About BMI and Eating Disorders

The Body Mass Index (BMI) is a tool used by doctors and healthcare professionals to determine if someone is at a healthy weight or not. While weight is not the only indicator that someone may be struggling with an eating disorder, it can be a helpful tool for some people when it comes to measuring their health.

What is BMI?

The BMI is a chart that looks at someone’s height and weight to determine if they are at a healthy weight. The BMI is used to tell doctors approximately how much muscle, fat, and bone someone has [1].

Depending on the chart, someone’s weight can be classified as underweight, normal weight, overweight, or obese [1].

How to Measure BMI

BMI is measured by taking someone’s height and weight. Once someone knows their height and weight, BMI is determined by a mathematical equation.

This equation attempts to measure someone’s size. BMI does not determine the amount of body fat, bone, or muscle someone has [1].

What is Considered a Healthy BMI Range?

The chart below uses information from the CDC to show the different BMI ranges and how they are classified [1].

BMI Measurement Weight Category
Below 18.5 Underweight
18.5-24.9 Normal weight
25.0-29.9 Overweight
30.0 and above Obesity

Is BMI Accurate?

There has been a lot of controversy about whether BMI is an accurate measurement of someone’s health. This can be really triggering and shameful for people who don’t fall into the “normal weight” category. This is unfortunate because even the government acknowledges that the BMI does not measure someone’s health! [1].

The BMI was created about 200 years ago by a man named Adolph Quetelet who was trying to determine what an ideal man was like. Quetelet was not a doctor or even a scientist. He was a mathematician.

When creating this theory of the BMI, he only studied White Europeans [2]. His work excluded so many different kinds of people that it can’t be considered an accurate measure of someone’s health.

However, doctors and health insurance companies use it because it’s a way to determine risk. The idea is that if someone has an unhealthy amount of body fat, then they are likely to have a high BMI.

If they have a high BMI, then they are believed to be more at risk for certain medical problems. Insurance companies use this information to determine how much they should charge someone for medical insurance. If the risk is high, the cost of insurance goes up.

Further Reading

This is unfortunate given that BMI is not a measure of health, nor does it accurately measure someone’s muscle mass, bone mass, or amount of body fat [1]. At best, the BMI is a guess and not a reliable measurement.

This is especially true given that there isn’t just one body shape and size that is healthy or normal for humans. There is a wide range of bodily differences that are based on gender, race, ethnicity, or age [1]. While there are separate BMI charts for adults and kids, all adults are grouped together.

This means that a 20-year-old woman’s health is compared to an 80-year-old woman’s health in the same way. The BMI does not account for the natural differences that occur.

Unhealthy BMI: When to Be Concerned 

While BMI is not an accurate reflection of someone’s health, it might be a warning sign that something is wrong. This may be especially true if someone’s BMI is low. A low BMI can mean that someone does not have enough fat, muscle, or bone. This can cause several health problems.

BMI Charts

Low BMI Risks

Low BMI comes with the following health risks:

  • Nutritional deficiencies
  • Menstrual irregularities
  • Irregular heart rhythms
  • Dehydration
  • Abnormal blood count
  • Dizziness or fainting
  • Thinning hair
  • Constipation
  • Dry skin
  • Bone loss
  • Mortality [3]

This list certainly doesn’t include all the health risks. A lot of attention is given to the health risks that are believed to be associated with being overweight. However, being underweight is also a serious matter that needs to be talked about.

Anorexia BMI

The American Psychiatric Association (APA) has guidelines for how to diagnose eating disorders. In order for someone to be diagnosed with anorexia, someone has to meet multiple criteria. One of the criteria is based on someone’s BMI.

Depending on BMI, someone is determined to have mild, moderate, severe, or extreme anorexia. The following chart demonstrates this criteria [4]:

BMI Severity of Anorexia
Greater than or equal to 17 Mild
16-16.99 Moderate
15-15.99 Severe
Less than 15 Extreme


Sometimes someone is considered to have a healthy BMI, but their eating behaviors are disordered and typical of anorexia. This reliance on BMI in order to diagnose someone with anorexia may get in the way of someone getting the necessary treatment that they need.

Anorexia vs Bulimia BMI

The APA does not have BMI requirements for bulimia or other eating disorders. It’s only anorexia that BMI is considered important. This means that someone with bulimia could have a normal BMI, but could still be unhealthy because of the impact of bulimic behaviors on their body.

It’s important that diagnosing and treating eating disorders isn’t based on BMI or weight. These two factors do not give an accurate depiction of what is going on with someone’s health, let alone their relationship to food, exercise, or body image.

Paying attention to the way someone feels about food, their eating patterns, how they feel about their body, and how their body physically feels may be more reliable in assessing for an eating disorder than weight alone.

Low BMI & Eating Disorder Treatment

If someone is struggling with an eating disorder, there are several treatment options available. For someone who has a low BMI and is dealing with an eating disorder, it’s possible that intensive treatment may be necessary in order to restore health.

Treatment options may include:

  • Hospitalization
  • Residential treatment
  • Intensive day programs or
  • Outpatient therapy and nutritional counseling
  • Support groups, such as Eating Disorders Anonymous


Author: Samantha Bothwell, BMFT

Page Last Updated & Reviewed By: Jacquelyn Ekern, MS, LPC on September 17, 2021
Published on EatingDisorderHope.com, Treatment Resources & Information on Eating Disorders