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Body Mass Index Vs Fat Mass Index … and why we like FMI

I find that while most people have heard of Body Mass Index (or BMI), not as many people have heard of Fat Mass Index (or FMI).

What is Body Mass Index or BMI?

BMI is a diagnostic tool widely used by doctors to determine if someone is in a healthy weight range for their height. It’s worked out by dividing your weight by your height and will position you on a scale of 16.5 – 40 kg/m sq which describes you along the range of being underweight, healthy, overweight or obese.

The problem with BMI as a measurement is that it doesn’t account for athletic populations with a lot of lean muscle. Because the more lean muscle you have, the more you weigh, and then the higher your BMI is going to be.

We don’t use the BMI for exactly this reason - athletic populations are too often incorrectly classed as obese.

So what is Fat Mass Index or FMI?

What we like to look at is your Fat Mass Index (or FMI). Using the FMI, we consider how much body fat you are carrying for your height… a much better way of determining if your weight is a risk factor for your health.

FMI is calculated in the same way as BMI but instead of dividing your total mass by your height, we divide just your FAT mass by your height.

To work out your FMI, you need to know your total body fat mass and the most accurate way to measure this is by using DEXA.

The female healthy range for FMI is between 5 and 9 and the male range is between 3 and 6 kg/m sq.

How we can use FMI

As well as considering the weight of lean muscle, FMI is also a much better tool for setting realistic goals for clients.

When people come in for a consultation, we work out their FMI and then look at their result on the scale with respect to ideal ranges. When we look at where clients sit in that scale, we also assess how much fat they need to lose to fall into the healthy range.

For people who are carrying excess body fat, this gives them a realistic goal to attain in terms of losing excess fat.

Take an example of someone who weighs 120kg and their doctor has told them they need to lose 60kg to have a healthy BMI.

Realistically, that amount of fat loss can be very unlikely because depending on the total amount of muscle and bone in their overall weight.

So for that same client we look at their FMI, and we can determine that means they need to drop to 85kg to fall into a healthy range, this gives them a much more realistic goal that is based on what is healthy for them, and not an arbitrary number.

That’s why we like fat mass index….

Because it accounts for lean muscle mass in athletic populations AND because it gives clients a more realistic goal in terms of the body composition change they need to achieve.

As a dietitian, I would like to see the reliance on BMI change move and become far more focused on FMI as the key diagnostic tool, especially amongst doctors.

If you want to know your FMI, book in for a consultation with one of our dietitians to request a DEXA scan today, and ask to determine your FMI.

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