Several times Harry, Ron, and Hermione have had almost nothing to eat:
The fact that Hermione was getting better at identifying edible fungi could not altogether compensate for their continuing isolation, the lack of other people’s company, or their total ignorance of what was going on in the war against Voldemort. “My mother,” said Ron on night, as they sat in the tent on a riverbank in Wales, “can make good food appear out of thin air.” He prodded moodily at the lumps of charred gray fish on his plate.
But why couldn't they simply ask Kreacher to bring them some? I know that in the beginning, he would not have cooperated, but once they gave him Regulus' locket, he would be happy to help. Why would they eat edible fungus when Kreacher would bring them a full meal?