I also carry food – something that’s not going to freeze like an energy gel, some dried fruit or cheese.
You don’t want this to be too heavy, particularly on summit day, but it contains everything you need for the day, from food to oxygen (I’ll also have a spare pair of mitts in my bag). I carry one litre of water or a flask of hot liquid (one litre is not really enough, but it’s a tradeoff between hydration and weight). I also carry food – something that’s not going to freeze like an energy gel, some dried fruit or cheese. People often lose their appetite at altitude – I lose my appetite for chocolate but crave savoury things, so I normally carry Pringles, too.