Scale fat percentage

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Merkavar
Merkavar Posts: 3,082 Member
in Health and Weight Loss
can anyone explain this.

Also no need to post if all your going to say is scale are inaccurate etc

So scales, they check your body fat % by electrical impedance since fat and muscle transmit electrify with different efficiency.

So say I stand in a scale and it says I weigh 100kg with 30% body fat.

Why if I move slightly or pick up an object like a phone does the scales say 100.5 and bf% 30.2

Why does a change in weight change the body fat %. Is it adjusting the % based on weight?

To me I would think weight and bf% are seperate. Like say it calculated 30% and then I pick up a besser brick, how does this change the electrical impedance or why does this increase the body fat percentage? Do bricks contain fat? :)

Wrinkling my brain

Replies

  • bpetrosky
    bpetrosky Posts: 3,911 Member
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    It's a formula based on the bioimpedance and weight reading. Shifting your weight on the scale shifts the formula that the scale is using to generate the number.
  • ninerbuff
    ninerbuff Posts: 48,535 Member
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    When parameters are put into the scale, it's making it's best "guess" while staying true to what bio impedence is currently reading. It's rare to gain any weight (even if most of it is muscle) without gaining fat and vice versa.

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  • Merkavar
    Merkavar Posts: 3,082 Member
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    bpetrosky wrote: »
    It's a formula based on the bioimpedance and weight reading. Shifting your weight on the scale shifts the formula that the scale is using to generate the number.

    That's what I thought. But why does weight influence the %

    Say me and some 10kg lighter than me had the same impedance figure. For argument sake we both had an impedance of 120 and this means 30% body fat for me.

    Why wouldn't it also mean 30% for someone 10kg lighter. If we have the impedance?
  • bpetrosky
    bpetrosky Posts: 3,911 Member
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    I don't know the exact formula used, but think in the reverse. If two people have the same weight but one is athletic and the other fat, the bioimpedance would be affected.

    So if you have the same bioimpedance, but change your weight (by holding a heavy object, for instance), that would change the parameters in the calculation.

    Same principle, if you use tape measurements to calculate BFP, the scale weight is a factor in the formulas.
  • senecarr
    senecarr Posts: 5,377 Member
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    From Wikipedia on it: "BIA[1] actually determines the electrical impedance, or opposition to the flow of an electric current through body tissues which can then be used to calculate an estimate of total body water (TBW). TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat."
    It needs to know the weight because it is actually calculating the amount of water.
  • Merkavar
    Merkavar Posts: 3,082 Member
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    senecarr wrote: »
    From Wikipedia on it: "BIA[1] actually determines the electrical impedance, or opposition to the flow of an electric current through body tissues which can then be used to calculate an estimate of total body water (TBW). TBW can be used to estimate fat-free body mass and, by difference with body weight, body fat."
    It needs to know the weight because it is actually calculating the amount of water.

    Ok that makes sense I think.

    Thanks

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