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So let's talk about body mass index (BMI).
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This is a term that's often discussed and so let's first figure out how it's calculated - how do we figure this out?
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So, if you have a little stick person, what you could do is have them step on a scale.
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And you could get their weight. So, the first thing you need is their weight.
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And then you go on and you ask them if they would mind if you would take their measurement of height.
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And so you get their "H" or height.
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And a BMI is basically just taking those two numbers and using them in a little equation
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where you take the weight divided by the height squared.
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Very simple, right?
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So now, let me calculate my BMI.
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But before I do that, I have to tell you one more thing
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Which is that, the weight, this is in kilograms (kg)
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and the height is actually in meters squared.
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So, when I was actually figuring out my own height and weight
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I realized the problem and I'll show you what that was.
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So, my weight is 160 pounds and my height is six foot, one inch.
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Which is the same as 73 inches.
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So, I would love to use this easy equation, but I have pounds and inches - I don't have kilograms and meters.
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So what do I do? Well, what I need to do first is convert over, right?
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So I can say, "Alright, well one pound equals 0.454 kilograms. So far, so good.
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And now one inch, which is pretty small, is going to be a small number relative to a meter, which is huge, right?
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And so, it is, it's 0.0254 meters."
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So, now I've got my conversions and actually, what I can do, is I can take these numbers and plug them in here
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to help me easily convert from kilos and meters over to pounds and inches - make my equation much simpler for me to use.
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So I can say, "Okay, well now how about weight in pounds and height in inches squared?"
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And I have to multiply, right? Because I have to multiply by the conversion.
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And so, I'll multiply by 0.454 divided by 0.0254 squared.
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And to make it even easier, I can actually take this whole thing, plug it into my calculator
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and it gives me the number 703. So I can say, "Okay, so really what I have here is, again, weight in pounds
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divided by height in inches squared, multiplied by 703."
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So going back now, I can finally throw in my own numbers, right?
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I can say, "Okay, so my BMI is 160 divided by 73 squared times 703."
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Let me erase that just to make it clear.
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Okay, so this math works out to 21.
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So my BMI is 21.
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Wonderful. The next question is: well, what does that mean?
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You know, if I have a BMI of 21, is that normal or is that good or how should I feel about that?
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So, if you put a scale on here starting with zero to, let's say 30
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and I have 25 here and let's say this is about 18.5
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What physicians have done is basically divide up the BMI into catergories
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and they said, "Okay, well if your BMI as an adult is somewhere between zero and 18.5
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somewhere in this range. I'm going to do it in yellow.
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Then you're underweight. I'm just going to write "under." And if you're between 18.5 and 25
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if you're in this range, you're in a very healthy range.
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And so I'll write a little smiley face for that.
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And if you're above 25, let's say 25 to 30, then you're in the overweight range.
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And finally, if you're above 30, let's say you're out here, then you're obese."
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And so that's where the word obese really comes from.
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It comes from a category of BMI.
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And so, going back to my BMI, I'm right here at 21
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and I'm doing pretty well. But, the question might come up in my head,
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"What weight would I be if I was overweight and what weight would I be if I was obese?
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How many pounds away am I from being in those categories?
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So I can go back to my equation:
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BMI equals W over H squared and I can just rearrange it
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I can say, "Well how about W equals BMI times H squared over 703, right?"
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Because that's the conversion. So if I rewrite the equation like this
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then now I can solve for my weight. I can say, "Okay, well let's say that I wanted to figure out what to do
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if I have a BMI of 25 and I know that my height probably won't change,
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I'm not going to grow anymore and 703 is the number we always have to use when we're using the conversion from
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pounds to kilos and inches to meters. So, what does that equal?
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Well, in this particular case, using 25, I can see that for me to be overweight, I would be around 190 pounds.
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190 pounds. So that's actually really, really good to know
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That's actually 30 pounds above what I am today.
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So that gives me a sense for how far away I am from being overweight
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and I can even do this for a BMI of 30.
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I can say, what about a BMI of 30?
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My current height, again, is 73 inches. That's not going to change.
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And I can do the math and find out that, in this case, my weight would be 227 pounds.
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So if I was 227 pounds, given my height, I would be considered obese.