AIBU to believe scales claiming to show body fat % are a load of rubbish

[fanjotastic]

Lighhearted

How is it possible those scales that claim to measure your body fat percentage can show me as 10% different depending upon inputting different gender or age.
They claim to use some sort of clever electrical current to accurately measure your body fat. If this was true, surely the fat percentage would not change depending upon inputting different facts like age.
Are they just rubbish and a con?

Yes of course they are rubbish. Body fat cannot be measured without actually being well......measured!

I don't understand how they work, but there are lots of different methods for measuring body fat and I come out within 1% the same on all of them, so they must be fairly accurate IMO.

I don't believe they can be very accurate - not the versions for home use at least.

Perhaps the fancy pants scales they use in medical/fitness settings are especially accurate for this type of measurement.

Marphe
But if I stay the same weight and height but change my age the scale must be making assumptions about my body fat based upon averages for my age as opposed to actually measuring it.

My home scales have a fat% on, but I step off before that bit comes on. I dont understand how they work. Would love to know what my fat % is though, even though its not what I would want to see/hear, but it might make me a bit more motivated.

Yes, probably. Perhaps it uses the same calculations the YMCA method uses? I believe professionals consider callipers the most accurate method, but the scales are easier and YMCA cheaper. For me, they all give the same result.

[[http://fitness.bizcalcs.com/Calculator.asp?Calc=Body-Fat-YMCA YMCA body fat calculator)

If you put the numbers in you can test it.

They work by sending a very mild electrical current through your body, and seeing how long it takes to get the whole way around. Different types of tissues conduct electricity at different speeds, so the scales then - yes - make assumptions about the quantities of different tissue types, including fat.

So for example, water is a good conductor or electricity, as is fat (while muscle is a bad conductor - it takes electricity longer to travel through it). If you're carrying a lot of water, then the machine won't be able to tell the difference between the water and the fat. Although if you're dehydrated, that also affects the results, so having a glass of water half an hour before using will usually increase the accuracy.

Men and women will typically have different proportions of different types of tissues, so the scales make assumptions based on our gender about those proportions. And the ageing process affects our tissues - e.g. older women typically have less muscle, and the muscles become softer when we're over 50 - so assumptions based on age are also used to understand why the electricity is travelling at the speed it is.

They will never therefore be 100% accurate, but few of us really need to know completely accurately - a good estimate is fine.

The most accurate way to measure body fat is with calipers, but personally I'm never going to do that so body fat at-home scales tell me if I'm going up or down & need to do something about it.

Sorry that wasn't a very light hearted answer...

thanks for that LordEm

However, how can I step on and it measure me then step on again a few seconds later and just by changing my age or gender it comes out up 10% different.

whatbI am saying is that if the electrical current is acvtually measuring anything why would it be so different for different input settings.
To me that shows the electrical current is not measuring much.

Callipers aren't the most accurate way of measuring body fat as they only measure subcutaneous fat.

Measuring body composition accurately is actually quite hard - dual energy Xray (DEXA) is probably the gold standard currently.

Well - if you are changing your age by 1 year, then I don't know.

Changing gender or changing your age by 20 years, or your height by 10am - then it changes the assumptions it makes about what the speed of the current means.

You're right to say they're not accurate, and it might be even that body fat has no connection to health (where you store fat is more important than how much of it there is). I don't think they're a con though, just not the bible of weight loss. I have some & I find it helpful but I find they're very consistent for me - though I haven't tried stepping off & stepping straight back on again...

They measure body fat using an electric current, or electrical impedance, but it doesn't actually travel around your whole body, it just goes up one leg and down the other. It doesn't measure the top half of your body, and assumes an average distribution of fat over the whole body. Thus if you are very pear shaped, it is likely to over estimate your body fat, and underestimate it if you are very Apple shaped.

It needs to know height to work out how much "body" it needs to add to its measurement, and it needs to know sex and age as the distribution of fat differs in upper vs lower body depending on these factors, so there is a bit of guess work involved based on statistical norms.

If it measured the whole body, it wouldn't need to know these things.

However, if you put the right info in and are fairly averagely shaped, they are surprisingly accurate!

LordEm I'm going to be pedantic here and say that actually fat is a pretty bad conductor of electricity (contains relatively little water).
But that doesn't change your basic description of why they work (thank you!)

Happy to be corrected!