I was eating tons of fast food construction season and when laid off work for the winter (5 months usually), I would lose a little weight in winter sitting around playing video games (and drinking every weekend) and gain it back when I started working. I have a very high metabolism.

I know it was muscle mass that I gained when working based off my complete lack of any body fat at 120lbs and at 155lbs (I am 5'5"), and because i could tell my strength increased by a lot in that time, when I started I struggled to lift a 88lb bundle, was dying after loading 7 of them. by the end of the season I could load around 100 a day on a roof and if the roof was low enough I would throw them from the trail up onto the roof.

This was when I was 21-23, I am 35 now and since then I haven't had any hard labour jobs. I still fast food or processed food most of the time but not as much as when I was younger, 2 mcdoubles and large fries is a full dinner for me. But I still cannot gain weight, in fact I have been losing weight the last 2 years I went from 125 down to 115. I do barely any physical labour and don't work out. I just game most days.