I recently discovered from a video and a Thread on Reddit, that for example cerials are making you more thirsty.
Another reason why you need so much food and drinks could be that you are sick. You can get sick for example after a zombie hit you and you are bleeding, then you have to desinfect your wounds with alcohol and then after some time you get messages that "your wounds are less hurting" and so on until you are healthy again, but because the alcohol don't work instant you can become sick before become healthier, just give it some time to work. During the workingtime of the alcohol you are sick then and have to drink and eat a lot more.
Another reason for becoming sick could be that your "clothes are discovered in blood", in this case you need some deinfection spray to desinfect your clothes very fast so you have a chance not to get sick.

Link to video: https://www.youtube.com/watch?v=ixn6DwuKQkg

Link to Thread: http://www.reddit.com/r/dayz/comments/1tc20t/stayin_alive_have_not_died_since_1st_hour_of/

I think the thirsty system is good and realistic, if your are jogging the whole time, you are getting thirsty very fast, and you can fill your stomach at a well.

At the hunger system I don't agree, you are getting too fast hungry. I didn't find much food, but perhaps to hold balance it could be a good idea to low food spawnrate if you wouldn't be getting hungry that fast.