Body fat percentage

[Ngala]

I've been with my Personal Trainer for 8 months now and we are working on reducing my overall body fat percentage and measurements. at the beginning we would do the weigh in before a training session and then we switched to doing it after a training session as it seems that my body fat % is LESS at the end of a session than at the beginning.
Today was my weigh in, and we went in before my session as the room was free. Everything was good but my body fat was at 25.7% We decided not to record that as it would have to be wrong because my weight came down as measurements down too. So we did a 30 min intensive workout and went back in to try the body fat. Second time round it came down to 24.2%. So in a matter of 30mins I lost 1.5% body fat?!! My trainer is a bit stumped on this one and said he will try it with some of his other clients.
Does anyone now why this would happen? I've tried googling it but have not found anything..

[RhiChiChi]

How is the body fat measured? Just on scales? If that is the case I would think it was more fluid loss from the workout than body fat....

[Ngala]

My gym has one of those 'things' with handles that meant to send 'waves' through you body to calculate the body fat.
So the trainer puts in my weight, weight, age & sex. I hold it out with my arms in line with my heart and then hold for a few seconds and it calculates the body fat.

[AnyDream]

The thing that calculates it is based on electrical resistance...something to do with muscles holding more water than fat, when combined with a weight measurement it yields an indication of body fat. I'd say it's more likely that the fluid/electrolyte balance in your body changed while you exercised (and drank water during exercise) and that would be reflected in a different reading.

Your trainer should understand that stuff otherwise it's a pretty meaningless kind of number to give you.

[Ngala]

Yes you would think so wouldn't you Marydean?!! But he is a male trainer, so probably doesnt understand the very complicated female body! Especially mine as it seems to be very very strange at times...
So what would be more accurate - the reading before or after my sessions?

[RhiChiChi]

Based on what MD said, then I would say beforehand. You may sweat more/less or drink more/less during every workout

[Rouge]

I was just about to say it depends on how hydrated you are... yes I'd be more concerned that he didn't know that too.

[Ngala]

Hmm well my weigh in this morning doesn't make sense!! I lost 1.5kg and bout a cm but gained 1.6% body fat?!!

[appletree]

Aaahhh, so that explains it! I just had another check and we did it after i had already done a session at the gym and it was nothing like the first reading i had done! I decided to just ignore it and concentrate on other measures.