Thursday, November 21, 2013

Waist-to-Height Ratio [EBP 4]

This was really the best picture I could find that wasn't a slim-waisted model with a casually draped measuring tape. Source.


I'm sure I don't have to tell you that obesity has become problematic in North America.  In Canada, it could prove to be an especially difficult problem due to the health consequences associated with obesity coupled with the publicly-funded nature of our health care system.  Another problem is that defining or measuring "obesity" is particularly difficult.  The Body Mass Index (BMI) has been widely used since the 1970s, and it is proportional to a persons weight divided by the square of their height.  This has worked fairly well over large populations, particularly for identifying where mortality risks increase, though it has the obvious weakness of not taking muscular or skeletal structure into account.  So, is there a better way?

Really and truly, the thing with which we should concern ourselves is body fat percentage.  This would automatically take into account muscular and skeletal structure and give an accurate picture.  This is also difficult or expensive to measure.  One way is to use calipers to take a flap of skin and measure the fat tissue underneath, but to get an accurate picture one needs to a) own a set of Vernier calipers and b) have the proper training to convert this measurement.  Another way is to pass a small electrical current through the body (usually through the feet) to measure the resistance.  Muscular tissue will have a lower resistance than adipose tissue (or perhaps vice versa), and the net resistance can be converted to body fat percentage.  The problem with this method that it varies not only by gender but also by race, and the test is only calibrated well to white males.  The final common way to measure a persons body fat percentage is to suspend them in a sling/chair sort of apparatus attached to a scale, not unlike the tray you can find in a grocery store to weigh your fruits and vegetables.  The person sitting in the sling is then submerged in a pool and the change in weight due to buoyancy force is recorded.  Since adipose or "fatty" tissue occupies more volume with less mass, it will contribute to the water exerting a greater buoyancy force on the body.  So the greater the change in weight from dry to submerged, the greater the body fat percentage.

It's tricky to describe in words.  Source.

Unfortunately, all methods of measuring body fat percentage involve specialised training, specialised equipment, or both.  And even then, there is speculation that the distribution of fat on the body also appears to play a role in mortality risk.  While I couldn't [easily] find scholarly articles to support the assertion, the hypothesis is that fat in and around the "greater omentum" (i.e. "beer gut"), encases the internal organs and can lead to health complications.  As an interesting side note to this, apparently it also makes surgery very difficult.  But, as always, I digress.

So as it stands, what we should concern ourselves with is body fat percentage, and the most widely-used metric, BMI, doesn't really attempt to approximate this.  At this point, I'm tempted to hypothetically ask if there would be a better way, but my distinguished and top-hatted readers have no doubt already read the title of this post, and know darn well that's where I'm going.  "I bet it's the waist-to-height ratio!" you're probably yelling at the screen, excitedly and inadvertently popping out your monocle.

Excellent guess, it is.  The waist to height ratio (WHtR, and I have no idea why that's the abbreviation) doesn't concern itself with the weight of the subject at all.  The lean waist doesn't tend to fluctuate much with muscular or skeletal structure, so with the exception of very narrow-waisted females, it's a pretty good analog to body fat percentage.  That's all there is to the waist to height ratio, by the way.  You divide the circumference of your waist by your height (using the same units for both measurements).  It's unitless, so you can use centimetres, inches, furlongs, leagues, whatever your heart desires.  Just make sure to use the same units.  The only point of note is to measure your waist as the figures do in the top of this post.  Fashion would define your "waist" as being below the anatomical waist.  The anatomical waist is either the narrowest part of your torso, or about an inch above your navel.

Waist-to-height ratio is also well supported in the literature as a metric of mortality risk, by the way.  An excellent review appears in Nutrition Research Reviews (2010), 23, 247–269.  The data suggest that, though the WHtR has a couple fringe weaknesses like an inability to predict diabetes mellitus in Taiwanese males, it is overall a much better indicator than BMI.  Further, a lot of references pop up with WHtR being a superior metric to BMI when monitoring children's health.  Something to think about.

Finally, it's probably worth discussing what ranges WHtR usually fall in to.  The best summary chart I have found is on the relevant Wikipedia page:

I'll also mention that an excellent discussion with pointers to great references can be found here.  So really, the healthy and normal WHtR for people usually falls between 0.42 and 0.50.  If you're above that, you may be healthy, but WHtR would suggest that you are at an increased risk of health complications, and you may want to consider action.  Similarly, it cautions that should your WHtR fall below 0.42, you are at an increased risk of [albeit different] complications, and you may want to consider action.  It is interesting to note that the body requires a certain baseline body fat percentage in order to function properly.  If you're curious about the complications of low body fat percentage, read up on body building side effects, it's a little disturbing.

So there you have it.  The evidence would suggest that WHtR is a superior metric to BMI for measuring the health of individuals, and should probably see more use as a result.  This is particularly true if you have broad shoulders, a barrel chest, or even wide hips, because your BMI will likely be skewed.  Plus, a cloth measuring tape costs a lot less than a bathroom scale, and never needs to be adjusted.


Update (2015-03-16): I want to include the latest table on the aforementioned relevant Wikipedia page, because it now contains some new, entertaining entries at the low end.