BF% from Scales

[Gainsdalf the Whey]

So yesterday I found out that we have a scale in our gym that measures body fat % by sending a small electric current through your body and measuring the resistance. I assume it gets body fat percentage by taking the resistance and dividing by distance the current traveled, probably the average distance from the foot, up to the pelvis, and back down to the other foot. It would figure out this distance using whatever height you enter. This would give a resistance per length, and that value I assume would vary with different body % as fat would have a lower resistance than muscle due to its high water content.

Now, how accurate are these? I was assuming i was 30-35% body fat from my pictures, and maybe self image I guess, but the scale told me I'm currently at 26.2% body fat. That gives me 184 lb of lean body mass, much higher than I expected. Also, on another note, I broke 250 lb for the first time since last October.

Finally, it told me I have a BMR of 2380 kcal, which is about 300-400 lower than other sources have said for my weight. How would it calculate this?

[Guest Snake McClain]

I have heard these CAN be quite accurate but it depends on how much water is in your body. if you are dehydrated it won't be accurate and if you have had too much water (whatever that means to the machine????) it can be inaccurate as well. But as a whole i've heard reasonable thing. Just what I have heard/read though.

[Gainsdalf the Whey]

Yeah, the reason water really affects it is because water is very similar is resistance to fat because fat cells are full of it. The more water weight you have, the more fat it will think you have. However, this is heartening for me, because I had 3-4 liters of water on the day before I weighed in on it. I'll try it out again on a weekend when I go in the morning and don't have much water yet.

[bigm141414]

So yesterday I found out that we have a scale in our gym that measures body fat % by sending a small electric current through your body and measuring the resistance. I assume it gets body fat percentage by taking the resistance and dividing by distance the current traveled, probably the average distance from the foot, up to the pelvis, and back down to the other foot. It would figure out this distance using whatever height you enter. This would give a resistance per length, and that value I assume would vary with different body % as fat would have a lower resistance than muscle due to its high water content.

Now, how accurate are these? I was assuming i was 30-35% body fat from my pictures, and maybe self image I guess, but the scale told me I'm currently at 26.2% body fat. That gives me 184 lb of lean body mass, much higher than I expected. Also, on another note, I broke 250 lb for the first time since last October.

Finally, it told me I have a BMR of 2380 kcal, which is about 300-400 lower than other sources have said for my weight. How would it calculate this?

Depends on the brand, some are better than others. But usually they can be 5-10% off or more depending on hydration levels.

And as to how they work, they just calculate the resistance of your body (Resistance = voltage sensed/current) and then use all that other stuff you entered and plug it into a formula which approximates your body density. They don't look at distance traveled. The formulas used aren't standard.

[Gainsdalf the Whey]

Depends on the brand, some are better than others. But usually they can be 5-10% off or more depending on hydration levels.

And as to how they work, they just calculate the resistance of your body (Resistance = voltage sensed/current) and then use all that other stuff you entered and plug it into a formula which approximates your body density. They don't look at distance traveled. The formulas used aren't standard.

Why would it need my height then? Resistance would increase with distance traveled, so a 5'0" guy at my weight and same bf% would have a different resistance than me at 6'0".

[Guest Snake McClain]

I've heard that they would normally only be off by a couple % not 5-10. yowsers.

[bigm141414]

Why would it need my height then? Resistance would increase with distance traveled, so a 5'0" guy at my weight and same bf% would have a different resistance than me at 6'0".

A little light reading:

​http://www.ake-nutrition.at/uploads/media/bia1_Kyle_et_al.pdf

Contains forulmas used in some analyzers as well.

[Naturally]

Yeah, the reason water really affects it is because water is very similar is resistance to fat because fat cells are full of it. The more water weight you have, the more fat it will think you have. However, this is heartening for me, because I had 3-4 liters of water on the day before I weighed in on it. I'll try it out again on a weekend when I go in the morning and don't have much water yet.

Don't think so... fat cells are full if fat. That's kinda the point of them a cell membrane, a few organelles squashed around the side, and a big droplet of fat taking up all the space.

I think if you use it like any other measuring device, and use the one device consistently, having drunk the same amount of water every time you get on it, and take an average of 3 readings (i.e. don't panic if you're % shoots up one week, take it over an average of that week, previous week and next week's reading), then you'll at least know which direction you are travelling in.

[bigm141414]

Don't think so... fat cells are full if fat. That's kinda the point of them a cell membrane, a few organelles squashed around the side, and a big droplet of fat taking up all the space.

Yup. Drinking water = decreases the impedance = decreases the BF% number. Muscles are full of water and electrolytes (Brawndo: It's got Electrolytes!) so they are good conductors. Water is also a good conductor which means more water in your system the better conductor your body is which means the analyzer thinks you got mad muscles. Fat, not so much at the whole conducting electricity thing. Stupid fat. Not good for anything.

[Waldo]

Now, how accurate are these? I was assuming i was 30-35% body fat from my pictures, and maybe self image I guess, but the scale told me I'm currently at 26.2% body fat. That gives me 184 lb of lean body mass, much higher than I expected. Also, on another note, I broke 250 lb for the first time since last October.

Finally, it told me I have a BMR of 2380 kcal, which is about 300-400 lower than other sources have said for my weight. How would it calculate this?

Without knowing the specific accuracy of these things...

Pictures might not be a great reference for bodyfat %. If you are built like me (kinda appears that way), the vast majority of your bodyfat is in one place. By the time I had lost 30 lbs I had great muscle definition in arms, legs, chest, and heck even beginning to get visible upper abs. This despite weighing in the 230 area. The more I have lost the more this effect has been magnified (If you saw a picture of me right now with a black bar 'shopped in between 2" above my belly button and where my pants sit on my waist, you'd think I was skinny).

Last night I was really pondering just how much more I have left to lose, since I really don't know. The two different body fat % estimate formulas I use are sufficiently far enough apart that it could be as little as 20 lbs, and as much as 35 lbs, to get to 10% bodyfat. I can see 20 lbs (2 1/2 milk jugs full), but 35 lbs I just don't see that much. Thus at least for now I am thinking that the Navy formula is giving the best estimate and my lean body weight is in the mid 170's. And if this is the case, 184 lb for your lean body mass is definitely not out of line. Just by looking at your avi pic, you do have a good bit more muscle mass than I do, but also seem to have my big frame and crazy ape arms (6'1" with 35" arms).

I have personally found for me the basic BMR calcuation out there (the one used by myfitnesspal) is pretty much dead on; it has always put me in the 2500-2600 area (exactly 2500 right now). When I put calculated trendlines on my weight graph based on target calorie intake, it is darn near dead on for the slope in stable periods (initial crazy loss and plateau after initial crazy loss being unstable periods). If it was off, the slope would not be correct.

[CaptainCat]

The Idiocracy reference made me chuckle, good times!

Like most methods of measuring body fat, you can't get much better than "a good estimate", and if you use it at the same time, and often you will get a picture of your overall average trend.