The BMI, or “body mass index,” is a common method to assess body composition. But how accurate is it really, and can it be used to predict health outcomes based on its weight/height ratio? There are other, less well-known tests that can accurately gauge fat composition of the body, but how useful are they? In this review, we’ll look at BMI and several alternatives to see which ones actually win the battle of the bulge.
Stepped on the scale this morning? Were the results to your liking? Well, you’re not alone: Everybody’s interested in their weight. Here in the U.S., it seems we all want to optimize our health, and weight is a huge part of that. We want to have a positive ratio of lean muscle mass to fat, so a reliable way to determine that ratio would be very helpful as we set our health and exercise goals.
But do we already have such a method? Before we get to that, let’s look at a little history.
Hippocrates, Adolphe Quetelet, and the Invention of BMI
People have worried about their weight ever since humans in ancient times moved from a sustenance diet to one that permitted overeating. Hippocrates, the 5th century BCE Greek “Father of Medicine,” was among the first to declare that obesity was not only a disease in itself, but that it could lead to other disorders – and he credited the Egyptians before him for that insight.
But it wasn’t until the mid-19th century that such concerns led a Belgian statistician to ponder how to devise a scale that could measure people’s size and weight compared to others – potentially many others.
Adolphe Quetelet developed a formula for standardizing individual measurements into a simple number. These numbers formed a tool that could be employed by public health officials and others to understand the range of human dietary and health experiences. Many years later, in the 1970s, this tool was codified as the body mass index, and we have been using it unchanged since it was first developed by Quetelet.
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What Is BMI? And What Are Its Limitations?
The formula for assessing one’s BMI is simple. It’s so uncomplicated, in fact, that you see it used in gyms all around the world to gauge progress in aerobics or body-building. The formula takes your weight and divides it by the square of your height to compute a number, and this number is your BMI. (It’s very straightforward when using kilograms and centimeters. But if you do the calculation using pounds and inches, you need to multiply the results by 703 for the BMI number.)
But why trust your own math? The National Institutes of Health has its own BMI calculator. Just click on this link to get your own number.
This method is all well and good, as far as it goes. However, when you pull it apart, BMI has numerous problems as a predictive or diagnostic tool. Here are seven of them:
- BMI assesses height and weight, but not fat. BMI gives no information regarding fat distribution on the body, which, according to medical professionals, is where the important information on health outcomes emerges. BMI does not distinguish among fat, muscle, and bone mass, all of which play key roles in determining health and potential illness.
- BMI is not a reliable method of measuring body composition. It is often used inaccurately for this purpose.
- Age, sex, and ethnicity play no role in defining BMI, but play a huge role in assessing health. As we age, we tend to add more fat to our bodies; this can sometimes – but not always – be associated with obesity. Women have more fat than men and a different fat distribution, so their body measurements can lead to unreliable BMI readings. It is also troubling that BMI is biased in favor of Caucasian men of “normal” height and weight. For example, people of Asian or African descent can have BMI numbers that do not reflect their actual body fat proportions.
- Athletes and bodybuilders also skew inaccurately. Their body mass is compact, and lots of lean muscle tends to be misread by the BMI scale as overweight.
- People who are not considered obese by BMI may be obese by more precise testing. In large studies, nearly 40 percent of participants classified as overweight were actually obese. Since BMI does not distinguish muscle from fat, someone with “central obesity,” that is, an apple-shaped body, can have a normal BMI reading. They would potentially miss out on early diagnosis and treatment, which can be effective in preventing cardiovascular illness.
- BMI does not accurately reflect “visceral fat” accumulation, and it’s this type of fat that experts believe is the key to most metabolic and clinical consequences of obesity. For instance, BMI is not useful in determining the cardiovascular risk associated with obesity.
- BMI focuses on averages, not individuals. For a population-level study, BMI can be used productively to make generalizations; however, as explained above, BMI can fail spectacularly at the individual level. This is especially true if you’re tall, short, young, old, female, non-white, muscular, or have an apple-shaped torso.
Want to know what diseases might be overlooked due to an inaccurate BMI? The list is long, so hold tight: heart disease; high blood pressure; type 2 diabetes; gallstones; breathing problems, including sleep apnea; certain cancers, including endometrial, breast, colon, kidney, gallbladder, and liver; cholesterol and triglyceride levels; stroke; osteoarthritis; and chronic inflammation. Whew! Please see your doctor if you’re concerned about the health risks of being overweight.
Measuring Body Composition – 6 Better Approaches
Given what we know about the limitations of the body mass index, you might be wondering if there are other tests that are more accurate than BMI for individuals. Indeed, there are numerous methods of testing body composition and body fat that are accurate. These include:
- Waist circumference (WC), and sometimes WC divided by height, does indicate fat distribution, and can be used as an indicator of health risks associated with visceral obesity. Wrist circumference may also be used for this purpose, but testing is in early phases.
- Waist-to-hip ratio can be used to assess risk of cardiac events.
- CT (computed tomography) and MRI (magnetic resonance imaging) scans can show fat distribution.
- DEXA (a measure of bone density) can also distinguish fat from bone and muscle mass.
- Full-body water immersion (underwater weighing) can measure fat and muscle composition.
- Medical history and health risk evaluations performed in a doctor’s office are highly recommended along with any of these other tests.
So why aren’t people flocking to these methods? Well, they’re just not that easy to do. For example, information about body measurements is still officially “investigational.” Most of these tests have to be done and interpreted by medical professionals, and they’re expensive and complicated. When it comes to underwater weigh-ins, have you ever seen one? No, neither have I.
Getting Back to BMI
After all this, you might think I’m working up to a big conclusion – BMI is tripe! It’s not worth the effort (or the math)! So, you might be surprised when I tell you that, for all its limitations, the lowly BMI has some uses that are still valuable.
For starters, it’s been around for about 150 years, which gives it a rather long head start against the newer arrivals. We (now) know what it doesn’t do – and what it does, which is to give a simple number to assign to our general health. That number can give us a sense of how we’re progressing over time. (But, to be fair, a simpler measurement of our waist circumference can help just as much.)
The CDC recommends BMI as being:
- Simple, inexpensive, and noninvasive.
- Usable over time to show progress.
- Useful to test for overweight and obesity (but remember, BMI can’t offer you a reading of your fat percentage).
- Revealing in studying large groups (more so than individuals) over time, region, and population subgroups. That is, it’s an effective macro tool, but not so much at the level of the individual.
- Additionally, the National Heart, Lung, and Blood Institute suggests that a full assessment of weight and health should ideally include three information sources:
- Waist circumference
- Assessment of risk factors for conditions and diseases associated with obesity
Plus, it’s a simple, quick measurement to take, and the result is a single number that can generally place people at high, medium, or low risk of weight-related medical issues.
Stick to a System that Works for You
We know now that BMI is frequently used and frequently misused. It’s a test that gets more accurate when you view it from farther away; viewed too closely, at the individual level, it can be an invitation to skewed results. But it’s an easier way to get your bearings in the world of health and fitness than, say, just weighing yourself daily.
Based on what we’ve discussed, it is probably better for you to make an appointment with a physician than to accept a personal trainer’s offer to test you for BMI. You’ll get a better, more reliable answer about your health and weight-related risk factors. And you may sleep better too, knowing you have a reliable, comprehensive picture of your health, your weight, and your overall body composition.
Remember, BMI does one thing well: It measures weight-to-height ratio. The rest of what you might want to know (such as a body fat analysis) cannot be revealed by BMI. So jump on your scale as often as you like, but for an accurate assessment of your health and any associated risks, you might be better off keeping tabs on your waist size.
Kevin Martin is Senior Writer for MagellanTV. He writes on a wide variety of topics, including outer space, the fine arts, and modern history. He has had a long career as a journalist and communications specialist with both nonprofit and for-profit organizations. He resides in Glendale, California.