We’ve all heard about BMI – body mass index. Your physician has probably mentioned this at some point or another, telling you what your BMI is and whether it is in the range for a healthy body weight or under or over weight. We’ve pretty much been led to believe that BMI is what we should all be focusing on.
But is this really true? Okay it gives us a rough idea of whether we’re underweight, overweight or just right, but how accurate is it? Are there other ways we could weigh ourselves to figure out how healthy we’re likely to be in terms of the amount of fat we’re carrying around?
Why doesn’t BMI work?
Okay so we’ve all been led to believe that our BMI number is set in stone. Whatever it is, it’s accurate and it tells us exactly what we need to know. Except it doesn’t, and in fact it doesn’t work very well at all.
You can get an idea of why this might be the case when you think about how BMI actually works. The basic calculation is this – you weigh yourself to work out your weight in kilograms. You then have to figure out how tall you are (assuming you don’t already know) in meters and square this figure. Divide the weight by the height you came up with and that will give you your BMI.
The glaring error here, however, is that two people could weigh exactly the same and be exactly the same height, but they might have totally different body shapes. It’s well known that some parts of the body are better to carry fat on than others. So person A could be carrying fat in the better areas, while person B might be carrying it all in the wrong places. But they would still have exactly the same BMI. So you can see that while it works to a certain extent, it fails miserably in other areas. We should only ever think of BMI as a broad brush stroke – a basic way of getting a rough idea of whether we’re a healthy weight or not. Clearly we need some other way of weighing ourselves to get more accurate information.
Step forward the hydrostatic body fat test
Okay it sounds like a mouthful but the clue to how this test is performed is in the name, HYDRO… Yep you got it – it involves water. Clearly this isn’t the kind of test you’d have done on a normal visit to your physician, but it can be done and it might be worth considering if you really want to know more about your weight and body composition. It’s more involved than the ‘measure your height, weight and calculate your BMI’ approach, but it is also said to be a lot more accurate.
It requires some serious equipment too – namely a hydrostatic water tank. You can’t just do this in a swimming pool, because proper measurements need to be taken to get an accurate result. The whole thing is done using a principle that was discovered by Archimedes. He’s the guy that jumped out of his bath and ran down the streets stark naked shouting “Eureka!” He did this because he had realized that the act of him getting into his bath displaced the water in the bath already by an amount of water that was equal to his own volume. Everyone knows the Archimedes story, and now you have probably just realized how it relates to hydrostatic weighing.
So first of all you would be weighed before you got into the pool. Once this has been done you would sit on a special chair that is designed for the purpose of weighing you once you go into the water. You have to wear the smallest amount of clothing possible to reduce the weight of it, and you also have to drive all the air out of your lungs. The chair is then lowered into the tank and you will go right underwater. Obviously you cannot breathe while this is being done and you’ve already expelled all the air you had in your lungs. But fortunately the process doesn’t take long. The only downside is that the process is normally done more than once, just so the most accurate measurement of weight can be ascertained.
The results will give you your body density, according to the Archimedes principle. In addition to this the amount of body fat you have is worked out afterwards using the information already gained. This is worked out as a percentage, so you can see whether you are within healthy limits.
Is it worth it?
You’ve probably noticed that the hydrostatic body fat test is pretty labor intensive. You can’t do it in a matter of minutes and you’d need to go to a proper center or venue that had the equipment ready to do the test. You will also have to pay for it, and the average cost is between $50 and $150
So is it worth having the test done? Well, it might be if you want to know just how healthy – or not – you are in terms of the amount of body fat you have. It can give you a greater understanding of the composition of your body. You should remember though that if you are dieting and you want to keep an eye on your body fat percentage, you’ll need to go back for repeat weigh-ins from time to time. This can make it a bit more expensive.
You also have to consider whether you’d be happy to sit underwater for a short period of time having already expelled the air from your lungs. I don’t mind going underwater but personally I’m not sure if I would feel particularly content to sit still in this situation – much less pay for the experience! Maybe one day they’ll invent a way to do hydrostatic body fat testing without the water.
You can see though that if you want to get an accurate measurement of your weight and your percentage of body fat, this could be the best route to take. A bit labor intensive, but more experts agree, it is a much better option than just figuring your BMI to determine body fat percentages.