They had rat poison. Doug didn't bring them fresh food every day - they had to ration it, but Ben had poisoned it all - that's why he stopped eating and Doug made the comment (while inspecting the food that had been left for days) that he needed to keep eating to keep his strength up.

I agree. I'm fairly sure Ben poisoned Adam - which led to his illness. Doug figured it out and realized that's why Ben stopped eating the food in the cave.