DEXA and Bioelectrical Impedance – Just like the name – not the same

Heard of Bioelectrical Impedance (BI)? DEXA and BI are definitely not the same thing. Only DEXA gives you truly accurate body composition measurements. Here’s why.

We all know gym training can lead to greater fitness, strength, fat loss and health. But how do we measure these changes?

That’s why body composition scanning is such a valuable addition to any training regime. It takes us way beyond the old bathroom scales and tape measure methods of the past, helping us understand for every person specific changes in fat mass and lean mass and also where those changes have occurred.

And of course the precise measurements contained in the scan results are another way to keep you motivated on your fitness journey. Scanning also makes it easy to measure progress on group ‘challenges’ on fat loss and muscle gain.

But…body measurement technologies differ and they can produce quite different results. So we want to clear up some confusion.

DEXA and BI are two well-known types of body measurement. BI brand names include Inbody, Global Bodies and Evolt. Despite anything you may have heard, DEXA and BI are not the same thing.

Often we hear people saying they have had a DEXA scan previously, the question we then ask is were you standing up or lying down? If the answer is standing up, then it was definitely a bioelectrical impedance test and not a DEXA. A DEXA scanner is a highly accurate, finely-calibrated and expensive piece of medical equipment. It takes a properly trained person to operate one and the patient is lying down whilst the scanning arm passes over them. A BI machine isn’t actually a scanner at all, instead it measures the body’s resistance to electricity. A BI machine is a fraction of the price of a DEXA scanner and pretty much anyone can use one. But in practical terms, does it matter which you use?

If you want the results to be meaningful, then frankly yes.

The reason why is down to Biological Variability, or BV. Our bodies are constantly changing due to food and water intake, time of day, levels of exercise and rest. BV affects BI scanners much more than DEXA.

DEXA scanners are ‘smart’ and with a skilled operator can minimise the effects of BV to less than 1.5%. But because BI machines are measuring impedance, which is greatly affected by how much water is in the body, biological variability can produce results which are, well, variable. The margin of error with a BI machine can be as large as 15% – a huge discrepancy.
An error of this size in a 70kg person means as much as 10kg of tissue could be misinterpreted. So someone with a true fat mass of 20% could show as low as 17% or as high as 23%. This degree of error makes the result of little value when you’re trying to chart someone’s body progress accurately. And those percentages are for fat mass, the measurement of tissue mass can be even more variable.

These are not our figures. They are taken from a study released at the recent American College of Sports Medicine Meeting in Boston, which highlighted the impact of measuring body composition using DEXA vs Bioelectrical Impedance1. The figures are the non-standardised – that’s to say, real life – results.

To be fair, under the ‘lab conditions’ of strict standardised testing, both technologies give a good result (though DEXA was better). The problem with that – of course – is none of us live in a lab.

MeasureUp is Australia’s leading provider of DEXA scanning. To see how a DEXA can change the way you track your progress or find out what you are made of in terms of composition, book your DEXA scan appointment today.


1  Validating InBody® 570 Multi-frequency Bioelectrical Impedance Analyzer versus DXA for Body FatPercentage Analysis. Ryan M. Miller, Toby L. Chambers, Stephen P. Burns, Michael P. Godard, FACSM. University of Central Missouri, Warrensburg, MO. (Sponsor: Michael P. Godard, FACSM) (No relationships reported)

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