Can I take my fitness tracker with me on a flight? should it cause any issues while passing through security?
Assuming we're talking about a watch-style or pedometer-style fitness tracker, it should be no problem. Just take it off and put it through the X-ray, like you would with any other electronic device. In rare cases, it's possible the security agents will ask you to demonstrate that it's functioning, at which point you could show them the LCD display, but it's pretty unlikely they'll ask.
Source: personal experience. Also the TSA's "can I bring" feature says a Fitbit is fine.
I took my Fitbit HR on a flight from Australia to New Zealand in January 2016 and there were no problems at all either way - since the device looks like a watch it was not questioned, and its function was unaffected by X-rays. The main thing to consider (and prepare for) is the possible loss of a bit of data due to the change in time zones.