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Body Fat Percentage vs Fat Mass Index

Almost everybody who comes for a DEXA body fat scan wants to know their body fat percentage. In fact, I would say that is far and away the one single figure they want to know.

But percentage body fat, while a valid measure and data point to have, is not necessarily the best – and certainly not the only – measure of fat.

Why? Because body fat percentage is simply a comparison of  fat to non-fat. In isolation it doesn’t give any indication of HOW MUCH fat you’ve got, only the proportion that is fat COMPARED TO EVERYTHING ELSE.

A better number, or one to use in conjunction with your fat percentage, is your fat mass index (FMI), which is your total fat mass divided by your height squared.

The strength of your fat mass index is that it is based purely on the absolute amount of fat in your body. If your FMI goes down, it means you have definitely lost body fat. And, conversely, if you lose fat your FMI will definitely fall. It’s a two-way relationship.

You can’t say that about your body fat percentage. You could lose fat and your body fat percentage could stay the same or actually go up. How? If you lose more muscle than fat, that’s how (as has happened with one unfortunate client).

Your fat mass index, however, is related purely to fat mass AND it is independent of muscle mass. If you stack on a few kilos of muscle but your body fat remains unchanged, your FMI will stay the same. It only moves if the amount of fat you carry changes.

Take a look at this real life Bodyscan bodybuilding client. His body fat percentage (subtotal, without the head) is 15.1% and his age-matched percentile is 3. That means only 3% of men his age have a lower body fat percentage. That great number is achieved because so much of him is muscle (ie, the proportion of his fat to everything else is very small).

But his fat mass index, at 5.13, achieves an age-matched percentile of only 25, ie a quarter of men his age have a lower FMI.

Why? Because FMI takes into account the actual amount (mass) of fat he’s carrying. And when we take into account his height, that fat mass is not quite as amazing as his percentage body fat.

So don’t obsess about your body fat percentage. Read it in the context of other fat and lean mass indicators.