BMI Facts and Fallacies

by Brad Pilon

Body mass index

 

Let’s start with the obvious – There are many people who disagree with the BMI.

They consider it to be almost entirely useless as a way to measure… well just about anything to do with the human body.

To understand why so many people have dismissed the BMI we first have to delve into its history.

For starters the name itself is a bit confusing. BMI stands for ‘Body Mass Index’  which is confusing to me since it doesn’t measure mass but rather relative weight. (I guess Relative Weight Index was already taken? )

BMI was the name given to the measurement by Ancel Keyes in his 1974 paper “Indices of relative weight and obesity”.

The measurement was designed in an attempt to remove the relationship between height and weight, since all components of weight (Lean body mass, skeletal mass, organ size and to some degree body fat) all increase with height.

In other words, the BMI was an attempt to remove height as a confounder when examining the relative weights of a population.

How is height a confounder you ask?

Imagine you are 6’4″ and I am 6’1″ and we both weigh 215 pounds.

Do we have the same body? the same health? the same risk factors? No we don’t, and I’ll explain why.

In this example if you were 3 inches taller than me, you should expect to have as much as 21 pounds more lean mass. Part of this mass would be your organs, and specifically larger organs than I would have.

Said another way, the proportions of the human body do not remain constant with increasing length.

Almost a century and a half ago a scientist from Belgium named Adolphe Lambert Jacques Quetelet (considered an early pioneer in astronomy, anthropometry and statistics) explored the concept of a relationship between weight and height squared back in the mid 1800s. It was called the “Quetelet Index”.

For reasons I have not yet discovered, Ancel Keyes found, verified, and then renamed the Quetelet index the “Body Mass Index” in his 1972 paper Indices of relative weight and obesity -  Again, an odd decision to me since it doesn’t actually measure Mass, but is rather a measure of relative weight – which is a way to determine a persons weight independent of their height.

In spite of such a confusing name and story about how it became the “BMI”, I still find it interesting why so many people have such a strong and often negative reaction to the BMI, specifically towards it’s validity as a tool for assessing health risk (you’ll notice I didn’t say obesity – more on that in a minute).

It seems that after Keys renamed and repopularized the Quetelet index, people started using this simple equation in research – and with good reason – now that height was removed, you could study populations in the thousands and tens of thousands and look for relationships between relative weight and health and disease.

As an example, the 1977 paper published in the American Journal of Cardiology titled “Relation of body weight to development of ischemic heart disease in a cohort of young North American men after a 26 year observation period: The Manitoba Study” studied almost 4,000 men who were either pilots or pilots in training for the Royal Canadian Air Force from 1946 to 1948.

In this study it was found that the men who suffered a myocardial infarction, sudden death or coronary insufficiency or suspected myocardial infarction (these are all various forms of cardiovascular disease) all had significant increases in their BMI over the 26 years they were followed.

Time and time again research conducted on large populations of men and women from all over the world of all different ages have found the higher the BMI, the higher the risk of morbidity and mortality.

So, why do people dismiss the BMI if it can give us useful information about relative disease risk?

The answer I most often hear is because it doesn’t differentiate between lean body mass and fat mass, which is true.

BMI became an international standard for weight measurement in the 1980s. And the general public learned about BMI in the late 1990s, when the US government launched an initiative to encourage healthy eating and exercise.

At first, the thresholds were established at the 85th percentile of BMI for each sex: 27.8 for men and 27.3 for women. This means that 85% of the population will likely be at this number or lower.

These BMI cutoffs represented the sex-specific 85th percentile of the BMI distribution for persons aged 20–29 y in NHANES II (NHANES is a really big scientific research study that looked at thousands of people). The rationale for selecting this age group as the reference population was that young adults are relatively lean and the increase in body weight that usually occurs with age is due almost entirely to fat accumulation.

Then, in 1998, the NIH (National Institutes of Health) consolidated the threshold for men and women, and added another category, “overweight.”

The new cutoffs—25 for overweight, 30 for obesity—were nice, round numbers that could be easily remembered by doctors and patients, and everything between 25 and 30 was considered to be varying degrees of overweightness.

This was also the first time the word ‘obesity’ was associated with the BMI, before that it was simply overweight and severe overweightness. “Severe overweight” was based on the 95th percentile BMIs for men and women aged 20–29 y. This means that if you were in the top 5 percent of weights for height, you were classified as ‘severe overweight’.

How much of a difference did these new cut offs make?

When applying the BMI cutoffs of ?27.8 for men and ?27.3 for women to the National Health and Nutrition Examination Survey data, the percentage of overweight US adults (over 20 years old) is 33.3% for men and 36.4% for women.  In contrast, at a BMI ? 25.0, the numbers jump to 59.4% for men and 50.7% for women.

By simply changing the definition of the “overweight” cutoffs, the estimated number of overweight adults increases from 61.7 million (BMI ? 27.8 and 27.3) to 97.1 million (BMI ? 25.0), representing a difference of 35.4 million overweight adults. This example calls to attention the actual effect that a shift in BMI criteria can have on determining the population at risk.

The BMI started off as a way to measure relative weight, it made it’s way into health policy, was ‘tinkered with’ for the better part of 4 decades and now is one of the most widely used health markers in population based health, specifically for determining “overweight” and “obesity”.

This now brings us to the debate about muscle mass.

Critics of the body mass index will point out it’s failure to distinguish between lean vs fat mass. And it is often stated that very muscular people are often misclassified as overweight or obese.

Herein lies the problem – Obviously a well-muscled person with low body fat but a high relative weight is being misclassified if we call them obese – But classifying them as overweight is an accurate statement. The BMI is supposed to reflect what is “normal” relative weight within the society.

If you are above this ‘normal’ weight then you are still technically over-weight regardless of some of that extra weight is muscle mass.

The studies I mentioned that found an increased risk of disease with increasing BMI scores also did not differentiate between muscle mass vs fat mass.

The findings of those studies were simply that relative weight, regardless of composition was associated with morbidity and mortality (death and disease) in large populations of people.

This means the possibility exists that the make up of the ‘extra weight’ doesn’t matter – in other words, too much weight relative to your height could be dangerous to your long term health even if some of it is from larger muscles.

Since this research deals with large populations, and because people with extremely high lean body mass and low body fat are still very ‘rare’ this is all likely a moot point

Screen Shot 2012 10 22 at 6.10.31 PM 249x300 BMI Facts and Fallacies

(remember the BMI was originally supposed to used on populations, not individuals).

However, as odd as it may sound, being built extremely large like Fat Albert or Ronnie Coleman may bring with it many of the same health risks. Maybe not ALL the same risks, but there is a chance the relationship between weight and cardiovascular disease could be related to total weight and not just overall body fat, especially up in the extreme ranges of relative weight.

It is true that as a measure of body fat of an individual, the BMI is mediocre at best. A more accurate measurement would be waist circumference to height ratio since waist circumference is an excellent marker of bodyfatness and risk of disease.

As a measure of total body composition a combination of waist-circumference-to-height, shoulder-circumference-to-height and BMI is probably as good as it gets without using a technical and very expensive measuring device like a DEXA or BodPod.

All of this information leaves us with the following question:

Should we assume that adding excessive amounts of muscle bring with it zero/no health risks as the people who dismiss the BMI in the name of muscle mass suggest?

Or

Does relative weight matter just as much as the make up of that weight when we get to higher extremes of relative body weight (BMIs of over 30)?

However you phrase the question we will likely not have the answer until a long term study is conducted that specifically looks at people who are overweight/obese by BMI classification and then divide them between those people who are overweight due to excessive fat, vs excessive muscle. My guess is a study such as this is not going to happen any time soon if ever as it is still very rare to find people with significantly higher muscle mass than the average, and even harder to herd them into a research study.

In the mean time I’ll suggest the best of both worlds – aim for a BMI well below 30, a waist circumference below 50% of your height, and instead of only using these kind of metrics to determine your disease risk, take your health into your hands and get regular blood work completed, at least on an annual basis… oh and fasting once or twice a week wouldn’t hurt either icon wink BMI Facts and Fallacies

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