Came across an interesting new study the other day, where a team of researchers from Texas compared a number of different body fat tests to see which one was the most accurate .
There was a lot about this study to like:
2. The 4-compartment model was used as a benchmark. This involves dividing the body into four compartments (mineral, water, fat, and protein) and measuring each one independently. It’s currently the gold standard against which other body composition tests are measured.
3. The researchers also listed the 95% limits of agreement (LOA) for each method. This gives you a much better idea about the accuracy of a given body fat test for an individual rather than a group.
Why does that matter?
Let’s say you round up a group of people and measure their body fat. Let’s also assume whatever body fat test you’re using overestimates body fat percentage in half the group by 5%, and underestimates 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.
Just because a body fat test does a decent job at estimating group averages doesn’t mean it works equally well for individuals. That’s why it’s good to know the 95% LOA.
I don’t have a very high opinion of body fat testing in general, and this study did nothing to change my mind.
Body Fat Scales vs DEXA
In men and women, the 95% LOA for body fat scales was 7.6%. For DEXA, it clocked in at 5.4%. In this context, the lower the number the better.
So what do those numbers mean exactly?
- If body fat scales put you at 15% body fat, it could really be anywhere between 22.6% and 7.4%.
- If a DEXA scan puts you at 15%, your true body fat could be as low as 9.6% or as high as 20.4%.
DEXA was better than body fat scales, but still not too useful. All it tells me is that my body fat percentage is somewhere between 10 and 20%. I could have worked that out for myself just by looking in the mirror.
The argument in favor of body fat testing is that even if a given test isn’t accurate, at least it’s consistent.
In other words, it doesn’t matter if a body fat test is out by a few percentage points here or there. As long as it’s consistently inaccurate, you can use it to track your progress over time.
But that isn’t always the case.
DEXA, for example, 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.
Here’s how the researchers sum up their results:
All single assessment techniques used in this study (DEXA, bioeletrical impedance spectroscopy, electrical impedance myography and bioelectrical impedance analysis) produce limits of agreement large enough to make the utility of these body composition estimates questionable at the individual level. When these techniques are used in individuals, practitioners and researchers are advised to use substantial caution when interpreting and using body composition estimates.”
All of which is why I don’t trust any of the body fat tests out there. Using the results to guide your training and diet decisions can easily send you off in the wrong direction.
Christian Finn, M.Sc.
Founder of Muscle Evo
NOTE: I’ve listed the 95% LOA for body fat percentage estimates in both men and women below, along with the make and model of the device that was tested.
– GE Lunar Prodigy scanner (DEXA) 5.4%
– ImpediMed SFB7 (bioimpedance spectroscopy) 5.9%
– Seca mBCA 514/515 (multi-frequency bioelectrical impedance analysis) 7.6%
– Skulpt Chisel Scanner (electrical impedance myography) 7.7%
– Tanita TBF-300A (single-frequency bioelectrical impedance analysis with foot-to-foot electrodes) 7.6%
– Omron HBF-306 (single-frequency bioelectrical impedance analysis with hand-to-hand electrodes) 7.9%