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DEXA Versus Calipers for Body Fat Estimation

This is going to be another fairly short piece since I’m still entrenched in editing the book and can’t think of anything much more useful to write about at the moment.  In it, I want to look at some issues/comments/etc. that have come up regarding body fat percentages (BF%) and estimates.

There is a common occurrence online (my support forum has an entire thread) for people to ask for visual estimates on BF%.  No, it’s not a perfect method but folks who have done this for a while can give at least a ballpark estimate.  You can also find some neato graphics people have put together for men and women online.

Now, for a lot of years, methods such as calipers, bioelectrical impedance (BIA, crap IMO), underwater weighing, etc. were used to estimate BF% and I wrote a long series about body composition, numbers, problems, recommendations, etc.  James Krieger also did a really good series on the topic.

Now first realize that all body fat estimates are only estimates, which are only estimates.  The only truly accurate method to get body fat percentage is to dissect someone and measure it out and you can’t do that very often (it’s also messy).  All BF% estimates have some built-in assumptions and some amount of error so it’s always just an estimate.

But here’s where the recent problem has come in.

DEXA versus Calipers, etc.

One of the more recent approaches to measuring body composition is DEXA (Dual-Energy X-ray Absorbitometry) which does a full-body scan and can measure things like bone density (critical for women) and other tissues.  And it gives what is proposed to be a more accurate estimate of BF% than older methods.  It can even do something where it gives you one value for upper body and one for lower body. I suppose folks could use this to look at regional changes in BF% but I think whole body BF% is more valuable.

And here’s where it gets a little bit weird.  DEXA seems to give systematically different measurements than other older methods.  And it seems that they are usually higher.  Case in point, a number of contest lean bodybuilders (who calipers or visual estimates might put around 4%, the lower limit for males) have been DEXA’d at like 9% or thereabouts.

And what I’m seeing online is that someone will ask for a visual estimate, they will be very lean and maybe estimated at 8% and someone will comment “Yeah, such and such contest lean bodybuilder was estimated at 9% so no way are you 8%, lol.”   Lol indeed.

But here’s the thing, you can’t compare apples and oranges like this (well you can but why would you).   DEXA gives diferent values than calipers gives slightly different values than other methods.  Personally, I find it a little bit off that DEXA is putting someone who is at a BF% near death/their essential body fat levels (3-4% in men) at 9% but so be it.  And clearly DEXA is  measuring something else that is causing it to give these higher values.   I truly have no idea what (maybe someone in the comments can comment by leaving a comment)

Consistency vs. Accuracy

But here’s the thing, you can’t compare apples and oranges like this.  The old caliper/visual/etc. estimates are giving one value (and what most, including myself are using) and clearly DEXA is giving different (typically higher values).  But they aren’t comparable.

In the series I linked to above, one point I make is that consistency is more important than accuracy.  Make no mistake, it’s nice to have a semi-accurate estimate of BF% or whatever but it’s only an estimate.  And even if two different methods are 5% different, it doesn’t make much of a difference in the big scheme in terms of estimating lean body mass (LBM) or calculating out diets.  It’s just not that big of a deal.

More importantly, the differences don’t matter because it’s an estimate no matter how you cut it.  More than that, being able to track relative changes is far more important than true accuracy.  So say that calipers put you at 10% and DEXA puts you at 15%.  Whatever.  And let’s say that you diet for a month and lose some amount of fat.  Calipers say you’re 8% and DEXA says you’re 13%.  Yes, the values are different but both dropped by 2%.  They both picked up the relative change (that is they are consistent) even if they give different values.

DEXA versus Calipers Redux

More importantly, and this was a question that I got regarding some information on the site, all of my early books and articles use the earlier values via calipers or visual estimates.  So in articles where I list some number, say 4% for a contest lean male or 10-12% for a contest lean female, that’s based on older methods.  DEXA might put them around 9% and 16-18% respectively but I don’t care.

Or there is my diet Category system which divides men and women into one of three different categories based on their BF% (which determines a lot of underlying physiology).  I use, 15% or whatever for Category 1 males.  DEXA would say 19-21% or whatever.  I’m using the older methods.

In the two books I’m working on, I will make the above more clear and provide values for both.  But again, to chortle “You’re not 8% b/c DEXA says some contest lean guy is 9%, lol” is missing the point.  The 8% is via the older methods, and the 9% is by a newer different method.

As my favorite author once wrote

“A man with one watch always knows what time it is, a man with two is never sure.”

Ponder on this until you understand and you might earn entrance into the Illuminatty.

Update: Someone made me aware of an older study (2004) showing that DEXA, on average was higher than older caliper estimations by about 3%.  So in the range of this article.  But there is variability.



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