Body-Fat Percentage and BMI vs Fat Mass Index

Now I’m not a betting man, but i would confidently say that the majority of people who train (whether that be your everyday gym goers, professional athletes or bodybuilders) have had their body fat measured or estimated and their bmi calculated. I mean some gp’s still use the Body Mass Index (BMI) as a index of health in otherwise healthy people! The question is how meaningful are these numbers as stand-alone indices of body composition or risk factors for disease? For the general population, not very is the answer and here’s why.

Body fat percentage is the amount of fat a person has relative to their total mass or scale weight and doesn’t account for the ratio of fat free mass i.e bone and muscle (although muscle is the one we are really concerned with) to fat mass. Two people could have the same body fat percentage but have very different amounts of muscle mass. If person A had a decrease in muscle mass, after a period of inactivity for example, there would be an automatic increase in the BF %, without any actual increase in fat levels. This particular body composition is usually classified as “skinny fat”.

So what of the BMI ? This is whole body mass relative to height. Assuming the person in question has reached physical maturity, height will obviously not change. However differences in height will influence the BMI of two people who weigh the same. BMI measurements do not differentiate between fat and lean muscle. In theory someone could have low body fat and be extremely muscle bound, yet still be classified as overweight or obese based on the BMI. In my opinion, the BMI should only be used with individuals who are clearly obese or morbidly obese, as an easy to administer, non- invasive way of tracking body composition and health risks. For otherwise healthy individuals, the Fat Mass Index is by far a better tool for assessing ‘fatness’.

The Fat mass Index looks purely at the relationship between a persons fat and height, independent of lean tissue. Similarly to BMI, this means it is influenced by how tall or short a person is. However a decrease in this number will mean fat is actually being lost and not at the expense of muscle (all other factors being equal i.e nutrition, training, sleep).

FMI is calculated by taking your fat mass in kg’s and dividing this by your height squared. The best way to get a fat mass reading is via a DEXA scan (London residents check out which will also calculate the amount lean muscle you have. These markers can then be tracked to ensure you are making the right improvements in terms of body composition. See the image below for a DEXA scan showing Fat mass (orange), muscle (red) and bone (blue). The two scans are 4 months apart and show an increase of muscle mass of 0.6kg and a decrease in fat mass of 5kg. This is a succesful body recomposition!

What should be Fat Mass Index be?

Normal ranges for males and females are 3-6 kg/m2 and 5-9 kg/m2 respectively.

So the take home message is understand the limitations of bodyfat % and the Body Mass Index as tools for assessing body composition, especially with the general population.

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