Body Mass Index (BMI) is a mathematical formula that divides a person's weight by the square of their height to arrive at a number that falls into one of these bodyweight categories:
- Very severely underweight
- Severely underweight
- Underweight
- Normal (healthy weight)
- Overweight
- Obese Class I (Moderately obese)
- Obese Class II (Severely obese)
- Obese Class III (Very severely obese)
Beyond assigning one of these categories, a high BMI can be also an indicator of high body fat and therefore can be used to screen for certain weight levels that could lead to health problems. Even though it is often used as one, it is not a true diagnostic of body fatness or of an individual’s overall health.
As most of us fitness-focused folks have likely heard, BMI is far from a perfect measurement. Much of the time, and often when it really counts, the BMI measurement may actually overestimate or even underestimate a person’s body fat. And when it does, it really does.
Where Does BMI Go Wrong?
A common example that people use when talking smack about BMI is that it doesn't distinguish between body fat and muscle mass, which is important because a hunk of muscle weighs more than the same size hunk of fat.
Some good examples of this misinterpretation, according to an analysis of BMI’s Blind Spots, are former Olympians Usain Bolt and Michael Phelps, who just narrowly miss the 'overweight' camp. Then there is star NFL quarterback Tom Brady who’s BMI categorizes him as obese. Also basketball player Lebron James and NHL right winger, Phil Kessel, both have a BMI of 27.5 and as we will learn later, a BMI between 25-29.9 is considered overweight.
How does this kooky error happen? Well, imagine a sedentary person who is six feet tall and weighs 203 pounds. They would have a BMI of 27.
Now, imagine a sprinter who is also six feet tall but weighs 211 lbs. They would have a BMI of 28.
A glob of muscle weighs about 18 percent more than the same size glob of fat.
So, according to the BMI, the sprinter is more overweight than the sedentary person. But as I said earlier, a glob of muscle weighs about 18 percent more than the same size glob of fat, so this is clearly not an accurate statement.
This isn’t the only place where BMI falls apart either. BMI is also not reliable to use on elderly adults, who generally have lost some amount of muscle and bone mass. In this case, an elderly person’s BMI could be within a normal range while they might actually be overweight.
It’s also important to mention that the BMI calculation is based primarily on Caucasian body types and may not be appropriate for people of other ethnicities. When compared to white Europeans of the same BMI, Asians appear to have a four percent higher total body fat. South Asians, in particular, have especially high levels of abdominal obesity, which can throw off the BMI measurement as well.
Does BMI Correlate to Health?
A person whose BMI says they are overweight, or even obese, is generally considered unhealthy, while people with normal BMI are generally categorized as healthy. But research published in 2016 suggested that this was incorrect for 75 million Americans.
Researchers found that 54 million Americans had been classed as overweight or obese, but with further investigation, they found that other cardiometabolic measures (blood pressure, blood sugar, cholesterol, etc.) showed they were actually perfectly healthy. Another 21 million people were classed as "normal" in terms of BMI but were later determined to be unhealthy based again on those other cardiometabolic measures.
To determine this, the researchers examined data from the US National Health and Nutrition Examination Survey and analysed the link between BMI and a range of other normal health markers (like blood pressure, blood sugar, and cholesterol) and found that BMI incorrectly pegged people's health at both ends of the weight scale.
Other scientists, however, have suggested that although some people may appear to be overweight but healthy, the extra weight still puts them at higher risk of certain diseases as they get older. So there is some dissention in the ranks.
The History of BMI
Where did it all start? Interestingly, BMI is really not new. Way back in the 1840s, a Belgian statistician named Adolphe Quetelet devised what he named the Quetelet Index of Obesity, which calculated obesity using a simple mathematical formula. Specifically by dividing a person's weight (in pounds or kilograms) by the square of his or her height (in centimetres or inches). The formula is written out: w/h2.
Quetelet apparently famously wrote, "If man increased equally in all dimensions, his weight at different ages would be as the cube of his height."
Adolphe Quetelet had no interest in studying obesity when he developed the BMI.
He went on to explain that in the first year of life, our breadth is indeed larger in proportion to our height, but after that we grow taller in relation to our width. It is important to point out that when he developed this index, Quetelet had no interest in studying obesity.
Quetelet was actually interested in applying probability calculus to human physical characteristics and that is what led him to develop an index of relative weight. He then used this index to study the growth of a normal human, having established that during normal growth, weight tends to increase in relation to height in meters squared.
After World War II, when there were reports of increased mortality and morbidity of overweight and obese life insurance policyholders (errr, I mean people), the Quetelet Index validity was confirmed and brought into practice as a useful index of general relative body weight. It was then renamed to the Body Mass Index and adopted by the World Health Organization in 1995 as a simple tool to quickly and easily guesstimate levels of obesity.
What Was Used Before BMI?
BMI became an international standard for obesity measurement in the 1980s and the general public learned about BMI in the 1990s, when the United States government launched an initiative to encourage healthy eating and exercise. You may know this as the Clinton Health Plan, which is generally thought to be a failure.
Before 1980, doctors generally used what was called a weight-for-height table (a different one for men and women) that outlined ranges of body weights for each inch of an individual’s height. But these tables were also limited because they only used body weight, not body composition. If you are old enough, you may remember your doctor using charts that made you find your height along the left side and then slide your finger to the right to see your “ideal weight.” Ah, the good old days.
In 1998, the National Institutes of Health lowered the overweight threshold for BMI from 27.8 to 25 to match international guidelines. That move alone added about 30 million American citizens who were previously thought to be of a "healthy weight" to the "overweight" category. And that is where we still are today.
Why Do We Still Use BMI?
It's most useful in population studies, but if you try to apply it to individuals, it doesn't work.
In the paper Obesity, BMI, and Health: A Critical Review the authors state, “Clearly, obesity, as determined by BMI, is not a monotypic, age-invariant condition requiring a general public health 'preventative' approach. A BMI-determined categorization of an individual should not be used exclusively in counselling or in the design of a treatment regimen.” And yet, BMI remains a common tool for judging body composition.
A graduate researcher at the Children's Hospital of Eastern Ontario Research Institute in Ottawa named Travis Saunders says of BMI, "It's most useful in population studies, but if you try to apply it to individuals, it doesn't work."
This is due in part to the fact that it is as important where you store fat as it is how much fat you are storing. Abdominal fat, in particular a type called visceral fat, that accumulates between our internal organs (instead of being stored beneath the skin), is particularly problematic. Fat on the hips, buttocks, and lower body appear to be much less of a health problem.
For that and the other reasons we have covered, many doctors today now measure waist circumference as a proxy for visceral fat. Using this measurement, men’s waists should be less than 102 centimetres (or 40 inches) and women’s should be less than 88 centimetres (35 inches).
So What is Better Than BMI?
In a recent paper published by the Journal of Strength and Conditioning Research, it was concluded that although many BMI-based equations produced similar group mean values as a 4C model of body measurement (a skinfold caliper measurement of four parts of the body—and we’ll talk more about that later), a protocol called SF7JP (a skinfold caliper measurement on seven parts of the body) actually produced the smallest individual errors when compared to the gold standards of underwater weighing and Dual Energy X-Ray Absorptiometry (DEXA).
In the end, the 7-site skinfold test was by far the closest to the much more involved, invasive and costly underwater or DEXA tests.
That study took one hundred thirty adults (63 women and 67 men) between the ages 18 and 28 and calculated their BMI as weight (kg) divided by height squared (m2). Then their body fat percentage was predicted using four different BMI-based equations and then from the SF7JP, 7-site skinfold test. In the end, the 7-site test was by far the closest to the much more involved, invasive, and costly underwater or DEXA tests.
This is good news for physicians who are willing to learn how to use skinfold callipers and are able to spend the time necessary with their patients to measure all seven sites of their body. It is even better news for the elderly, the extremely fit, and people of ethnicities that do not benefit from the current BMI calculations. But since the study was only published in January 2018, we’ll likely have a bit of a wait before we start getting pinched in uncomfortable locations by our GP.
What is a Skin Fold Test?
This is one of the oldest and most common methods of determining a person's body composition. It is done by measuring the skinfold thickness at specific locations around the body. The thickness of these folds is a measure of the fat under the skin (subcutaneous adipose tissue). Once the thickness of the skin is measured, formulas based on gender and age are used to convert these numbers into an estimate of a person's percentage of body fat.
To say this test is not pretty or fun is an understatement. To perform this test, the technician actually pinches the skin with a metal caliper that measures in millimetres, and pulls the skin fold away from the muscle underneath. That way you can be certain that only the skin and fat tissue are being pinched. Two measurements are usually done at each site and then averaged and calculated using a formula.
The places on the body where you get pinched vary depending on protocol but usually include these seven locations:
- Triceps - The upper back of the arm.
- Pectoral - The mid-chest.
- Subscapular - The edge of the shoulder blade.
- Midaxilla - Under the armpits
- Abdomen - Right next to the belly button.
- Suprailiac - Top of the hip joint (AKA Love Handles)
- Quadriceps - Front of the thigh.
As we learned earlier from the Canadian Researcher, Travis Saunders, BMI is still useful for population studies where simplicity is key, if you want anyone to bother responding to your survey. Also, a person who has a high BMI is likely to have too much fat in their abdomen, along with other unhealthy locations like the heart and liver. And that is why, despite all its flaws, BMI is still useful to a certain extent.
Ultimately, when it comes to using BMI, we need to keep in mind that it is not an accurate measure of health or of physiological state (like blood pressure or a blood test) that can actually indicate the presence of disease. At its core, it is only a rough estimate of your size.
There are many people out there who have a high (or low) BMI who are also very healthy and, conversely, plenty of people with a normal BMI who are quite unhealthy. For example, a person with a perfectly normal BMI but who is also sedentary, smokes, and has an unfortunate family history (genetics) of cardiovascular disease, will surely have a higher risk of early death than another more genetically fortunate person who has a higher BMI but is also a physically fit, non-smoker. Right?
So I think it’s a really good thing to know your BMI but to also recognize its limitations.
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