Should you listen when somebody tells you you’re too fat because your BMI is high?

What about if your BMI is below 20, does that mean you’re lean?

The simple answer is NO!  You’d get just as good results by rolling the dice.

Clients and trainers alike regularly inform us that their Doctor has told them they are too fat or obese because their Body Mass Index (BMI) is over 25 or over 30. So we need a brief discussion of BMI, what it is and whether or not you should worry about it at all.

BMI is a very simple formula consisting of your bodyweight divided by the square of your height (kg/metre squared). Right off the bat you can see that this is going to distort for very tall and very short people. But it should also be obvious that the formula doesn’t distinguish whether you have more muscle or less muscle, or smaller bones, all the formula takes is the total weight of all of these combined.

We all know that fat weighs a lot less than muscle for the same volume, so it is very easy for this formula to give the same results for a fat person without much muscle as a lean person with toned muscles.

UP trainer and client same BMI
Both Trainer and Client in this photo have a BMI of 27. What do you think – does it make sense to use it as a guide?

For those of you with a more mathematical mind take a look at the chart below.

If BMI and Bodyfat percentage were directly related the chart should go from the bottom left to the top right. Instead there are a few points on the top left (lean muscular people) and many on the bottom right – skinny fat people. The lean muscular men can easily ignore a Doctor telling them that they are obese because it’s obviously nonsense. However, it’s those on the bottom right who are really let down by BMI as a tool. They are lulled into a false sense of belief that they are healthy, or even that they are lean. Unfortunately the rates of pre-diabetes and insulin resistance are on the rise in Singapore and it’s this group that gets that unpleasant surprise as they age because they were never told they were unhealthy.

Asian male BMI vs Bodyfat percentage
This scatter graph of BMI vs BF% for all the Chinese, Japanese, Korean and Malay men we’ve measured when they start their fat loss or muscle gain programmes at Ultimate Performance (before they have begun exercising). The line across the middle of this chart shows the correlation between BMI and Bodyfat percentage. The line is nearly flat indicating almost no correlation.

Why have we excluded those with a bodyfat percentage of 30% or more from the chart? Well obviously once someone passes a certain weight they are almost certainly getting fatter so by definition over 30% bodyfat we will find a strong correlation with BMI. This isn’t the issue – the issue is discrepancies for people with a lot of muscle or very little muscle. Anyone who is over 30% bodyfat already knows they are unhealthy!

So what should you do if someone tells you that you need to lose weight when you know that you don’t, or if you suspect that you should be in better shape but you get told that you are okay?  Easy – get someone to measure your body fat with callipers or even a bioelectrical impedance measurement which we don’t like but is 100x more accurate than BMI.

We’ll put up a separate post about BMI for Asian women because men and women have different bodies and less muscle on Asian women compared to men and even compared to Western women makes a big difference.