Why you shouldn’t just use body fat % to assess your body composition.

The first thing people are interested in when they get a DEXA scan done at Body and Bone is there body fat %. It is the number everyone obsesses about, as the lower the body fat % is, the leaner you look. However, it isn’t quite that simple there are also other very important variables to consider.

We know body fat % is a better measure of body composition than BMI or weight, it still isn’t the best, as it wraps everything up into one single number. Body fat % is the ratio of fat mass to your weight. This means it is affected by your muscle mass %, so one number trying to take account of two things.


FMI: Males 3 – 6     Females 5 – 9

LMI: Males 18 – 21       Females 15 – 18

As you can see both have exactly the same body fat % however very different body compositions. One has low fat and low muscle and one has high fat and high muscle. This shows body fat % is not the only thing to consider, FMI and LMI are important too. FMI/LMI is the amount of fat/lean mass you have in relation to your height. If you are at the lower end of the recommendations you might need to increase fat mass or muscle mass. If you are at the upper end or above for LMI this shows you have more than average for muscle mass in relation to your height.

Looking at both of these values is a better method at determining your body composition priority. The FMI is unaffected by muscle and the LMI is unaffected by fat, unlike body fat % which is affected by both. Therefore, you can see the one on the left should prioritise gaining muscle whereas the one on the right should prioritise losing fat.