Why Does BMI Carry SO Much Weight?

By: Katie Clarke MS, RD, LDN

BMI. Quite possibly the least favored topic of any patient.

Discussing a patient’s BMI can cause frustration because, quite frankly, most people feel judged when looking at what appears to be a grading scale for how well or poorly they have managed their weight. That frustration can then lead to a poorly developed relationship with your dietitian or provider which studies show can negatively affect a patient’s outcomes. So let’s review some facts about this topic to set you up for success.

Most people can answer what BMI stands for, Body Mass Index. But few can define it. BMI is a very simple equation of a person’s weight (in kilograms) divided by their height (in meters squared). That’s it. On its own, easy math which anyone can do given the correct conversion units. Interpretation of BMI, on the other hand, is not quite as straight-forward.

  • BMI is NOT a definitive indicator of health. There are individuals who have a “healthy” BMI who develop chronic diseases like diabetes and there are others with an “obese” BMI who have no major health issues to speak of. BMI is only one statistic in a combination of information we use to interpret overall health.
  •  BMI is NOT an estimation of attractiveness. Yes, I have had patients who have asked if they will be pretty at a certain weight based on BMI. No, I cannot answer that for them. As the old saying goes, beauty is in the eye of the beholder. This means that beauty or attractiveness is subjective and differs from one person to the next. It cannot be measured with objective data. In order to see yourself as beautiful, you should look at your qualities rather than your measurements.
  • BMI changes with age. As infants, our growth patterns aren’t based on BMI but on weight and length for age. As adolescents, BMI is based on an individual’s rate of growth as compared to a national average, again on a growth chart. As adults, BMI is based on the sliding scale of underweight to obese with a healthy range of 18.5-24.9. And as a geriatric patient, the same sliding scale shifts to a healthy weight of 22-29.9.
  • BMI IS a risk assessment tool. BMI is a quick assessment method of interpreting if someone is at elevated or average risk of developing a chronic disease. It should come as no surprise that there are diseases linked directly to a person’s weight. Excess of fat stores on the body alter the action or effectiveness of bodily functions. This, in turn, results in a decreased ability to keep up with the demand of the body which typically results in an external or medicinal need.

This brings us to a very interesting distinction… the current BMI scale didn’t start as a HEALTH assessment tool but as a COST assessment tool. That’s right, the BMI scale as we know it today only became popular in the late 1980’s when it was noticed that the higher (or lower) the ratio of a person’s weight compared to their height, the more costly they became in the eyes of healthcare. The more they needed specialty doctors, prescriptions, and had incidences of chronic disease. Hence, a scale was developed to reflect someone’s COST FOR HEALTH based on weight divided by height.

Does knowing these facts change your views on BMI? Feel free to revisit this point of discussion with your registered dietitian at your next appointment for more information.