So my little guy started fasting around the middle/end of October. I'm not exactly sure when, because he is prone to missing a few meals here and there. So I've kept track of his weight since Nov. 19, which looks like this:
2018.11.19 593g
2018.11.26 583g
2018.12.03 582g
2018.12.10 570g
2018.12.16 571g

Before I've started keeping track of his weight, he was 628g if I recall correctly, but had a pretty big poop, so the 593g should be realistic. He is about 1.5 years old, and I think he should weight a bit more. He was a problem eater when I got him, so there were quite a few missed meals while transitioning from live mice to F/T rats.

What keeps me calm (yet) is that he behaves like his normal self. Hides all day, then comes out at night for a bit of climbing and streching. Doesn't seem stressed even if I get him out (he is always pretty chill about handling). Also he is drinking normally.

Humidity is around 50-60%, I mist the enclosure about once every two days.
Ambient temp is about 26-27°C(78.8- 80.6°F), the temp in the warm hide is around 33°C(91.4°F).
He has 3 hides, uses all of them.
Substrate is cypress mulch, I've switched from aspen shavings recently.

I've tried feeding about once every 7-10 days, but most of the time he doesn't really care about the rat.

So is it realistic that he's having a winter fast at this age? Should I be concerned about this amount of weight loss, or is it considered normal? (He did pass some urates after I started measuring).