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DEXA: Why body fat percentage is not a good measure of fat

Almost everyone who comes to Bodyscan wants to know their body fat percentage (BF%) and it’s the number so many people obsess about.

But whilst BF% is a better measure of body composition than weight or BMI, it’s still not the best measure because, like weight and BMI, it wraps everything up into one, single number.

Body fat percentage is actually just the ratio of your fat to your weight, which means it’s affected by your non-fat mass (mainly muscle) as well as your fat. One number trying to take account of two things – not good!

To show the limitations of BF%, take a look at the DEXA scans of two men who have exactly the same body fat percentage – 21.6% – but very different body compositions.

One has low fat and low muscle, the other has high fat and high muscle, but their BF% is the same!

Can you see now why body fat percentage is not the best metric?

The restrictions of BF% are why a DEXA scan is such a superior methodology for measuring body composition. A DEXA scan measures your fat and lean mass separately and gives you a meaningful index for each – your fat mass index (FMI) and your lean mass index (LMI). These tell you, respectively, how much fat and muscle you carry in relation to your height. (In simple terms, DEXA allows us to break down BMI (body mass index, weight for height) into two components. If you add your FMI and LMI together, they don’t quite equal your BMI because your bone mass (a ‘bone mass index’, if there were such a thing) has been removed.

Crucially, your FMI is unaffected by muscle and your LMI is unaffected by fat. Unlike your body fat percentage, which is affected by both.

If we look at the FMI and LMI for the two Bodyscan clients above, we see what polar opposites they are! For fat, one man is in the lowest 30% for his age, the other is in the highest 35%. For lean mass and muscle the situation is reversed. In relation to his height, one man has more muscle than 90% of men his age, the other has less muscle than 90%.

Until you know your FMI and LMI and get the best picture of your body composition you can’t be really sure what your training and nutrition priorities should be.

These two new indices (and others on the report, such as A/G ratio) will no doubt be new to you, which is why a clear and professional interpretation of your DEXA report is crucial. Without an experienced explanation of all the numbers in your report, your scan results is just a lot of numbers. This DEXA information video explains the need for a detailed interpretation. Bodyscan’s experience and database of more than 17,000 DEXA body composition scans means we understand every data point and can explain what they mean to you as an individual (age, sex, ethnicity, lifestyle, etc).

To get a firm grip on your body composition and make changes to it, a DEXA scan with Bodyscan is the best first step. If you have any questions, please take a look at our comprehensive set of FAQs or write to us at [email protected]. All our bookings are taken online and, if you’d like to bring a friend or your partner, we offer a 2-person Baseline scan, which can help with motivation as well as reduce the cost!

Philip Chant

Kevin Garde

Rob Webster