Earlier this week, I came across a forum post about a guy who got a DEXA scan for the first time.
He’d estimated his own level of body fat, based on the way he looked in the mirror, at around 18-20%.
But the DEXA scan told him that his body fat was 28.6%, and he couldn’t understand why it was so high.
One of the replies he got suggested ignoring the number, and focusing instead on how that number changes over time.
In other words, it doesn’t matter if DEXA is off by a few percentage points here or there. As long as it’s consistent, you can use it to track your progress over time.
However, DEXA scans aren’t cheap. The guy I was reading about paid $100 for the scan. If he goes back again after a few months, that’s another $100. Two hundred dollars is a lot of money to spend if the results aren’t telling you anything useful.
How accurate are DEXA scans when it comes to tracking your progress over time? Are they worth the money?
DEXA Scans and Body Composition
DEXA, short for dual-energy X-ray absorptiometry, was originally used to determine bone density, and over time has become popular as a way to estimate how much fat and muscle you have.
When you get a DEXA scan, which usually takes somewhere between 10 and 20 minutes, a large scanning arm is passed over your body.
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This scanning arm sends out two low-dose X-rays, which are absorbed differently by bone and soft tissue. The denser the tissue, the more X-rays pass through. The density profiles from the X-rays are then used to generate an image of the scanned area.
How Accurate Are DEXA Scans for Body Fat?
The short answer to this question is that DEXA scans are not an accurate way to measure your body fat.
That said, no device can accurately measure how much muscle and fat you have.
The only way to truly measure your body fat is to have all of it stripped out, placed on a scale, and weighed. This method, known as carcass analysis, is highly accurate. The only downside is that you have be dead in order for it to happen.
A better question to ask yourself is this:
Are DEXA scans accurate enough so that I can use the information to make better decisions about what to eat and how to train?
DEXA might overestimate body fat percentage in half your group by 5%, and underestimate it in the other half by 5%. When looking at the group results, the average error size is zero. But the individual results are way out.
The fact that a body fat test does a good job at estimating group averages doesn’t mean that it’s equally as good at measuring individual progress over time.
What Is the Margin of Error With DEXA Scans?
The size of the error with DEXA is around 5% (although some studies put it much higher).
So what exactly does that mean for you?
You download Gutless, follow it for a couple of months, and get down to 15% body fat.
But when you go back for another scan, DEXA might tell you that your body fat percentage is still 20%.
Same thing holds true for muscle growth.
Let’s say you gain 5-6 pounds (around 2-3 kg) of muscle. Which, depending on the length of time you’ve been training, could take many months of hard work.
But a DEXA scan might show that you hadn’t gained any muscle at all.
You’d come away with the impression that whatever you’d been doing to generate those results didn’t work, when actually it did.
And you run the risk of ditching a training and nutrition program that’s working and replacing it with one that’s less effective.
One of the arguments in favor of body fat tests like DEXA is that it doesn’t matter if it’s out by a few percentage points here or there. As long as the size of the error remains the same, you can use it to track your progress over time.
Problem is, the extent to which DEXA overestimates or underestimates your body fat percentage seems to change depending on how fat you are.
In one study, DEXA overestimated body fat percentage in 13 of 16 individuals with less than 15% body fat, but underestimated body fat percentage in 9 of 11 individuals with more than 15% body fat .
It wasn’t just inaccurate, it was inconsistently inaccurate.
DEXA does a better job at estimating your body composition than devices that rely on bioelectrical impedance analysis, such as body fat scales. But you’re still left with an error of around 5%.
Which means that if a DEXA scan puts you at 15%, your true body fat could be as low as 10% or as high as 20%. That’s a big range. You could get a similar rough estimate just by looking in the mirror.
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