I had a patient ask me that the other day. Is it important? We use it to classify patients every day: “normal”, “overweight”, “obese”, “morbidly obese”. Most people in the normal category have no objections, while those in other categories often want to poke holes in the science. So is it reliable? Let’s take a look.
What is BMI?
BMI is short for body mass index, and it’s a standard measure of height-for-weight. Obviously, throwing everyone in categories based on their weight alone doesn’t account for the differences that height plays, so we relate the two measures to determine one’s fatness. BMI is measured in kilograms per meters squared.
How to calculate BMI
BMI = ( Weight in Pounds / ( Height in inches x Height in inches ) ) x 703
Or, just click here
What does it mean?
< 18.5 Underweight
18.5 – 24.9 Normal
25 – 29.9 Overweight
30 – 35 Obese Class I
35 – 40 Obese Class II
40+ Obese Class III
BMI represents risk of obesity-related disease on a population-wide scale. Generally, people in the normal category are at less risk for diseases like diabetes, high blood pressure, and cardiovascular disease than those who are in the overweight category; overweight people are at less risk than obese; etc.
What is it good for?
BMI is a quick and easy-to-measure method of representing a person’s risk. To get more information than the BMI gives, blood work is required and that takes a hell of a lot more time and money than stepping on a scale and standing next to a ruler. The cutoffs of each category are not random, they were chosen to correlate with population data on risk factors for disease. In the greater population, those with a BMI in the Obese Class II category are likely to have some health issues related to their weight-for-height. The simple truth is that you’re probably healthier (i.e. free of disease) if you are within that normal class.
What is it not good for?
There will always be errors when using one measure (BMI) as a proxy for another (disease risk/body fatness). By using weight-for-height to indicate risk for other health problems, the accuracy is nowhere near 100%. There will be people who are overweight who do not have comorbid conditions, and who may not develop them in the future if they remain in the overweight category. Likewise, there will be people in the normal BMI category with clogged arteries poppin’ statins to make sure they stick around for a long time. Taking a generalization about a group and applying it to the individual is termed the “ecological fallacy” by behavioral economists, and it explains why BMI cannot be the sole measure of health.
Tall people tend to get shorthanded a little bit with BMI. The same body shape and composition (in terms of % body fat) on a tall person will result in a higher BMI than in a short person, simply because of the way the math shakes out. Sorry tall people, but at least you can always see at concerts.
Additionally, it does not take into account body composition – a 5’3″ woman could weigh 150 (overweight), be totally jacked, and have super healthy insides. Or, a 5’3″ woman could weigh 120, have no muscle, rarely exercise, and have a heart attack tomorrow. Muscle weighs more than fat, but fat is much more dangerous to our health. This imperfect correlation of weight with body composition can misrepresent muscle-y people as being overweight (which we equate to having a greater risk of disease) when in fact they are perfectly healthy; and can categorize a person as “normal” who actually is overfat though not overweight, and at increased risk of disease.
Should I pay attention to my BMI?
Yes, you should. It really does matter! For the vast majority of us, we’re not walkin’ around with so much muscle and so little fat that we can’t get to the normal range by shedding unnecessary fat. I know it’s easy for me to say as a twentysomething who has an efficient metabolism and who has never had kids, but at the very least it’s a goal to strive for. There is nothing to be gained by writing off an elevated BMI as “not applicable”. I see this used much more as an excuse than anything else “Oh yeah the BMI told me I’m obese but those things aren’t really accurate, right?” Actually, “those things” were created by some super smart people using a shitload of data on a gigantic population, so yeah they’re probably pretty accurate. In 99% of the situations where I hear something like this, the person definitely would be healthier with weight loss, and definitely has the ability to do it themselves. It’s tough, but you have so many resources available to you! Take advantage!
For those who are below the normal range, you definitely are aware of this fact and you need to know that being too skinny puts you at increased risk for disease as well. In fact, most of us would see someone with a BMI of 18.5-19.0 as “skin and bones” even though they’re technically in the normal range. So if you truly are somewhere under 18.5, consider the fact that we’re all gonna think you’re super hot and awesome even if you do gain a little poundage to get yourself to a healthy place. If the thought of weight gain makes you nervous, you should seek help from a professional when you’re ready. I want you to be happy and healthy – it’s not a one or the other kind of deal.
What should I do if my BMI is outside of the normal range?
If you’re below, you’re at risk too. Focus on healthy fats like avocado, seeds, nuts and nut butters, olive oil, and fatty fish as a way to pack in the calories without suffocating your heart (binge eating pizza and candy bars, while it might sound fun at first will not be great for you in the long term).
If you’re above the normal range, you can always talk to an RD 🙂 we want to help. Otherwise, first start by taking a closer look at your health: getting a lipid panel done will give a more full picture of how at risk you are for disease. You could have stellar cholesterol numbers and be super healthy, but we want to make sure. As far as taking action goes, start with something as simple as making sure to have a vegetable with every meal. Walk after dinner. Track with My Fitness Pal or join Weight Watchers in order to get a sense of how much you should be eating compared to how much you are eating. The possibilities are endless, and you can always email me for help getting started.