Medically reviewed by Suzanne Fisher, RDMedically reviewed by Suzanne Fisher, RD
The body mass index (BMI) is commonly used to categorize weight ranges (from underweight to obese) because it is accessible, no-cost, and quick. Using a formula or BMI calculator, it is relatively easy to plug in height and weight measurements and get a figure meant to represent a measure of body fat and, by extension, related health risks.
While there is a noted correlation between high BMI and obesity-related health risks, such as cardiovascular disease and type 2 diabetes, BMI has flaws and is not meant to be a diagnostic tool for individuals. BMI does not consider other variables that influence health risks, such as muscle and bone mass, lifestyle, sex or gender, ethnicity, race, and family history.
The American Medical Association (AMA) recommends against healthcare providers using BMI as a sole indicator of health and as a means to diagnosis obesity. Rather, the AMA argues, it should be used along with other measures, such as waist circumference, cholesterol tests, and other tools to gain a more comprehensive picture of an individual's health and care needs.
This article will discuss the accuracy and inaccuracies of BMI, how to assess health beyond BMI values, and how to determine a healthy BMI for an individual.
Use of Gendered Language
Overwhelmingly, the data and research on BMI use binary gender measures of male and female, and does not account for gender diversity. This article uses binary gender language to reflect the current research, while recognizing that more research is needed that considers how BMI affects those who are not cisgender.
BMI Accuracy: What Is True?
BMI is calculated using weight and height measurements—namely, weight in kilograms divided by the square of height in meters.
BMI is not a precise measure of body fat or health but can be used as a screening tool. It gives a measure that corresponds to weight categories (underweight, normal weight, overweight/pre-obesity, obesity classes 1, 2, and 3). BMI levels alone do not indicate a person's health status, but BMI is moderately correlated with more direct measures of body fat.
Higher BMI levels are also correlated with elevated mortality risk, and with an elevated risk for conditions such as cardiovascular disease, type 2 diabetes, and other health issues associated with overweight and obesity.
BMI is not an individual diagnostic tool, but it is useful for examining overall population trends and can be used as a starting point for determining an individual's obesity-related health risks.
BMI Ranges
The World Health Organization (WHO) categorizes BMI for adults as:
Underweight: Less than or equal to 18.5
Normal weight: 18–24.9
Overweight/pre-obesity: 25.0-29.9
Class 1 obesity: 30.0–34.9
Class 2 obesity: 35.0–39.9
Class 3 obesity: Greater than or equal to 40
These values are based on cisgender white men, and may need to be adjusted according to factors such as race and sex.
BMI is calculated differently for children and adolescents than it is for adults. Adult BMI measures and ranges cannot be used to assess BMI for people under 20 years old.
BMI Inaccuracy: What’s Untrue?
BMI does not give a precise measure of body fat, and it cannot be used alone to determine an individual's health. It is not a diagnostic tool; it just gives a general idea of body fat.
There are many factors that can affect BMI, but BMI is calculated using only height and weight. In addition, a BMI figure alone can have different implications depending on other contributing factors. Two people can have the same BMI, but have different levels of body fat and/or different health risks.
For example, at the same BMI:
Women tend to have more body fat than men.
Older adults tend to have more body fat than younger adults.
Athletes tend to have less body fat than nonathletes.
People of different racial and ethnic groups tend to have more or less body fat than other racial or ethnic groups.
Some of this discrepancy stems from the origins of the BMI. The concept for a calculation like BMI was invented by a mathematician in Belgium about 200 years ago. He based the data on European White men, without accounting for variables in sex, race, and ethnicity.
Several studies have found that the current BMI ranges do not correlate accurately with some health risks, such as type 2 diabetes, for Asian Americans.
The WHO has noted that people of Asian descent can have a higher risk for some metabolic conditions at lower BMIs than the current classification ranges indicate.
BMI may be less accurate for certain ethnic groups, such as:
Aboriginal and Torres Strait Islander peoples
Chinese people
Japanese people
Maori people
Pacific Islanders
South Asian people
Available data also suggests that the association between BMI and body fat, dyslipidemia, metabolic syndrome, and all-cause mortality may be weaker for Black Americans than for White Americans. This may affect how BMI should be interpreted for this racial group.
More research is also needed on the accuracy of BMI with other racial and ethnic groups, such as people of Middle Eastern descent.
In addition, BMI does not take into consideration body makeup, such as the weight percentage due to fat, muscle, bone density, etc.
In general, BMI tends to be a less accurate tool for people such as:
Pregnant people
People with a physical disability
Certain athletes, such as bodybuilders, weight lifters, or some high-performance athletes
People under 20 years old
How BMI is used can also be problematic. Because it is a simple, easy-to-use tool, there can be a tendency to use it as a sole indicator of health or to make health recommendations.
This may lead to important assessments being missed, such as dismissing heart health risks in someone whose BMI is in the normal range, or unnecessarily pressuring someone with a BMI above the normal range but without additional risk factors to lose weight.
Other potential downsides to BMI include:
Fostering of weight-based shame, which may discourage some people from seeking medical care
Possibility of mistrust for the healthcare provider, if too focused on BMI
Missed or misdiagnosis if healthcare provider incorrectly attributes symptoms to weight
Arbitrariness of the BMI cut-off points in terms of identifying health risks
Overreliance on BMI to the exclusion of other measures of health
Why Is My BMI High If I Am Not Overweight?
BMI does not account for body makeup. A person with increased muscle mass and lower body fat may have a BMI that classifies them as overweight or obese, but their body size may be smaller than someone with the same BMI who has a higher amount of body fat.
Assessing Health Beyond BMI Values
BMI is one of many tools that can be used to assess health related to body fat.
Waist circumference is another tool commonly used to assess potential health risks. Excessive abdominal fat is associated with an increased risk of conditions such as cardiovascular disease, type 2 diabetes, and high blood pressure (hypertension).
To measure your waist, do the following:
Use a tape measure over bare skin or one layer of light clothing.
Place the tape measure in the area between your lowest rib and the top of your hip bone (about belly button level).
Breathe out normally and wrap the tape around your waist snugly (without squeezing the skin) to measure.
You may have a higher risk of obesity-related health conditions if your waist circumference is more than:
Men: 40 inches
Women: 35 inches
As with BMI, waist circumference is another screening tool that can be used as part of a health assessment, but not as a sole indicator of health. Waist circumference measurement is not appropriate for:
Pregnant people
People under 18 years
People with a medical condition that causes a very enlarged abdomen
People from certain non-European backgrounds
The AMA recommends additional measures for diagnosing obesity, such as:
Body adiposity index (hip and height measurement)
Relative fat mass
Measurements of visceral fat (found deep in the abdominal cavity)
Body composition (percentage of fat, bone, and muscles)
Genetic/metabolic factors
Tools healthcare providers may use include:
Caliper measurements of skinfold thickness
Underwater weighing
Bioelectrical impedance
Dual-energy X-ray absorptiometry (DXA)
Isotope dilution
These are less readily available, and some can be expensive. They should be performed by professionals trained to use these tools.
As part of a comprehensive health assessment, healthcare providers may also do other checks, such as:
Blood pressure
Cholesterol
Blood glucose (sugar)
What Is a Healthy BMI for Me?
What a healthy BMI is for any individual depends on more factors than just which range the number falls in, including genetic factors, fitness levels, lean mass, health behaviors, environmental risks, and more. BMI alone should not be used as a health assessment but rather one screening tool among many.
Rather than rely on the one BMI reading, it's best to see a healthcare provider for a comprehensive health assessment.
To calculate your BMI, visit the Centers For Disease Control and Prevention (CDC) website for the following calculators:
Summary
BMI is a popular tool used to categorize body mass based on height and weight. It is helpful as a screening tool when used alongside other measures of health, but it can be inaccurate or misleading when used on its own to assess an individual's health risks.
BMI doesn't take into consideration factors such as body composition, age, sex, race and ethnicity, genetics, and other variables that contribute to an individual's health status and health risks. Instead of relying solely on BMI, see your healthcare provider for a comprehensive health assessment.
Read the original article on Verywell Health.
