In another thread (UL Road Blocks), there was some discussion about the difference between carrying water in your stomoch (i.e. cameling-up) and carrying water in a bottle (presumably in your pack). It has been claimed that one advantage of a filter (which allows you to drink water immediately) is that you can camel-up at a water source and thus not have to carry as much water. Those who use chemical treatments will have to carry their water for the time it takes for the chemicals to work before they can drink the water. It was suggested that carrying water in your stomach is still carrying water and this does not represent an advantage that filters have over chemical treatments.
I would suggest that the way in which things are carried has a great deal to do with the energy exerted and is a topic that is under appreciated by those wishing to lighten their load, where the discussion mostly focuses on counting weight. There is no doubt that carrying more weight up hill takes more work (Work = mass x acceleration of gravity x vertical distance). But walking is a very complicated process. Much of the energy consumed is not just to lift mass against the force of gravity but also to move forward, and how you carry loads does make a difference.
Biometric studies have found that African women can carry loads of water equal to 20% of their body weight on their heads across level ground without burning any more calories than walking walking with no load. This was not true for people carrying the same load on their backs or by hand (in buckets). The secret is that having the load centered over your spine does not disrupt the walking mechanics. In fact, it even improves the efficiency so the energy saved through increased efficiency offsets the extra energy needed to carry the load (up to the 20% point).
http://discovermagazine.com/1995/aug/noskycapsneeded554
Extrapolating from this study, it would be reasonable to expect that carrying a kilogram (liter) of water in your stomach would be easier than carrying a kg of water in your pack, as it will be centered over you center of mass. On the other hand, one might also find that carrying a kg of water in front of you (say on a shoulder strap holster) might act as a counter balance for your pack on your back and make you pack easier to carry (fewer calories burned). So carrying more weight may actually "lighten your load". I don't have any data to prove that last suggestion, but it is something to think about. Maybe I will ask some of my friends in our University's Movement Science Department to look into it. They have the tools.