Astronauts in microgravity don't use their legs much. This has negative health effects in the long run. But I wonder if it doesn't have the good side of lowering human need for food and water and thus the mass of supplies? Do people eat less in space than on Earth?
Almost certainly not.
First of all, astronauts work out for hours every day, specifically to prevent the deterioration of bones and muscles.
Moreover, the human energy budget is dominated not by the energy cost of moving around, but by a) maintaining body temperature and b) the brain.
Furthermore, just removing gravity from the equation isn't necessarily going to save you any energy when it comes to moving around: While you no longer have to fight against gravity in some situations; you do have to replace it in others.
(This answer only addresses calories: Water requirements are invariant wrt. gravity.)
From one NASA study which examined several space shuttle crews: "Energy expenditures were similar before and during flight." However, there was a decrease in energy intake (also noted in this journal article).
So the energy requirements seem to be the same, but for whatever reasons the astronauts seem to take in less food and water. So yes, people eat less in space, but they shouldn't!
Apparently the sense of taste is dulled in space. Thus favorite food stuffs are things like Tabasco sauce, and other spicy things. Thus they may eat less, due to reduced taste and desire to eat. (Partial answer at best, I know).
Here is confirmation from Chris Hadfield (video).
In the short term on long micro g missions, because blood no longer pools in the legs due to gravity, too much blood returns to the heart. This will make the kidneys actually reduce blood volume by increasing urine production for a while. I suppose you can say that during this time, we need less water.
FWIW, this loss of blood volume causes all sort of issues when returning to normal g after prolonged missions