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Fat measuring machines, are they reliable?

I used a weight and fat measuring machine in Boots. I had lost 1.5 lbs since last week. But gained fat percentage, up to 34.5 from 33.9 last week. Does it matter than I had just finished my run and had drunk some water. I have been more active than normal and had even done some weights so it doesn't seem to make sense that it has increased. Are they reliable? Perhaps they are not that sensitive?

If anyone can shed any light......

10 Replies

Very curious! How does it measure fat?


Sends an electric current through the body and measures impedance. Fat and lean tissue differ. I suspect that it is not very accurate though...that's my excuse.


ooooo - sounds spooky! I bet it isn't as accurate as it likes to make out! Just because it gives results with decimal places doesn't mean it's that accurate overall.

Just did some googling: "You must not be dehydrated - do not use first thing in the morning or immediately after a workout or after consuming alcohol. Use 3 hours after a meal and after voiding your bladder. Women may experience fluctuations with retaining water, and those on diuretics may see inaccurate readings. You need to weigh yourself without clothes."

Bet you didn't weigh yourself without clothes in Boots! Sounds as though it's more of a ballpark thing rather than being all that precise.

Now I want to go to Boots (dressed though!) and try one out - might do that this afternoon, just out of curiosity.


Thanks. Didn't think to google it. If you do go to boots then take the right change, 70 p. don't do as I did and take your shoes and socks off then have to walk the length of the store to ask for change!


:) A good tip! Get the change ready first.

I just found this too:

rather technical (I'm used to reading science papers, but this one had a lot of unfamiliar jargon) - but I think it implied that they can be quite accurate, but, not necessarily meaningful on a short-term basis, as variables such as when you eat, drink, exercise, and for women, monthly cycle (and menopause) can all have quite an impact one way or the other. So maybe weekly is a bit too often to show anything much useful.


I think you're right,perhaps once a month might be more sensible. Now I'm doing waist measurement and waist to hip ratio. It's quite simple really -ijust have to lose two stone, ho, ho ho.


I get weighed and body fatted at the gym each week. It's generally around about the same figure (or has been since I hit target weight at the beginning of December) but as the lady who measures me says, it varies depending on how hydrated you are, whether/what you've eaten/drunk that morning, your age (it goes up when you go up a birthday!!!) and possibly the phases of the moon and whether you were out dancing round with the fairies last night. So I wouldn't take a single reading as gospel, but if you go in at the same time/ place/hydration each each week or so you should see a trend.


I guess over time it would be meaningful. I don't know why I am bothering really because I only have to look at my tum to see there's too much fat there! I was just hoping that it would be another way to measure progress.


Measurements of body fat tend to fluctuate a lot more than simple weighing, and different methods of estimating body fat yield very different results. Just as different scales may give different results, different body fat analyzers can provide very different body fat estimates.

Even with the same scale the numbers will vary because:

Weight loss tends to produce substantial, continuous, and unpredictable changes in body water content. Because body fat analysis is determined by water content in the body, results can vary considerably from day to day.

Hydration status affects body fat results. If you've just worked out, there is less water for the electrical current to flow through. This may result in a higher body fat result. In the same way, if you measure your body fat after drinking a lot of fluid, it may appear that body fat is lower than it really is.

Skin temperature can have an influence too. Measuring body fat in warm, humid weather when skin is moist will yield a different result than if when skin is cold and dry.

Results may not be accurate for persons under the age of 16, or persons with an elevated body temperature, diabetes or other health conditions.

As with weight, when your goal is to change body composition it is better to track trends over time than to use individual daily results.


Many thanks for taking the time to reply. I'm not going to focus on it too much and just stick to weight and waist measurements I think.


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