You’re on the receiving end of “the talk” about your physical condition as you wrap up your visit to a healthcare professional. He or she, with a concerned look, raises the topic of your weight. You instantly cringe, thinking “Woo boy, this is not gonna be fun.” In a moment, a number – or shall we say “the number”- is delivered. It is your Body Mass Index or BMI for short. Whatever it is, you are told this number is very important for evaluating your health, and it ought to be in the range of 18.5 to 25 (kg/m2).

After some thought of what you learned at the doctor’s office, you decide to join a gym and commit to taking off some excess weight. At your first training session, a bulky chiseled trainer welcomes you to unfamiliar territory. You are sweating on a treadmill like never before, but at the same time feeling pumped up and proud. The trainer is earnest and encouraging enough for you to discuss weight and nutrition. In a moment, “the number”, BMI, pops up again. But instead of hearing what the doctor told you, you might surprisingly find yourself listening to criticisms of BMI. You are told, based on your trainer’s own experience, that this number is fairly useless if not a complete sham.

You’re probably confused, for a good reason: How can the views of two professionals on BMI be polar opposites? A quick Google search to learn more about BMI merely worsens your confusion. On the one hand, science news reports that BMI can be linked to various diseases and health conditions, which appears to support the arguments that it is to be treated with importance. But on the other hand, blog posts are sharing convincing personal stories and theories on the use of BMI being deceptive or even dangerous.

In a 2009 mini-interview on NPR, Stanford mathematician Dr. Keith Devlin went so far as to use such terms as “lying,” “wrong” and “bad” in relation to BMI. It’s unlikely, however, that many healthcare professionals would agree with Dr. Devlin’s word choices. In any case, your skepticism over BMI remains valid at this point. Is this number good or bad? Or is it even worth giving your attention?

The first step to answering your questions is to view BMI as a handy tool in a large scientific toolbox. When Belgian mathematician Adolphe Quetelet created the formula to calculate BMI (actually known as the Quetelet Index back then) some 200 years ago, he was in his late twenties and engaged in passionate research into measurements of the human body. Quetelet wanted to know how height and weight increased as the human body matured from a baby to an adult, and how he could best express this process mathematically. Truth be told, he was not thinking about body fat at all. However, Quetelet’s work inspired American physiologist Dr. Ancel Keys at the University of Minnesota, who demonstrated in a 1972 article that BMI could be used as a satisfactory (but not perfect) guide for assessing human obesity in large groups of people.

This discovery resulted in BMI being added to the scientific toolbox that’s still in use today.

The emergence of BMI opened new doors to human health research. Scientists, armed with measuring tapes and scales, a handful of technicians, and a measuring process that takes less than three minutes, can quickly assemble BMI data from tens and thousands of people with relative ease and at low cost. They can compare and study people with various BMI numbers, through which nuggets of useful health information are revealed. For example, people with BMI numbers higher than 30 (kg/m2) are more likely to suffer from high blood pressure, high cholesterol, type 2 diabetes, heart diseases, stroke, breathing difficulties and some types of cancer. People with BMI numbers lower than 18.5 (kg/m2) are more susceptible to infections, low muscle mass, anemia and lower pregnancy rate.

These discoveries have pushed forward our understanding of human health, thus making BMI a valuable tool for scientific research.

As knowledge concerning the relationship between BMI and various medical conditions becomes available to the public, misunderstandings over this number also arise. You may have heard someone worry about having developed diabetes or heart disease due to his or her BMI. No matter how authoritative that may sound to you, please note that is *not* how BMI is used in the medical field. To paraphrase the opinion on BMI posted on the Center for Disease Control website, the number itself cannot directly measure your body fat, nor can it determine if you have a certain disease or not.

If BMI doesn’t diagnose any disease then, how is it applied in the medical field? The short answer is that BMI is a quick and useful screening tool for healthcare professionals. By calculating a person’s BMI, the healthcare provider can determine if he or she requires further medical evaluations. For instance, a BMI number of 30 (kg/m2) raises a red flag in your medical chart. With this red flag, your dietitian may need to take other measurements (e.g. waist size, or skin-fold thickness) to evaluate your body fat. If you are as lean as a bodybuilder, the high BMI will be treated as a false alarm. However, if body fat is indeed the main contributor to your high BMI number, your doctor may want to gather more information and run tests to rule out potential medical conditions such as type 2 diabetes and heart diseases. At some point in this process, you will probably receive “the talk” about your BMI and weight issue.

Whether any diagnoses will be confirmed by other tests or not, your high BMI will continue to fly as a red flag, warning of potential threats to your general health.

Despite BMI’s usefulness in scientific research and most medical screenings, it has its limitations as a basic tool. For starters, BMI cannot be used as a reliable guide for people having an exceptional amount of muscle mass, such as active bodybuilders and athletes. So, it is not surprising to see such people disregard use of BMI. Secondly, BMI cannot be used as a reliable guide for individuals with certain medical conditions, including people after amputations, on dialysis or with certain liver diseases. Lastly, BMI is not designed for infants under two years of age. It can be utilized for children and teenagers from ages 2 to 19, but the interpretation of the BMI numbers in such cases is significantly different from adults.

So now you have it – all the key points surrounding the debate over the “good” or the “bad” of BMI. If you’re unfamiliar with the concept of this tool and want to know more, please consult a dietitian to learn how to use, and interpret, your BMI properly. I do hope this article helps in dispelling some of the confusion.

This article was contributed by Wei Chen, Ph.D., RDN.