You’ve probably heard of body mass index, also known as BMI, before.
It is among the quickest and easiest measurements available to offer a rudimentary assessment as to whether or not an adult’s current weight may be increasing their risk for negative health outcomes ( ie: chronic disease).
It is calculated as follows:
BMI = Weight in Kg/ Height in M2
As a practitioner who specializes in weight management and healthy eating, calculating my clients BMI gives me easily accessible ( albeit incomplete) insights into where they are at from a risk management perspective.
It also easily transferable among healthcare practitioners and can be measured very reliably.
When your BMI value falls between 18.5-24.9, you are considered, statistically, to be at the lowest risk for a variety of chronic diseases such as heart disease, type 2 diabetes and some types of cancer ( it’s a bit more complicated though – more on this soon).
The opposite is true for BMI values >25 and especially if those values are > 30.
If your BMI falls below 18.5, that isn’t a good thing either and you may be at risk for an insufficient intake of vitamins/minerals which could result in anemia, osteoporosis, malnutrition, issues with your heart and so on.
BMI does not take into account anything else aside from your height and weight.
Not your gender, not what you eat, not your waist size, not the colour of your eyes.
Obviously given how relevant these other factors are, it’s inevitable that using BMI has some serious limitations.
No one is questioning that.
But does that make it a useless measurement?
I don’t believe it does, and the reason I am writing this article today is because I’ve found that I was more likely to encounter ridiculing of BMI, rather than an objective account of its strengths and limitations.
I actually believe BMI is a useful measurement in many cases, despite its flaws.
Before we get into that though, let’s take a closer look at some of the popular arguments against the use of BMI.
Three Popular Arguments Against BMI
#1 The Muscular Individual Argument
There are a number of arguments that people use to detract from the validity of using BMI.
The most common argument that you will encounter on the internet is that BMI does not account for muscle mass and that muscular individuals will have very large BMI scores despite the fact they are in impeccable condition and have low body fat%.
That’s absolutely true, but I am quite exhausted of hearing this argument because the hard reality is that the great majority of people who walk through a dietitian’s doors are not at an elevated BMI category because of their extensive musculature (unless you work with athletes!).
Obviously every client demographic is different but I can say that many of my clients do not engage in any sort of resistance or strength training at all.
Only a relatively small portion of the population of exceptionally “fit” individuals fall into the category of being so lean at heavy weights that their very high BMI values are essentially meaningless.
#2 The Obesity Paradox Argument
Here is the main thing that you need to realize when it comes to BMI.
Using BMI we can confidently say that someone in the 18.5-24.9 range is LESS LIKELY to end up with a chronic disease than someone with a BMI > 24.9
It does not tell us that EVERY SINGLE PERSON in the 18.5-24.9 range is healthier than EVERY SINGLE PERSON in the BMI > 24.9 range.
There’s obviously a significant limit to how much someone’s weight can tell us, and the quality of an individual’s diet is extraordinarily important when it comes to determining chronic disease risk.
Case in point, a 2016 study from the International Journal Of Obesity found that around 1/3 obese ( BMI>30) individuals actually had good biochemical indicators of heart health, whereas a similar % of people within the “healthy weight” range ( BMI: 18.5-24.9) had less than ideal scores in those same indicators.
#3 The Anti-Weight Focus Argument
Let me start by saying that I am MUCH more interested in what you eat than what you weigh, but it would be foolish to suggest that a modest weight loss isn’t valuable in certain contexts.
Clients come into my office wanting to lose weight, and I always tell them that I respect their goal and that I hope they will also respect my goal for them, which is always to improve the nutritional adequacy of their diet.
In most cases that I see, these two goals are completely and utterly coherent.
When people who are classified as overweight or obese ( BMI >25) lose 5-10% of their body weight, they tend to see significant improvements in blood cholesterol, blood pressure and blood sugar management.
I’ve actually had someone online tell me that while losing 5-10% of body weight is strong advice, having individuals strive for a reduction in their BMI categorization is not.
While I get the argument, and it is certainly valid in some contexts, here is why I have a problem with it:
Let’s say I have a male client who is 6’ 200 lbs. He is not a muscular individual.
That puts him at a BMI of 27, right thick in the middle of the overweight range.
If this man loses 10% of his body weight it brings him into a BMI category of 24.4, which is within the “healthy range”.
The moral of the story is that when people lose 10% of their body weight it does actually often entail them dropping into a new BMI category.
Three Reasons Why BMI Is Still Useful
Aside from being utterly convenient and easy to calculate of course..
1. BMI overestimating lean mass is, in reality, a bigger concern than it overestimating fat mass: A cross sectional study by Shah and Braverman demonstrated that, in a group of adults with a mean age of 51, BMI was an inaccurate means of classifying obesity for the reason that it underestimated body fat levels. This was especially true in women and seniors. This is quite a stark contrast to the usual internet argument that BMI overestimates body fat in muscular individuals ( which again represent a much smaller portion of the population).
2. BMI plays a modest role in determining mortality risk: Its flaws and limitations already noted, BMI exists in the first place because the data supports that high BMIs are, on the average, associated with an increased risk of mortality. As you will see in the next section, gathering some additional information can go a long way to making BMI a more useful measurement.
3. The association between BMI and mortality is found across the globe: As per work by The Global BMI Mortality Collaboration, the higher risk of all-cause mortality ( death by any cause) associated with an increasing BMI is something occurs worldwide, and is not a North American exclusive phenomenon.
How To Make The Most Of BMI
You may be confused about the utility of BMI after reading all this.
That’s completely normal.
I will leave you with one final consideration to keep in mind.
Measuring the circumference of your waist is a great way to shed more light on the validity of your particular BMI score.
If you are a male with a waist measurement that is greater than 102 cm, you are at greater risk of health issues even if your BMI is in the healthy range.
If you are a male with a waist measurement that is less than 102 cm, you are at a lower risk of health issues even if your BMI is in the overweight range.
For women that magic number is 88 cm.
BMI is and always was a flawed measurement but also happens to be a victim of undue criticism ( in my opinion!).
Relative to how easy it is to calculate, it offers reasonable value and insight to the average individual.
It is NEVER all that matters about a person and their health.
Hopefully today you learned way more about BMI than you had ever hoped to and and are in a better position to understand the strengths and limitations of the measurement.
Until next time,
Andy De Santis RD MPH