I worked on trail crew for the National Park Service a while and found that I couldn't eat enough food to keep up my riding habits. They only summer jobs, and it was always "go go go" with both work and recreation; we were always hiking and riding whenever the weather was nice, and California summers had lots of nice weather. I just started dropping weight, and I don't weigh much to start with. I was eating tons of pasta, avocados, oatmeal, etc, but just couldn't seem to keep weight on, and I started to lose muscle. I didn't have the time to put the effort into eating properly -- we were too busy having fun!
So, I started drinking weight gain shakes daily, and that helped a lot. I never actually gained any weight, I don't think, but I didn't waste away like before.