What do a svelte, health-conscious athlete and a sedentary person weighing 200 lbs have in common? A high body mass index (or BMI).
This is no joke; athletes, fitness fanatics and those who are considered overweight or morbidly obese often have a similar BMI. You might find this surprising, but the real surprise should be that we’re still using this method to measure individual health.
The BMI debate rages on, so let’s find out what this calculation really measures and why we should take BMI with a pinch of (low-sodium) salt.
What Is BMI?
BMI was invented way back in the 19th Century by Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist. Quetelet wanted to devise a way to measure obesity in a population, and thus the body mass index was born.
The BMI is calculated by taking a person’s weight and comparing it to their height squared. Like this:
- A person who is 5’10” and weighs 200 lbs has a BMI of 28.7
- A person who is 175 cm tall and weighs 65 kg has a BMI of 21.2.
These measurements put the first individual in the category of “overweight” and the second in the “normal” weight category.
Typically, BMI is classified into four groups:
- Underweight = <18.5
- Normal weight = 18.5–24.9
- Overweight = 25–29.9
- Obesity = BMI of 30 or greater
Problems with the BMI
The measurement fails to account for the degree to which fat, bone mass, and muscle contribute to overall body mass. Bones and muscle are denser than fat, so a person with strong bones and a good amount of muscle mass may have a similar BMI to someone with more porous bones and a high degree of body fat.
The bluntness of this tool has been noted time and again by physicians and others working in public health. BMI can be useful for measuring population health as it is unlikely that a high average BMI can be attributed to an incredibly toned and muscular population with strong bones. When it comes to individuals, however, BMI is largely unhelpful, so why is it so popular?
The easy answer, of course, is that the calculation is relatively simple and easy to figure out.
Confused Logic and the BMI
The BMI is uni-directional. This means that a person with a lot of body fat will probably have a high BMI, but that a person with a high BMI does not necessarily have a lot of body fat. At best, the tool is descriptive of something you probably already know. At worst, it’s thoroughly misleading and lazy.
Again, a high BMI could mean that an individual is overweight or obese. It could also mean that a person is fit and healthy, with plenty of muscle mass, strong bones, and little fat. Conversely, a person could have a low BMI because they are largely sedentary, have low muscle and bone mass, and/or are sick, but have a fairly high proportion of body fat.
Even the best-intentioned health authorities fall prey to the twisted logic of the BMI. The US Centers for Disease Control noted that “the BMI is a reliable indicator of body fatness for people.
So, what can we use instead of BMI as a fairly robust and simple measure of health?
Alternative to the BMI
A better option is waist measurement (because central adiposity is highly correlated with poor health) and your waist to height ratio. Recommended waist sizes are as follows (and will vary for people of European, Asian, Indian, and African-American descent):
- No more than 39 or 40 inches for men
- No more than 34 or 35 inches for women.
To measure your waist, place a tape measure around the top of your hip bones at your lower back and around to the belly button.
For waist to height ratio, the aim is to have a waist circumference that is less than half of your height (i.e., 0.5). For example: if a person is 177.8 cm tall (around 5’10”), weighs 200 lbs, and has a waist size of 80 cm (around 31.5″) their BMI would be 28.7 and put them at the upper end of the overweight category. However, their waist-to-height ratio would 0.45, which is under the recommended 0.5 ratio.