I have been an elite athlete in my life and know a fair amount about this, including coaching distance athletes. Losing 10 lbs will not only reduce your energy levels during the trip, it can result in serious illnesses during and afterwards. Fasting is extremely dangerous, simply do not do it. When you have too little nutrients your body uses your muscles. Not good.
If you are concerned about saving weight follow the guidelines Braeton discusses regarding calorie dense and BALANCED foods. You can save allot of weight this way. Your body needs everything, fats, carbs, minerals, vitamins, etc. Don't skimp here. Eat ALL the time, I mean all day. Drink plenty of fluids AT THE SAME TIME YOU EAT to avoid hyponutremia too. (low electroyte condition).
Depending on your age, condition, pack weight and body weight, and total effort each day give yourself enough calories. Your ability to accomplish this will soar!
Most long distance folks, myself included have found the real secret to these events is to learn how to start out at the pace one will finish, to allow the body 3 days to acclimatize both to the effort and altitude, and to be steady with mini breaks for stretching, yoga, and micro naps.