100 lb. Club - How does body fat % work???

View Full Version : How does body fat % work???

09-02-2009, 12:07 PM
Bare with me here...I have a scale that measures bodyfat...embarassingly it tells me I'm between 47% and 48% fat, attractive huh? Well, I got to wondering how this relates to my body composition and I found this site (http://www.healthchecksystems.com/bodyfat.htm) and decided I'd calculate my stuff out the way they do in the example. Here's what I got:
266lb woman, 48%fat, wants to lose 131 pounds
--inital body fat: 266lbsx.46= 122.6lbs of my body comp is fat
--lean mass: 266lbs-122.6lbs= 144lbs
---I therefor couldn't lose 131 lbs because I don't have 131lbs of fat to lose... so I went on with the next part...
I'd like to be at say 20% body fat...
--144lbs lean mass + 53.2lbs fat= 197.2lbs...WTH??
Okay, so I thought BOGUS! So I tried it a simpler way... I'm 48% fat now, I want to be 20%...I need to lose 28% of the fat...so 266x.28=74.48lbs need to be lost...which puts me at 192.
I am completely lost and confused. Does this mean I'll never see 135 or are my calcuations wrong? Or maybe I'm really like 70% fat (I found a site with a calculator where you take measurements of various body parts and it too came out about 48%) and once your over like 40% fat no fat measurments expect the super scientific come out right????
Or is fat supernatural somehow and none of this would even apply??

09-02-2009, 12:20 PM
Some of your lean (nonfat) mass is fluid and other tissue associated with your current size. So some of that will go down. Also, it's very hard to retain *all* all muscle mass when losing that much weight. Some of that muscle exists because you're carrying around 130# or so of extra weight a day!

Also, this part of your math is wrong:
"I'd like to be at say 20% body fat...
--144lbs lean mass + 53.2lbs fat= 197.2lbs...WTH??"

If you want to end up at 20% body fat, you have to apply the 20% to your goal weight, not your current weight. So if your goal weight were 135, 20% body fat would be 27# of fat not 53. Also at total weight 135, lean mass of 80% would then be 108#.

If you kept *all* your lean mass (theoretically) and shot for 20% body fat, you'd weigh 180 # (144/0.8). As you have already guessed, a 5'4" woman weighing 180# with 20% body fat would be very, very muscular.

And of course, the 47-48% (144# LBW) is just an estimate. At 267#, every 1% error in body fat estimation is 2.67#.

09-02-2009, 12:34 PM
i think Juli has that one covered!! I would like to say you are being very smart and reasonable trying to attain a "body fat" goal rather than a scale weight goal. Body fat is such a better measure of overall health and fitness, and the number on the scale, and especiall number slike BMI dont mean anything. For instance, at 5'4 i am 133 pounds. Thats is just BARELY within the acceptable BMI range. However, i am only 14% body fat, with only 19 pounds of body fat and 114# 's of FFM, or fat free mass. I cant physically really lose anymore fat, and if iw anted to drop to a lower number on the scale, i would have to sacrifice lean muscle mass in order to do so!!!

09-02-2009, 12:35 PM
Ty for clearing that up...I was terribly confused.
I don't want to be She-Hulk, :D so I'm good with saying bye to some of the extra mass my body's added to help lift the bulk and I didn't take that into account. You always here about needing to lose fat..not muscle, so I never thought of it the way you put it. It makes sense!
Guess I have a number of pounds to go before I stress too much anyways :) but it was a momentary "bummed out" moment. I am really excited about my goal (I've never set one this low and thought it was pretty reasonable for my height/bone structure, if higher is healthier I'd be fine with that too, but 190!?!) and it depressed me to think I might be fooling myself!
TY TY TY, again.