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Last updated January 16, 2020. 5 minute read

Body Mass Index or BMI: what this number means

While BMI can be a good indicator of high body fat, it’s not without its limitations. According to the CDC, factors like age, sex, ethnicity, and muscle mass can influence the relationship between BMI and body fat. It also doesn’t consider bone mass, nor does it provide any indication of how fat is distributed among individuals.

Screen Shot 2020 01 28 At 2 52 04 Pm Written by Maria Del Russo
Reviewed by Tzvi Doron, DO

Whether you’re just starting out on a journey to a healthy weight, are actively trying to drop some pounds, or just remember a handful of lessons from your middle school health class, you’ve likely heard the term BMI. BMI stands for Body Mass Index, and it’s a screening tool used by healthcare providers to determine a person’s weight range using height and weight. It’s considered the go-to screening tool because it’s simple and non-invasive.

That said, plenty of healthcare providers, researchers, and even the Centers for Disease Control And Prevention (CDC) have noted some issues with using BMI as a sole indicator of health, or as a diagnostic tool. Because of this, BMI is just considered one small part of your overall health, and not the final word in body weight management. 

But if you’re still curious about BMI, how it’s calculated, and what it means for your health, scroll on. We’re getting into the nitty-gritty of this number and what it means for you.

Vitals

  • BMI stands for Body Mass Index and is a screening tool to determine a person’s weight range using their height and weight.
  • Your number is not an overall indicator of your health, but it can be associated with certain health risks.
  • BMI doesn’t take your age, sex, ethnicity, or muscle mass into account, which is why the CDC notes it should only be used as a screening tool — not a diagnostic tool.
  • Other measures of body fat, like your weight circumference, can be more invasive than a BMI test.

The basics of BMI

As stated above, BMI is a tool used by healthcare providers to screen for overweight and obesity. According to the CDC, it’s calculated as weight in kilograms divided by the square of height in meters. While it’s considered an indicator of body fatness, it actually measures excess weight over excess fat. This is part of why some people consider it an imperfect screening tool — but we’ll get to that in a second.

BMI measurements are broken down into four categories: normal, underweight, overweight, and obese. Normal weight in adults is classified as a BMI between 18.5 and 24.9. Underweight is classified by a BMI of under 18.5, while overweight is classified by a BMI of between 25 and 29.9. A BMI of over 30 is classified as obese. Obesity is then frequently broken up into classes: Class 1 is BMI of 30 to 34, Class 2 is BMI of 35 to 39, and Class 3 is BMI of 40 or higher. Class 3 is also categorized as “extreme” or “severe” obesity.

While BMI is considered only part of your overall health, research suggests that a high BMI is associated with health risks. These risks include type 2 diabetes, heart disease, high blood pressure, certain cancers chronic back pain (Gutin, 2017), and more health conditions. 

You could calculate your BMI on your own, but the CDC also provides BMI calculators for adults, as well as a separate one for children and teens. This is especially useful for folks who have a hard time converting their weight in pounds to kilograms, and their height in meters to feet. 

That said, it’s important to remember that your BMI doesn’t tell the full story of your health. Speaking to a healthcare provider could give you better insight into your health, as well as provide tips for improving your lifestyle.

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Issues with BMI

While BMI can be a good indicator of high body fat, it’s not without its limitations. According to the CDC, factors like age, sex, ethnicity, and muscle mass can influence the relationship between BMI and body fat. It also doesn’t consider bone mass, nor does it provide any indication of how fat is distributed among individuals. 

Athletes with a lot of muscle can find issues with their BMI because their BMI might be high, but their body fat percentages could be low or normal. Since BMI doesn’t take muscle into account, it could classify a person with low body fat as overweight and obese. And while BMI is associated with health problems, it’s important to remember that high body fat is the problem that most higher risk health issues are connected to. 

These misclassifications go both ways, too. Research suggests that BMI tends to underestimate the prevalence of obesity in the general population, especially in women with high levels of the hormone leptin, which is part of the system which controls hunger in your body (Shah, 2012).

Ethnicity is also an issue, as the majority of data behind BMI is based on Caucasian populations. But different ethnicities can respond to weight gain differently. For example, research has shown that weight gain can be particularly detrimental in Asian populations, which is why Chinese and Japanese medical organizations have established lower BMI ranges for obesity in these countries (Shai, 2006). 

There’s also the issue of sex and age. Because of their body compositions, women tend to have greater amounts of total body fat than men with equal BMIs. Older adults also tend to have more body fat than younger adults, even if their BMIs are equivalent. 

According to the CDC, BMI is calculated the same way for children as it is for adults. But it should be interpreted differently. For children and adolescents between the ages of 2 and 20, BMI is relative to a person’s age and sex. That’s because the amount of body fat changes dramatically with age. It’s also affected by sex. 

Child BMI is calculated the same as adult BMI, but it’s then broken up by percentile considering the child’s age and sex. Anything less than the 5th percentile is considered underweight, where a percentile between 5th and 84th is considered a healthy weight. The 85th percentile to the 94th percentile is considered overweight, and anything equal or greater than the 95th percentile is considered obese.

Other measures of body fat

According to the CDC, there are other forms of body fat measures. These range from skinfold thickness measuring, to waist circumference measuring, to underwater weighing. 

The best way to assess your body fat percentage, though, is the dual-energy X-ray absorptiometry (DEXA) test. In this test, x-ray beams measure your density at different parts of your body. 

The issue with these other tests, though, is that they can be invasive, expensive, or just not widely available. That’s why BMI tends to be the go-to for healthcare providers — it’s an easy way to screen for body fat. 

That said, the CDC is quick to remind that BMI should not be used as a diagnostic tool for weight loss. Instead, it should be used as a screening tool to identify potential weight problems, and then health care providers can give medical advice from there. Other factors, like evaluations of diet, physical activity, family history, and other health screenings, should be taken along with BMI to get the full story on a person’s health.