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Memories of the 28th Century

Body Density Index

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After my recent medical situation I noticed that my waist was smaller; I had to put more holes into my belt, and my pants are quite loose when the belt isn't buckled, so I checked my weight. My weight was almost unchanged from what it was when I was in the hospital. And my body mass index is almost 32, which is in the obese range. The funny thing is that I'm not actually fat, but I am quite muscular. The sad thing is that many people take body mass index seriously, even though it doesn't take into account whether someone is lean and muscular or fat and flabby.
The Body Mass Index is a simple calculation that divides weight by height, so someone who weighs 225 pounds and is six feet tall has a BMI of 30.5, and that puts that person into the obese category. Now, if we had two people who weighed 225 and were six feet tall, then both of them would be classified as obese, even if one was a blimp with a 45 inch waist and the other had a 36 inch waist and was built like a champion bodybuilder; they would still be classified as obese. This is a major problem with BMI. We could greatly improve the calculation, if we also used the Body Density Index, BDI. Density is weight divided by volume, or for comparison purposes we could use specific gravity. Specific gravity is density divided by the density of a standard (usually water).

Consider that people usually float in water; although lean people can sink, and fat people float very well, high in the water. The calculation could be the weight of a person divided by the weight of the water that person displaces. The principle is the one that Archimedes discovered when he was trying to discover whether Hieron II, the ruler of Syracuse was cheated in the purchase of a golden crown. It was supposed to be pure gold, and Archimedes discovered the concept of comparing densities of materials by comparing the water they displace. One ounce of gold displaces less water than one ounce of a gold alloy, because gold is one of the heaviest (densest) metals. He thought of this when he got into a bath and noticed the water level rise in proportion to his volume.

This could be put into use with a large tub being installed in physicians' offices, and their customers could get in the tub to determine their volumes. It might be wet and sometimes messy, but it isn't something that would have to be done often. The tub could simply have a connected tube for determining the depth, and the tube could have marks that would indicate the volume about the rest level.

A Body Density Index already exists, but it is more complicated, and I am uncertain whether it is meaningful. It includes waist and buttocks measurements, so it takes fatness into consideration. Tweaking this index might work as well as using actual BDI.

We would want to add a factor that was an indicator of bone mass and something that gave more of an indication of muscle mass. For bone mass the girth of a wrist, or that compared with something else might be useful. Different people strengthen different muscles, so a single measurement that would be an indicator of muscle mass is difficult; I will keep thinking about it, but if a reader has a suggestion; that would be good.

I am surprised that no one has come up with a solution to this, and maybe the person who wrote the linked site did, but it doesn't look like it works all that well, but it gets around the problem of the tub of water.