Nutrition Science and Everyday Application - beta v 0.1

384 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN
the risk of developing type 2 diabetes, cardiovascular disease and stroke, and even cancer
increase, as well as other complications of obesity, such as osteoarthritis. 2,3,4 Carrying
extra weight not only puts a mechanical strain on the body, but it also negatively impacts
metabolic health and increases inflammation. 5
Limitations of BMI
The advantage of BMI is that it’s simple and easy to calculate, but it also has several
important limitations. Since it’s only based on weight and height, it doesn’t distinguish
between muscle mass and adipose tissue. It’s not unusual for muscular athletes to be
classified as overweight based on BMI, but this can be misleading, because they may have
little to no excess body fat. On the flipside, BMI can underestimate body fatness in someone
with very low muscle mass, such as a person who is elderly and frail. In addition, BMI can’t tell
us where body fat is located in the body, and as we’ll learn, this is a major factor determining
its impact on health.
BMI’s accuracy in predicting body fatness is also affected by biological sex and race—not
surprising given the natural diversity in shape and size of human bodies. At the same BMI,
women tend to carry more body fat than men. Also at the same BMI, a Black person tends
to have less body fat, and an Asian person tends to have more body fat, compared to a
white person. This means that a high BMI may overestimate health risk in a Black person
and underestimate health risk in an Asian person. BMI is also not useful for estimating body
fatness in a pregnant person, because pregnancy weight gain includes placental and fetal
tissues. 1
All of this means that BMI is not particularly useful for comparing one individual to another
individual or even one population to another, because this one number simply doesn’t
account for all of the underlying diversity in factors like body type, muscle mass, biological
sex, and race. And for an individual, one BMI measurement at a single point in time may
not be all that meaningful—other measurements and clinical assessments are needed to
diagnose obesity and more accurately assess disease risk, as we’ll discuss in a moment.
BMI is perhaps most useful for tracking changes in body composition over time, whether of
a population or an individual. For example, the data on average BMI in the U.S. show a clear
increase over the last several decades, and the most likely explanation for that is not that
people in the U.S. are all gaining several pounds of muscle each year, but that we’re putting
on more fat. On the other hand, someone may have a BMI classified as overweight, but if
they’ve been at that BMI their entire adult life and are active and metabolically healthy, that
may just be the natural size and shape of their body.
MEASURING BODY COMPOSITION
A person’s body mass is made up of water, lean body mass (including organs, bone, and
muscle), fat, and other components like minerals. The weight on the scale does not
distinguish between these different components, but body composition measurements can.