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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?

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 two numbers tell you how much fat and muscle you carry in relation to your height.

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 men, we see what polar opposites they are. For fat, one man is in the lowest 30%, 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, 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.