1832 (originally it was termed the “Quetelet Index.” He observed that an individual’s weight seems to increase relative to their height squared. It was forgotten about for centuries, but in the early 1970’s, Ancel Keys, a prominent cardiologist in America (who also recommended low-fat diets – now that’s a whole story on its own) promulgated the use of BMI to measure obesity in America. These days, America has agreed on the following ranges for BMI in adults:

  • 18 – 24.9: Normal
  • 25 – 29.9: Overweight
  • 30 – 34.9: Obesity (Stage 1)
  • 35 – 39.9: Obesity (Stage 2)
  • 40 or more: Obesity (Stage 3)

However, some countries use different cut points. For example, some countries in Asia define obesity as a BMI of 25 or more. The World Health Organization defines obesity as “Excessive or abnormal body fat that affects health. BMI may be used as a crude population measurement.

Does BMI Matter?

Yes and no. To compare one state to another, or one country to another, yes, BMI can provide a lot of information. However, for the individual, it leaves a lot to be desired. The problem with BMI is that it doesn’t tell you how much of your body is fat vs. muscle vs. bone. A well-muscled individual will often have a BMI above 25 as a result of lifting weights. That being the case, I have never seen a person with a BMI over 40 because of having so much muscle.

At very high numbers, BMI correlates with significantly increased health risks including a lowering of the lifespan – for example, for a male age 20-39 with a BMI above 35, the lifespan is reduced by 8.4 years.

BMI is also used to determine the appropriateness of different treatment options. For example, weight loss medications are approved by the FDA if the BMI is above 27 with other weight-related problems, or above 30 without. Similarly, weight loss surgery is indicated if the BMI is above 35 with weight-related medical problems, or above 40 without.

Finally, during this latest COVID-19 scare, our CDC has recognized a BMI of 30 or more as a risk factor for developing severe complications of COVID.

Should Everybody Have a Goal of Achieving a “Normal” BMI?

Absolutely not. Telling a person with Stage 3 obesity that they must have a BMI of 24 is not appropriate. Remember, this is more of a population gauge than an individual goal. It is NOT recommended to use these BMI ranges to set goals with patients.

Instead, if a person is at increased health risk as a result of their elevated BMI, it is recommended to reduce the BMI by 5-10%. Greater reductions are possible (up to 20% with medical treatment, sometimes 30% with surgery), but the first 5-10% provides significant health benefits and is achievable with currently available tools.

I hope one day we have tools to normalize the BMI for all people! But, sometimes a dose of reality can help a person manage expectations depending on the treatment(s) chosen, although I have had the rare patient lose 30 and even 40% of their body weight.

Body Composition vs. BMI

At Clinical Nutrition Center (and most other Obesity Medicine Physician Specialists), we prefer to measure an accurate Body Composition in addition to measuring the BMI. We purchased a medical body composition analyzer made by SECA, and have found the test results very illuminating in helping a person understand their body composition and health risk.

In addition to measuring BMI, it separates the measurement into a fat mass index (FMI) and a fat-free mass index (FFMI). It further measures visceral fat (organ fat – the more deadly type of fat) and muscle distribution.

With this scale, we can see if the BMI is elevated as a result of too much fat or too much muscle. Many normal-BMI individuals still have an elevated fat mass index (FMI) and/or elevated visceral fat (so-called “normal” weight obesity). Or sometimes we will find a person with an elevated BMI, but the fat and visceral fat are normal, and the fat-free mass and muscle weight are high (body-builders, for example).

In addition, while losing weight, we can see what a person lost! Did they lose mostly fat weight? Has the visceral fat come down? And, if exercising, is the muscle weight improving? Also, in many people the muscle distribution is asymmetric – for example, in a person who mostly walks, we may see good lower-body muscle weight, but poor upper-body muscle weight. In that person instead of counseling to “exercise more” i.e. “walk more” we may recommend adding upper-body resistance or yoga.

Measuring My Body Composition

If you want to think beyond BMI, maybe it is time to check your body composition. Just let us know at your next visit and we are happy to perform your Seca mBCA. Leave about 10 minutes. It is non-invasive and doesn’t hurt (well, at least not physically). Review it with your provider or dietitian at your next visit, and use it to set compelling goals.

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