If we go to Mars, what people forget is that mars has very little in the way of nutrients in the soil or even nitrogen in the air. Almost every nutrient we need will have to come from earth in some way, even the nutrients that go into the plants that we grow there. What does this mean? It means that everything is going to have to be reused, everything. We can't afford to let bodies be put into the ground and waste 70kg of nutrients that we brought up from earth, humans will have to be eaten or else the colony won't be able to sustain itself. Everything has to be part of a sustainable cycle of nutrition.