What is BMI (Body Mass Index)?

Almost 200 years ago, a mathematician invented the BMI formula (a person’s weight in kilograms divided by the square of their height in meters) using data from just a few hundred people. Because of the limitations of BMI, it was never intended to be used as an estimate of body fatness. However, because employing methods for reliably measuring body composition in studies that follow thousands of people for many years is unrealistic, researchers use BMI as a stand-in (BMI <18.5 = underweight, BMI 18.5-24.9 = normal-weight, BMI 24-29.9 = overweight, BMI >30 = obese). The problem now is that many people are using the terms “obese” or “overweight” synonymous with “unhealthy.” This simply isn’t true.

Pros to using BMI

As a research tool, BMI has its benefits: it’s quick, easy, and globally accepted as a research standard (using standardized tools lets us compare studies from different research groups in an “apples to apples” capacity).

Limitations of BMI

There are many limitations of BMI. First, because it’s just a ratio of height to weight, it doesn’t consider body composition. This is a big problem when we think about using BMI on individuals. The health risks associated with obese BMIs are related to excess body fat, not high amounts of skeletal muscle. Therefore, someone with a large frame and good muscle development might have a BMI that places them in the overweight or obese category even though they don’t have the excess body fat that may increase their risk for disease. Second, the formula is the same for adults of all ages and genders, even though the relationship between body fat and health is different for men and women and changes as we age.

When is weight loss a healthy goal?

Having a BMI of 25 or more does not mean that you are unhealthy or that losing weight will improve your health. Restrictive and drastic weight loss attempts can actually hurt your health. If you have a BMI that is greater than 25 but have no other risk factors for poor health, I recommend that your focus be on living a healthy, balanced lifestyle (with plenty of veggies and physical activity).

Making dietary and lifestyle changes may improve your health even if you don’t lose weight if one or the more of the following conditions apply to you:

  • A waist circumference greater than 35 inches for women and 40 inches for men
  • Chronic disease or metabolic disorders (e.g. diabetes, heart disease, high cholesterol, hypertension)
  • Family history of chronic disease or cancer
  • Sleep apnea
  • Joint pain
  • Inability to be physically active due to discomfort related to body weight

Limitations of BMI: You are Not a Number

I want to reemphasize that you are not a number. Your BMI and your body weight do not define you as a person. If you’re feeling anxious about your body or are having trouble with your healthy relationship with food, please check out the following resources:

About-Face equips women and girls with tools to understand and resist harmful media images that affect self-esteem and body image.

Developing and Maintaining Positive Body Image from the National Eating Disorder Association

National Eating Disorders Association offers educational materials and prevention programs and sponsors Eating Disorders Awareness Week.

Men with Eating Disorders provides information and resources for and about men with eating disorders.