Why does BMI have units of kg/m^2 when we are three dimensional? Wouldn’t kg/m^3 or g/cm^3 be more accurate?
In: 100
It is basically just a very unsufficiant method to guess the area of your waistline in proportion to body height (which isn’t the worst method to estimate if someone is obese)
It is based on two assumptions that are both untrue but close enough to the truth that the result is not completely meaningless.
1st assumption: The human body has the shape of a solid cylinder
2nd assumption: The density of the human body is consistent (bones, muscles, fat all have the same density). Let’s assume the density is 1 for the rest of the argument but it works with any density in principle.
Density = BodyWeight / BodyVolume
When we replace density with 1 and BodyVolume with the formula of a cylinder, we get:
1 = BodyWeight / (BodyHeight * CircularAreaOfWaist)
Using Algebra we get
CircularAreaOfWaist = BodyWeight / BodyHeight
CircularAreaOfWaist / BodyHeight = BodyWeight / BodyHeight^2
CircularAreaOfWaist / BodyHeight = BMI
So if you would just measure your waist, calculate the area and devide it by your body height, you would get a much better version of BMI. There is never a good reason to use the BMI for individuals. According to it’s inventor it was only meant to be used to get estimations about large groups where you only have little data (weight and height are commonly available in big medical datasets). Like comparing all military conscripts this year with military conscripts one decade ago.
Yes, you are right. The human body actually scales with an exponant of about 2.3. So if someone is 10% larger, they should weigh 1.1^2.3 ≈ 1.25 times as heavy. The fact that you still need a chart to see what BMI is healthy at a certain length is a direct consequence of using the wrong exponent in the formula.
Yes, you are right. The human body actually scales with an exponant of about 2.3. So if someone is 10% larger, they should weigh 1.1^2.3 ≈ 1.25 times as heavy. The fact that you still need a chart to see what BMI is healthy at a certain length is a direct consequence of using the wrong exponent in the formula.
It’s infuriating that nobody’s answering your question.
Everyone’s just saying “because that’s the way it is”.
The true answer is: you’re right.
kg/m^3 (called the CI, or Corpulence Index) is an alternative to BMI which is known to be just flat-out superior.
BMI is fundamentally flawed, for exactly the reason (among others) that you have mentioned.
That is known information.
Anyone saying BMI just coincidentally happens to describe someone’s thinness accurately is misinformed: kg/m^3 is more accurate.
BMI was popularized at a time when nobody really cared about these things and there weren’t any popular alternatives, so it got established in the 1970s as just what people used.
Since then it’s been kind of caught in a feedback loop, where because it was popular, people published things about it, and because people published things about it, it got more popular.
In the grand scheme of things, whether you use kg/m^2 or kg/m^3 (or kg/m^2.5 ) doesn’t matter *that* much.
BMI is good enough at average heights, and only becomes problematically inaccurate when people are very short or very tall.
(Basically, if you’re very tall, you will always be misreported as being overweight by BMI when you’re actually at a healthy weight)
But…most people are of an average height, we mostly only care about BMI as a population statistic anyway (where we’re focused on the average person), so it continues to persist as fundamentally flawed, but, meh, good enough most of the time.
It’s infuriating that nobody’s answering your question.
Everyone’s just saying “because that’s the way it is”.
The true answer is: you’re right.
kg/m^3 (called the CI, or Corpulence Index) is an alternative to BMI which is known to be just flat-out superior.
BMI is fundamentally flawed, for exactly the reason (among others) that you have mentioned.
That is known information.
Anyone saying BMI just coincidentally happens to describe someone’s thinness accurately is misinformed: kg/m^3 is more accurate.
BMI was popularized at a time when nobody really cared about these things and there weren’t any popular alternatives, so it got established in the 1970s as just what people used.
Since then it’s been kind of caught in a feedback loop, where because it was popular, people published things about it, and because people published things about it, it got more popular.
In the grand scheme of things, whether you use kg/m^2 or kg/m^3 (or kg/m^2.5 ) doesn’t matter *that* much.
BMI is good enough at average heights, and only becomes problematically inaccurate when people are very short or very tall.
(Basically, if you’re very tall, you will always be misreported as being overweight by BMI when you’re actually at a healthy weight)
But…most people are of an average height, we mostly only care about BMI as a population statistic anyway (where we’re focused on the average person), so it continues to persist as fundamentally flawed, but, meh, good enough most of the time.
Your width would correlate more with how fat you are which is kind of what we are trying to work out. Also there isn’t an easy way of working out an intrinsic width.
I’m sure you could have a much better measure using a dexa scan that takes into account lots of measures. But that would be applicable to almost no one, since very few people can afford regular dexa scans.
BMI isn’t perfect but it’s a really good measure that can be easily applied to everyone.
It’s infuriating that nobody’s answering your question.
Everyone’s just saying “because that’s the way it is”.
The true answer is: you’re right.
kg/m^3 (called the CI, or Corpulence Index) is an alternative to BMI which is known to be just flat-out superior.
BMI is fundamentally flawed, for exactly the reason (among others) that you have mentioned.
That is known information.
Anyone saying BMI just coincidentally happens to describe someone’s thinness accurately is misinformed: kg/m^3 is more accurate.
BMI was popularized at a time when nobody really cared about these things and there weren’t any popular alternatives, so it got established in the 1970s as just what people used.
Since then it’s been kind of caught in a feedback loop, where because it was popular, people published things about it, and because people published things about it, it got more popular.
In the grand scheme of things, whether you use kg/m^2 or kg/m^3 (or kg/m^2.5 ) doesn’t matter *that* much.
BMI is good enough at average heights, and only becomes problematically inaccurate when people are very short or very tall.
(Basically, if you’re very tall, you will always be misreported as being overweight by BMI when you’re actually at a healthy weight)
But…most people are of an average height, we mostly only care about BMI as a population statistic anyway (where we’re focused on the average person), so it continues to persist as fundamentally flawed, but, meh, good enough most of the time.
A body with the least surface area and the most mass (density is constant) will be a sphere. Therefore, BMI measures how much of a sphere you are.
Your width would correlate more with how fat you are which is kind of what we are trying to work out. Also there isn’t an easy way of working out an intrinsic width.
I’m sure you could have a much better measure using a dexa scan that takes into account lots of measures. But that would be applicable to almost no one, since very few people can afford regular dexa scans.
BMI isn’t perfect but it’s a really good measure that can be easily applied to everyone.
Your width would correlate more with how fat you are which is kind of what we are trying to work out. Also there isn’t an easy way of working out an intrinsic width.
I’m sure you could have a much better measure using a dexa scan that takes into account lots of measures. But that would be applicable to almost no one, since very few people can afford regular dexa scans.
BMI isn’t perfect but it’s a really good measure that can be easily applied to everyone.
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