MRE? Those are the green bags you see the Army eating out of - they are VERY high in calories, and usually have all the vitamins you need - 2 are meant to keep a soldier in combat feed for a day - for days on end

I saw something the other day about actual firefights/special forces missions - the can't keep it up for too long - you actually burn so many calories, that it's almost impossible to EAT as many as you burn