Almost everyone who comes to Bodyscan wants to know their body fat percentage (BF%)
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 a 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 as well as your fat.
The DEXA scans of the two men below show they have exactly the same body fat percentage - 21.6% - but very different body compositions.
Can you see now why BF% is not the best metric?
A DEXA scan measures your fat and lean tissue separately and gives you meaningful indices for each, most importantly your fat mass index (FMI) and your lean mass index (LMI). These two numbers tell you respectively how much fat and lean mass 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 our two male subjects, 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 a clear picture of your body composition you can't be really sure what your training and nutrition priorities should be.