If you’ve ever been underweight, overweight or on the internet you probably heard about BMI or body mass index. You’ve probably also head there might be issues with using BMI as a measure of health. But is that even true? Is BMI the most important number indicating health or something you can basically ignore. Today we’re putting lots of other stuff to the side for the most part. We’re not really talking about exercise, what your diet consists of or you mental health. All of those things are also incredibly important in the whole picture of who you are and how healthy you are. I get that, for today we’re focused on this one thing and that is BMI, because it is important! There is a lot of information out in the zeitgeist right now about how weight, body shape and BMI really doesn’t matter. The thing is for the average person it does, a lot. Not because anyone wants you to conform to traditional beauty standards, look hot at the beach or buy your clothes at any one store over another. It matters because lots of people, like your family and your doctor want you to meet your grand kids, celebrate a 40th wedding anniversary and live a full life for as long as you can. It matters because of population trends like a downward spiral in the age of diabetes patients, the cost to public healthcare systems and that children born now might live shorter lives than their grandparents. Those things matter to me and they matter to me for you! Popular opinion or not, BMI has a role in all of it.
What is BMI and how is it used?
Here’s the thing with the number on the scale, it isn’t comparable from one person to another. One person who weighs 160 lbs maybe short or tall, very active or sedentary and they may have vastly different frame sizes. So around 1840 Adolphe Quetelet, a Belgian astronomer, mathematician, statistician, and sociologist invented the BMI formula which is:
Weight in Kg/ Height in m squared
This allowed at least some of those factors above to be controlled between patients. Notice that there is no difference between the formula for men and the formula for women. That’s because it truly doesn’t make a difference. Because it is a ratio of weight to height that should allow us to tell the difference between a 160 lb patient that is 5 foot and 6 foot tall because there is one. The use of BMI pretty much stayed true to old Dr Q’s vision. While BMI is a made up and meaningless number it’s used to give an easy estimation of body fat percentage, which isn certainly not meaningless. It’s a number that puts you into a certain category of ‘health’ and places you in a medical context. Rather than give you just a number on a scale being told your BMI gives you an idea of where you sit healthwise in a way that’s controlled for height.
Can you ignore it?
In some cases sure but more often that would be a poor life choice. There is a reason that the doctors take the time to calculate your BMI and talk to you about weight. Epic volumes have been written in the scientific community about health outcomes and their relation to BMI. If you ignore your BMI then you’re essentially ignoring all that potentially life changing and life saving research. What about all the criticisms that you’ve seen that BMI is useless or even dangerous is that to be ignored? Well no, not for most of us I would argue but in some cases, for some people it really can be a meaningless number.
BMI is not useful because it compares your height to your weight but rather because it gives a pretty good estimation of your body fat percentage. This is where the gender differences come in. Women will usually carry more body fat at a certain BMI than their male counterparts but how much is healthy is the same for each category by gender. Body fat is directly related to health because of the strain too much or too little can but on the body’s systems and organs. Some body fat is required for those same systems and to get you through tough times like a prolonged sickness. Too much however will stress those same systems. It builds up in your organs like your heart and liver causing heart disease and fatty liver disease. The second of which is a rapidly growing cause of death in western cultures.
BMI is most often used to place you into a certain category:
Under 18.5 = Underweight
18.5 – 25 = Healthy weight
25 – 30 = Overweight
Above 30 = Obese
Often the underweight and overweight categories are further subdivided detailing the degree of risk within that category. For all the criticisms of BMI floating around out there over the long term, for the vast majority of people (well over 90%) BMI is a really great indicator of future health outcomes.
Is it a good indicator of health?
Over the longer term the predictions that BMI makes ring true for most. You may say something like “Well my BMI is 35, has been for years and my doctor says everything is totally in order. Blood results fine, blood pressure fine and sugars totally good.” I’ll say come see me again in 15 years. It’s more complicated if you regularly run 10k’s with a BMI of 35 but we’ll get into that in the next section. For the vast majority of people your BMI and the increased or decreased risk percentages over the study probably represent you too. Here’s the thing these studies are made up of a cross section of the population so no matter how you think you’re different people like that were probably in the study too.
Skinny fat, fat but fit, fit-ish and fat-ish were all in the study too and none of those things are all that rare by the way. Any group and any individual is likely to track the progress in that study. Sorry that’s harsh but it is what it is. BMI is a stand in for body fat percentage without having to do a complicated caliper or water displacement test. For that it works pretty well. A special word of caution for those that push a higher than it should be BMI to the side and think none of that bad stuff is happening to me so I’m good. The body is an amazing thing it does everything in its power to keep you as healthy as it can for as long as it can no matter what you throw at it. Your body can handle a lot but not forever. If this is you I beg of you to ask someone at a similar BMI to you WITH those health problems how it happened for them. Please though do it in a sensitive and private way though. I’ll bet that they will tell you they knew they were overweight but everything was just ticking along just fine for a while or a long time. But then at some point one thing happened like joint pain, then something else like high blood pressure then diabetes. Seemingly overnight they were taking a cocktail of pills, testing their sugar and just not getting around like they used to. I also bet they’ll tell you to do something about you’re weight now that they thought they had time until the clock ran out.
However… BMI is important but it’s something else too and that is changeable! Almost no one was a perfectly healthy weight in high school and hasn’t added or dropped a pound since. Don’t worry about it though depending on your height no 5 or even 10 pounds ever sealed someone’s fate. My healthy BMI weight range is anywhere from 122 – 165 pounds, I’d be considered obese at 198. Does it really matter if I weigh 119 or even 170 pounds, no not really very much if at all. Are my health concerns different at 205 pounds compared to 197? Nope I’m still pretty much in the same boat as before. It’s a sliding scale and gaining or losing the pound that moves you up or down a category is not going to change your life or your health. Just be honest about where you are. What if I had been 250 pounds for three years or so does that mean I’m doomed? It probably wouldn’t register on my lifetime health risks overall. The longer you’re at an unhealthy weight the bigger a deal it is for your health but it’s never too lade for a course correction!
When is it a poor indicator?
So we’ve established that unless you truly are special that BMI means something for most people despite what the internet told you. But with any rumor that runs rampant there is a thread of truth and believability to it. There’s a few examples that are always presented for the ‘BMI is stupid’ crowd. The most popular is that of the jacked body builder. You know 6 foot tall 250 pound muscle guy with an obese BMI of 34. He’s clearly healthy so BMI is stupid right? Well yeah he’s way, way healthier than your average obese guy but his joints are still under strain and probably so is his heart. Wouldn’t is surprise you to know that in the general population BMI actually UNDERESTIMATES body fat percentages. In one study of 14 000 people BMI predicted that 21% of men and 31% of women were obese. But… wait for it… body fat measurements put 50% of males and 62% of women were obese. So in the real world BMI actually underestimates how many people have dangerous levels of body fat.
Let me give you a second one how about a female elite marathoner who wins world major marathons at 5’5” and a hundred pounds even. She’ll live forever right? Surely her heart is fine and her liver like has no fat in it at all. True but if her training or weight keeps her from having her period then she may struggle with infertility or osteoporosis later in life. It’s murkier for the pretty overweight regular exerciser. Sure they are insulating themselves but not entirely. Even for the most elite athletes for whom you may think BMI is meaningless there are still good indicators from the number.
BMI, specifically the formula has a few quirks in it. Because the denominator is height squared some have suggested that for short people it ends up being a bit low and for tall people it runs a bit high. This could be remedied by simply stating that very short people are better off at the lower end of the healthy range while very tall folks can safely stray out of it on the upper end slightly. Truth be told since we’re again talking about a 5-10 pound differential only it’s probably not a huge deal.
What’s the takeaway message then?
Here’s where I am on this if the only other number you have about your body other than your weight and height then it’s pretty much the important one. If you doubt it for some reason get a more direct measurement of your body fat percentage. No one likes to hear that they have a bad or dangerous trait but the fact that you don’t like it doesn’t make it untrue. I for instance have been regularly told I talk way too much sometimes even about myself, ouch. The thing is that even though that can hurt to hear it’s still true. We know that BMI correlates very well to poor health outcomes and early death on both sides of the scale and that it typically under estimates body fat percentage. So if you are well into the obese region and you’re not a body builder that’s not a good thing. If you regularly exercise but you’re still obese good on you and you’re already doing a lot of the hard work for staying healthy. I would still be better if you could bring that number down closer to the healthy range at least.
No one who is on the very edge of the healthy category is absolved completely of the risks in that category but a couple of pounds either way aren’t killing you. I get that for some people that genetics are a huge factor in weight, putting it on or taking it off. Some people are going to have to work way harder and more consistently than others on their weight and that’s totally not fair. But guess what the genetic lottery is what it is and that’s not fair either. I occasionally deal with alopecia and lose all sorts of hair, that’s not fair either but it is what it is. The best course of action, though it can be a really hard one is to get into that healthy BMI range with a few pounds to spare. Then when stressful times or happy times like vacations and holidays happen you have some wiggle room to let go a bit for a week or two before getting back on track. That advice goes both ways get to a bit above 18.5 or below 25 to do the best possible thing for yourself.
No matter what your BMI is it says nothing about the quality of person you are. Your BMI doesn’t make you kind, smart or interesting at all. It doesn’t make you beautiful or unattractive and all bodies are beautiful because they are all one of a kind. Every body out there deserves love and respect from everyone most of all it’s owner! BMI is really only one measure of hundreds about your body and your life it just happens to be a pretty important one. If you want to change yours then talk to your doctor, get support and prioritize making better choices for it because no one but you has to live in it!
What’s your take on BMI? Do you agree that it’s a very important number to well over 90% of the humans out there? Do you think it’s time has passed? Why is that?