Fooducate Friday: "Are BMIs Accurate for Me?"

October 14, 2016

This is a question that I received and is a common question asked. The Body Mass Index is a tool used to determine whether you are at healthy weight, underweight, overweight to obese. It is a calculation that utilizes your height and weight. That is all -- your height and weight. So let's think about that. Based on the result of the calculation, determines if you are underweight, normal weight, overweight or obese. For the everyday person, who does not participate in much physical activity, we could potentially see this as a standard identifier for those health risk categories. But what about individuals who exercise often, even young adults who are athletes; because, the BMI does not take into consideration your age or activity level. We know that the more athletic someone is, the more potential they have to be more fit -- having lean body mass. Muscle (lean) mass, does not weight more than fat mass. One pound of fat and one pound of muscle weigh the same. However, muscle is smaller, it takes up less space and is more compact. So, if you put 5 lbs of fat into a box and put 5 lbs of muscle into the same type of box, the fat mass takes up more space. And so, if you have a human body, and fill it with fat and one you would fill with more muscle, the one with muscle weighs more because you can fit more muscle in the body than fat. So, muscle does not weigh more than fat, you can just put more muscle in one space than you can fat. And therefore, the individual with more muscle at 5'7" will weigh more than someone else at 5'7" that has more fat mass. But that doesn't make the more fit individual overweight, if their BMI within the overweight category. That BMI calculation never took into account the lean muscle mass of the person. With that being said, BMI is a poor indicator of whether someone is underweight, normal weight, overweight or obese for the active individual. I understand many teens and student-athletes have approached me saying "My BMI is high and I need to lower it." No. We need to determine how honest that is, but discussing or finding out what their body fat percentages are and, what their diet is like. If you have concerns that your BMI is inaccurate, find out your body fat percentage using skin calipers, a bioelectric impedance scale, a BOD POD (air displacement plethysmography), Dexa Scan, or underwater weighing. Also, those body fat percentage numbers will help further determine which health risk (or non-risk) category you are in.   Jenna Stranzl, RD