body fat percentage calculations

[liksani]

hmm.. anyone know how accurate those things where you enter weight, height, age, and then putting your thumbs on plates are at actually calculating a correct body fat percentage? i was just wondering how I could find out.

[witneylaura]

BMI is a fairly hit and miss calcualtion in lots of ways- it is really just a weight and height calculation and there are lots of places on the net you can get it calculated. For example, http://nhlbisupport.com/bmi/
Anything, or anyone, that knows your weight and height can calculate your BMI, that is come up with a number. But it is the interpretation of a BMI calculation that is more complicated, and that can depend on why you want to know in the first place :) BMI itself in terms of healthy weight is not so great at taking into consideration ethnic issues or 'short people' for example. I'm short and my BMI is fairly high, because of the ratio of my legs and torso.
Hope this helps! And how do I know this stuff- I used to work in an eating disorder clinic where I had to know lots about BMI :rolleyes:

[Brina]

I have never seen the ones you put your thumbs on. I know there are scales that have special foot plates that are supposed to calculate your body fat and body water percentages, and according to some reviews that I have read they do provide a fairly accurate readout.

[velogirl]

You're referring to bio-electrical impedence. This is the same methodology used in the Tanita scales. It is highly dependent on your hydration level. So, take your weight in the morning when you're dehydrated, and your weight is lower but your body fat percentage will be higher. Take it in the evening when you're better hydrated, your weight will be higher but your body fat lower.

Bio-electrical impedence is best used to measure trends over time. For example, if I step on my Tanita scale the same time everyday and it says I have 19% body fat, then it begins falling to 18% and then 17%, life is good. If, however, it begins trending upward, I might be concerned. Now my BF% might not really be 19%, but I can see the trend -- the change over time.

The most accurate ways to measure BF% (not BMI which is simply a calculation) is with a skilled technician using fat calipers. You should try to find someone who does this many times a day. Most personal trainers will be able to do it, however the secret is to find someone who is skilled. Then, in three months or so, return to that same person to do it again.

There are many other methods (including a couple of displacement methods --- water and air) that can be more accurate, but they both depend on the clients ability to perform the test correctly.

Lorri

[RoadRaven]

I know very little about body fat...

but I do have a question, because some of the people I work with seem to think (obsess?) about body fat indicators...

If your body is fit, doing what you want or improving towards what you want, why does one need (or want) to measure ones body fat?

[Raindrop]

Velogirl was right on with her description of how bioelectrical impedence works and the only thing I would add to her excellent post is that, for those that want to watch the trends of their measurements, bioelectrical impedence is just one way.

As a personal trainer I have to work with a lot of different personalities that want to chart their results in different ways. For those obsessed with scale weight numbers I suggest weighing no more than once weekly. For others, they're satisfied with my caliper measurements taken at the end of each of their "terms". Others need to either rely on monthly inch measurements...and the best measurement...a pair of barometer jeans. Those jeans that fit best when you feel fittest! Try them on weekly. (Jeans are just an example. Use whatever clothing you deem as your barometer).

BMI isn't extremely useful in real life due to the difference of muscle to fat ratios. In fact, I believe it's in the same range as using scale weight as a measure of fitness.

Still, if you're looking to change your body composition, re-read Velogirls post, and seek the downward "trending" of whatever measurement method you choose.

Or...(a personal plug), hire a Personal Trainer or Nutritionist to guide you on your particular journey.