What's the Story With BMI?
By Julia Kenney
Body Mass Index (BMI) is a health metric used to measure body fat and its impact on population and individual health. While useful, BMI measurements alone may be misleading. Read below to learn why your BMI may not accurately reflect your health.
Measuring your height and weight is part of a routine check-up with healthcare providers. This data can be used to calculate your Body Mass Index (BMI) – a measure of body fat. Though simple and useful, BMI is not always a reliable predictor of your health and may oversimplify the relationship between body mass and your individual health.
Created as a way to gauge the health of a population, the index has become a common way to assess individuals’ health as well. Your healthcare providers may track your level of body fat over time to determine your risk for health complications such as high blood pressure, heart disease, and type 2 diabetes.
Though BMI has proven useful, when assessing health on the individual level, BMI may not tell the whole story.
What is BMI?
To find yours, use this BMI calculator. BMI measurements are used to place adults (20 and older) in one of the following categories. Note that the following categories may differ based on race/ethnicity:
- Underweight – Below 18.5
- Healthy Weight – 18.5-24.9
- Overweight – 25-29.9
- Obesity – 30 and above
BMI is calculated the same way for children and teens, but the interpretation depends on their age and sex. Learn more about youth BMI here.
What are the benefits of calculating BMI?
The benefits of BMI are numerous, not only for population-level studies but also as an initial tool for individual health screening to assess risk for body fat-related health complications.
“BMI is an inexpensive and easy screening method,” said Dr. Charles Alexander, an endocrinologist and diaTribe’s scientific and medical advisor.
For example, a high percentage of body fat is associated with an increased risk for type 2 diabetes. It has been proposed that fat build-up in the pancreas can increase insulin resistance and decrease insulin production. In addition to diabetes, body fat increases the risk for heart disease, kidney disease, high blood pressure, and other health conditions.
Using records of height and weight, researchers, health systems and governments can easily approximate rates of the different BMI categories in populations to study obesity and more effectively distribute health resources.
Is BMI Accurate?
Being categorized as “Obese” or “Overweight” based on BMI does not necessarily mean you are unhealthy. While the metric can be helpful in population-level health assessments and health screening, BMI oversimplifies the relationship between an individual’s body mass and their health. BMI does not diagnose and treat individuals, nor does it account for several factors that can influence a person’s health. The following are some factors:
Race, Sex, and Age – A person’s race/ethnicity, sex, and age can impact their level of body fat, regardless of BMI. According to the CDC, women tend to have more body fat than men, white people tend to have more body fat and a different fat distribution than Black people, and elderly people tend to have more body fat than young people. The BMI scale is often described as racially and sexually biased because it is based on research in populations of white European men.
Non-fat body mass – The body is composed of more than just fat. We are made up of muscles, bones, nerves, and organs, in addition to fat, that all contain mass. BMI does not distinguish between fat and non-fat body mass, meaning that a person with a high muscle mass could be categorized as overweight or obese even if they have low levels of body fat. The percent of body mass that comes from fat, not body mass overall, is the metric most relevant to predicting health complications.
Fat distribution – While those with high levels of body fat may be at risk for health complications, not all fat is harmful. Subcutaneous fat – the fat found just underneath the skin – is less harmful and is the main way that the body stores energy. Visceral fat – the fat located in and around organs – can be dangerous in large amounts because its excess is associated with many chronic diseases. BMI does not distinguish between subcutaneous and visceral fat, meaning that a person with large amounts of subcutaneous fat may be categorized as overweight when they are healthy. Fat distribution, determined by multiple factors including genetics, is key in identifying a person’s health risks.
Your weight and overall risk for health complications depend on many different factors. In addition, other factors such as income, level of education, stigma, and place of residence are all associated with risk for obesity and other health complications. This is part of the reason why BMI alone does not tell the whole story.
How is BMI used in healthcare?
With these factors in mind, healthcare professionals should be careful about how they use BMI. Relying solely on BMI measurements risks individuals getting treatment they do not need or getting denied healthcare they do need.
Clinical trials will sometimes measure the BMI of each participant, which can be used later during analysis of the research findings. For instance, in large-scale clinical studies that describe the relationships between diabetes, obesity, heart disease, and other related complications, having those BMI measurements allows the researchers to identify the association between a high BMI and the presence of these complications. This is why your doctor may use BMI as a quick method of determining health risks.
Additionally, some clinical trials will include a specific BMI range in their inclusion and exclusion criteria, or guidelines that determine whether someone can or cannot participate in a trial. When this happens, medications or other health interventions may not be tested across the full scope of people who could benefit, nor will they be used by those people if approved.
BMI also impacts the calculation of insurance premiums. While insurance companies cannot charge higher premiums to those with pre-existing health conditions, obesity is not considered a pre-existing condition at the federal level. People with a BMI over 30 can expect to pay an average of 25 to 50% more for health insurance than those with a BMI below 30.
BMI is still a useful tool if it is not used in isolation. This is also true of other metrics such as A1C, blood pressure, cholesterol, etc. No individual measure can tell the whole story – providers must combine these results to fully understand an individual’s health status. Healthcare professionals often use BMI to screen for potential health risks before performing further health testing.
For individuals calculating their own BMI, understand that those categorized as underweight, overweight, and obese may be healthy. Avoid making assumptions about your health and consult your healthcare providers with any concerns.
Here are some more resources on BMI and weight: