(Contributed by Linda Anegawa, MD, FACP anegawa-md.com )
The BMI, or Body Mass Index, is one of the most commonly used measures to identify people whose health is at risk from excess weight.
But despite its near-universal use by medical practices and hospitals, BMI can be one of the most misleading measurements when it comes to health risks. How is this possible?
First, let’s define what the BMI is. BMI is simply equal to your weight in kilograms divided by your height in meters squared. Typically, a BMI of 19-25 is considered normal. Those with a BMI of 26-29 are considered to be overweight, and those with a BMI 30 or above are considered to be suffering from obesity.
At first glance these categories may seem helpful to us. You might assume that the higher the BMI, the less healthy the person is, and sometimes that is in fact the case. However, as it turns out there are serious flaws with relying only on BMI.
As an illustration: let’s take the case of two individual men:
(1) A 22 year old, 6 feet 4 inch tall professional linebacker with the NFL who weighs 300 pounds.
(2) A 65 year old, 5 feet 1 inch tall secretary who weighs 190 pounds.
You might guess that these two men sound like they’d have very different bodies which is clearly the case. However, despite their vastly different physiques, these two men in fact have the exact same BMI – 36! They’d appear to be identical using a health scale that relies on BMI as a measure.
In addition, both the football player’s and the secretary’s BMIs of 36 would place them in the range of morbid obesity per standard guidelines. But as I mentioned, the BMI doesn’t tell us whether most of their weight is made up of fat, muscle, bone, or water weight, otherwise known as body composition. For example, the typical pro athlete has a body fat percentage under 10% making them extremely lean, regardless of BMI. As we get older, more of our body composition shifts towards fat, and we lose lean mass. BMI alone is unable to account for this.
Also, as it turns out, even if these two people had the same body fat percentage, where the fat is located on the body makes a very big difference in predicting health risk. For example, fat located around the middle of the body, known as ‘visceral fat,’ puts one at higher risk of cardiovascular-related death, even with a normal BMI . So just because one has a BMI under 25 does not necessarily mean one is safe from metabolic disease.
One final thing that BMI doesn’t account for is whether each person’s weight causes health problems. One could have a higher BMI and yet be free of health conditions. On the other hand, some individuals with lower BMIs have very significant weight-related diseases such as prediabetes and type II diabetes, hypertension, or abnormal cholesterol. We don’t know exactly why that is, but it’s likely that genetics, ethnicity, activity levels, and even environmental factors all play a role.
Using BMI therefore can make it extremely difficult to tell who is at risk from their weight, and who is healthy. Doctors especially Obesity Medicine specialists, have recognized this, and are turning to other measures to help. Here are some examples of other methods which are growing in use:
(1) Waist -to-Hip Ratio: this simple, inexpensive measure is easy to do at home or in your doctor’s office. To obtain it, you simply measure your waist at the level of your navel, and your hip at the widest portion, then divide. A ratio of 0.9 or less is best for men, and 0.85 or less is healthiest for women.
(2) Body Composition Analysis, or BCA: BCA readings are able to determine how much of a person’s makeup is fat, versus muscle, bone, and water. Healthy body fat percentages for men are anywhere from 10-20% and for women, 22-33%. Segmental BCA machines are also able to estimate the degree of visceral (belly) fat one has, which as we know raises health risks.
(3) Edmonton Obesity Staging System: not a measurement, but the Edmonton system allows doctors to estimate risk based on whether weight-associated medical problems are present and the severity. For example, a smaller person with a higher Edmonton Stage is at much higher risk of problems than a larger person with a lower Edmonton Stage. 
The BMI simply doesn’t tell the whole story. Fortunately, Obesity Specialists have taken the lead in advancing newer ways to assess weight-related health risks in order to provide critical attention to those who need it the most.