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Kill the BMI Fairy

For as long as I can remember, I've heard countless people claim they want to shed the kilos so they can reach their ideal weight as prescribed by the body mass index. This calculation is garbage, offers zero benefit and in certain situations can be dangerous to your health.

Before I get into the specifics I want to highlight some examples. Most of you know Connor McGregor and would agree he is quite shredded with minimal fat. At 1.75m, 77kg and 27 years old his BMI is 25 which according to this scale is overweight.

Arnold Schwarzenegger has maintained a fairly constant weight throughout most of his career, somewhere between 107kg and 120kg. At 1.88m he has a BMI of 32, making him obese under this measurement.

Chris Pratt AKA StarLord (1.88m, 101kg, 36 years) has a BMI of 28 which is considered overweight and quite close to obese. The list can go on all day but just from the above you can see how BMI is pointless for figuring out your "ideal weight".

 The Explanation

BMI was created by mathematician Adolphe Quetelet sometime around 1850. That fact should point you towards two issues. First the creator is not a medical doctor nor have any qualifications to do with the body. He was a brilliant mathematician and a founder of statistics and sociology. Second it was created well over a century ago, when they didn't understand the human body as well as we do now. It was meant to help the government quickly gauge the general obesity of the population to provide appropriate resources.

Let's look at the formula for BMI to see if we can find any issues.

Weight in kg divided by (height in metres times height in metres).

Apart from the logical issue with multiplying the height by itself, there are many key values missing. The most important would be waist size as that is where the majority of people store their excess weight.

There is no inclusion of bone, fat and muscle proportions. Bone is denser than muscle, which in turn is denser than fat. So if you have strong bones, a nice muscle tone and low fat you will have a high BMI. This is why the above examples are so high even though they are fit and healthy.

Logically speaking the formula is wrong. Quetelet had data for obese people. He needed a formula that ensured those people had a result that was high. So while the formula matched the data, the reverse didn't work out. To understand this, assume I told you I bought a car. Logically you can conclude that it has wheels. But going the other way, if I told you I bought something with wheels you need more data to conclude what it is. I could have purchased a bike, a pram, a moveable filing cabinet or even a fridge.

The biggest thing to remember is that this was created to get an average person, meaning it wasn't meant to apply to an individual, but to a large group. Again this is useful for governments but not someone wanting to determine their target for fitness and weight loss. You don't have 2.4 children and wouldn't struggle to get that 0.4 of a child, but that is exactly what people are trying to do with BMI.

The Alternative

So what can we use to figure out our ideal body? We need something that can apply to individuals, is based off measuring the locations where fat is primarily stored and accounts for bone and muscle mass. The best thing for this is your percentage of body fat.

Assume you are female, 1.7m tall, 80kg and have 10% body fat. That means you have 8kg of fat and 72kg of lean body mass, meaning your bones, organs, muscles, blood, hair etc. Now your BMI would be 27.5 signalling being overweight. To get you into the healthy normal, BMI claims you need to be between 20 and 25. To get there you would need to drop 10 to 20 kg. So from the above example you will need to drop at least 2kg more than the total fat in your body. So BMI wants this person to lose lean body mass. This is beyond dangerous and may create eating disorders. As someone that had a loved one go through this I can attest to its seriousness. Thankfully its under control now.

To understand what is an ideal body fat percentage, take a look at this table.

table of ideal body fat percentage

Essential fat is the minimum amount the body needs to simply function. Women need a higher percentage due to the ability to get pregnant. Again, the example person is already dangerously low on body fat so they can't drop anything more. Putting some fat on might actually be healthy.

Athletes in this context are the professionals. Most people that just want general fitness or an amateur sports career would fit into the Fitness category. Acceptable is for the average Joe that just wants to be healthy. Anything above this is Obese. A good goal for the majority is to be towards the lower end of the Acceptable range. Those training alongside me at Infinite MMA in Ballarat should strive for somewhere between Athletes and Fitness.

So now we have the core principles, how do we work it out. There are several calculations for this but they all share the same idea. Measure the body locations where excess fat is primarily stored, and plug it into a formula.

There are three main ways to do this. The most accurate method is via water immersion in a very expensive instrument. Basically sensors measure the amount of water you displace when floating in a tank. If you can afford this best of luck to you, but there are simpler and cheaper methods. They aren't as accurate but they are close enough for our purposes.

The next method is to pinch the fat to take measurements with callipers. You may have heard this referred to as a skin-fold test. Doctors and fitness professionals are the most likely to use this method. It is quite good but generally we have to make an appointment. The callipers can be fairly cheap (around $30 but up to several hundred), however it relies on an exacting placement as well as the exact same pressure applied to the tool every time you measure. It takes training to do it properly, but as it generally takes 7+ measurements it is fairly accurate.

Which leaves us with the final version using a measuring tape. This is quite cheap and can be done at home. There are only a few measurements and it gives a good idea of how you are progressing. You only need to do this once every three or four weeks.

The Method

So how do we use this sucker? First we need to take a few measurements, all taken while wearing minimal clothing and in metric.

  • Your weight and height are the first ones which should be quite simple for you to obtain.
  • The next ones are your neck and abdomen.
  • The neck should be measured at the narrowest point, sloping slightly down to the front. This should be taken below the Adam's apple.
  • The abdomen is measured horizontally around your waist at the navel (belly button) for men, and above the navel for women. As another point of reference for women, you are after the midpoint between the top of your hips, and the lowest rib on the side. This should get the narrowest point for both sexes.
  • Women also need their hip measurement. Relax the buttocks and stand with feet at shoulder width. This should be the hips at their widest point.

Armed with these values we can calculate the body fat percentage. The equation to do this is a bit complicated, but we don't need to worry about doing the maths. There are thousands of websites and phone apps to do the calculation for you once you have the measurements. If you are interested in the actual formula, here they are (using inches for measurements):

For men: body fat % = 86.010 x log10(abdomen - neck) - 70.041 x log10(height) + 36.76

For women: body fat % = 163.205 x log10(waist + hip - neck) - 97.684 x log10(height) - 78.387

What we are using here is the US Navy Formula. It is considered the most accurate free method that everyone can use. It generally matches a skilled practitioner using callipers to within 1% . It has an error rate of up to 3% which is close enough for normal usage.

You can try this online calculator, or use this Android app to get a few more features. Remember to change the units to metric in the settings. Its generally accepted to take three of every measurement to help rule out any issues with how you took them. Once you have three body fat percentage values, get their average for the final figure (add together and divide by 3).

The Conclusion

As a society, we need to stop thinking about losing weight and instead call it what it really is: losing fat. Using BMI is horrendously wrong as its a calculation for average population meant to assess groups of thousands. Using the body fat percentage method is a far better way to understand your current position and where you are aiming.

Remember to set goals to achieve success. A good big picture goal would be to have a body fat percentage in the Fitness or Athletes ranges, but be specific with an actual percentage.

I recommend having a journal or electronic document to track your progress. You can use the above app to save if you upgrade to the paid version. However there are many reports on every available app that the history can be lost after a serious crash or upgrade. Keeping your own copy with a backup will ensure you don't lose your progress data.

I've created a simple Excel spreadsheet to track progress which also charts the results. Just put your desired body fat percentage at the top and enter the values. The dates are in three week intervals. This way you have a schedule to follow. Download it by clicking here.

Remember that losing weight doesn't necessarily mean losing body fat. You may gain 5kg of muscle but lose 7cm around the waist, which should reduce your body fat percentage by several points.

Please share your body fat percentages, and what your fat loss goals are, in the comments.

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© AJ Watson