The BMI (body mass index) is a simple mathematical equation that has been used for decades to assess a person’s body fat and health risk. It categories people as “underweight” (<18.5), “healthy” (18.5-25), “overweight” (25-30) and “obese” (>30). People who fall into the “overweight” and “obese” categories are often deemed unhealthy and advised to lose weight by their doctor.
While this simple tool is widely used, it has some major problems. Let’s explore where the BMI came from and why it needs to go.
The BMI was first developed nearly 200 years ago when Belgian mathematician Lambert Adolphe Jacques Quetelet wanted to identify the characteristics of the “average man”. To do this, Quetelet used height and weight data from French and Scottish armies. He developed the “Quetelet Index” (QI) to neatly graph this data along a bell curve to find the average weight. The QI is the person’s weight divided by their height squared, what we know today as the body mass index. It is important to note that the QI was not created to make conclusions about a person’s health. It simply helped Quetelet identify the average weight of a group of people.
So how did it become one of the most popular measures of health? Well, in the early 1900s health insurance companies found higher mortality rates in their larger bodied clients. They used this association to create weight tables that reported the ideal weight for a person’s age and height. These tables were created by applying the QI to data of 4 million people who were mostly middle class and male. Doctors started recommending weight loss for their patients and the diet industry was born.
In 1972, physiologist Ancel Keys, renamed QI to the body mass index (BMI) after testing the equation on a cohort of over 7000, mostly European, men. They found that the BMI was not great at measuring body fatness, but the convenience of the tool outweighed its failure to accurately assess health. Since then, the BMI has been adopted as an assessment tool in healthcare and research worldwide.
However, there are a number of issues with the BMI so lets take a look.
As you may have picked up, research behind the BMI relies on the weight and height of mostly European men. People who are not male or European are not accurately represented by the BMI categories. For example, people from a Polynesian background usually have larger body frames and applying the BMI scale will overestimate their level of body fat. The BMI scale also fails to consider that women genetically carry more fat than men and female life stages such as pregnancy and post-menopause are not accurately represented.
The point of the BMI is that it is a non-invasive method to estimate a person’s fat mass and disease risk. But at no point does the BMI differentiate between muscle, fat, and bone. Someone with dense bones or a high muscle tone may be considered “obese” while another person with low muscle tone may be considered a “healthy” weight. If we take this approach we would then assume the person with strong bones and good muscle mass is at a higher risk of disease than the thin person with low muscle mass.
The BMI scale assumes anyone with a BMI above 25 must have poor health or is at risk of developing disease. But this is not true. A study of almost 3 million people found that those with a BMI between 25-35 (those in the “overweight” and “obese” categories) had the same mortality risk as those in the “healthy” BMI range. And people in the “overweight” category actually had the LOWEST mortality rate of all BMI groups. I will repeat, people with an “overweight” BMI had a lower risk of death than those in the “healthy” range.
Another study of over 40 000 people assessed whether BMI is an accurate marker of metabolic health (cholesterol, blood sugars, blood pressure). They found that a significant portion of the “healthy” BMI participants had poor metabolic health and a significant number of the “overweight” and “obese” participants were metabolically healthy. The researcher’s concluded that BMI is not a helpful way to assess a person’s metabolic health.
Categorising people based on their body size pathologies their body and promotes weight stigma. I have been putting the terms overweight and obese in quotations as these terms are used to say a person’s weight is abnormal and is used to make assumptions about their health and character.
Weight stigma is when a person is devalued based on their body size and extensive research shows that healthcare providers change the way they treat patients in larger bodies. As the body size of patients increases, health care providers report being less willing to help the person. On top of this, people who have experienced weight stigma are less likely to seek medical help or attend regular screenings for breast, cervical or bowel cancer. This can have huge consequences for a person’s health.
A huge problem with BMI is that it encourages individuals and health providers to ignore health-promoting behaviours. A person’s BMI does not tell you how active they are, how many fruits and vegetables they eat, what their mental health is like, or whether they smoke. Assuming a larger person eats poorly and rarely exercises promotes weight stigma and can send someone down a path of chronic dieting and weight cycling. While assuming a smaller person eats well and exercises often can lead to missed diagnoses.
As you can see, the BMI is a pretty unhelpful tool when it comes to determining your health. Rather than focusing on weight, there are a number of other ways you health can be assessed such as your blood pressure, cholesterol levels, blood sugar, physical activity, and diet.