In one of Gary Taubes' books, he said (and I'm paraphrasing) that your body will continue sending hunger pains until it gets the nutrients (essential proteins) it needs. He added that it's possible that 800 calories is all you really need in a day if it's packed with nutrient-dense foods like animal protein and devoid of "empty" calories (or those you don't really need, like a lot of carbs, even "good" carbs).