It's a what?

It's a PEARlescent light bulb of course ............ groan

Went on holiday - put on 4llb
Week later, 4 llb lost
Week later went on holiday - put on 4 llb
Week later, 4llb lost
Week later - put on 4llb without going on holiday and its stayed there. That's not in the script. WTF is going on?

Decided it is as much to do with my exercise regime as what I eat. I had swapped spin, body pump and kettlercise for aerobic sessions in the gym but I just don't think they do as much good. So, recently had my first classes for about 3 months. Ouch! If the old adage no pain no gain is true, I am going to gain a lot of benefit from these classes.