It really depends on the individual trip with me. On shorter trips my eating doesnt really change, but my appetite shuts down on longer more strenuous hikes. The last few years have found me doing only a couple 3/4 day trips of shorter duration and I didnt see much change on those. Even brought some burgers, chili, and corn on the cob up Bear Creek last summer. Mmmmmm yummy!! On longer more strenuous hikes it typicall takes me a day or two to get an appetite. In 2001, I joined my brother for 6 days while he did the JMT. I was his resupply and carried our food for the 4 days we were together. Met him at Edison and came out over Hell for Sure via Mt Goddard/Martha Lake (last 2.5 solo). We basically kept to his schedule of no less than 10 miles a day (

which is alot for me

) and I basically didnt have an appetite the whole trip. I made myself eat, but I guess the physicallity of the hike just ate up me appetite. Came out with about 1/2 of my food, which always makes me grumpy as well.
One thing is kinda funny though. When I first started we always came out and wanted a Xl pizza, a jumbo burger, etc, etc. Stopped that along time ago because after I long trip I may want to eat like a horse, but I cant. Dont get me wrong. I still love a good burger from the Joly Cone or even more likley a piece of pie at the VVR (again Mmmmmmm yummy!!!) and one of my fondest memories is chowing down a burger at the store that was up by the Rock Creek lake Pack Station (not sure if its there anymore, been 20 years and they also had showers), but Ive learned to scale it down until I get back home and have a day to acclimate to the flat lands.