When I was in the infantry in '75 and going on field exercises, I remember eating every "Lifer Bar" that I could find in the C-rations; they were the chocolate covered cookie-looking "desserts" in certain C-rations. They completely stopped my digestive system from working during the FTX. To get it running again at the end of the FTX, I would eat all of the saved "John Wayne Bars"; they were the chocolate/peanut candy bars. They could unblock everything in my system and I'd spend the first few hours after an FTX in the latrine losing several pounds ...
I guess you guys had two enemies to worry about. The VC/NVA and the food.