This is a really fun discussion. Thanks all for contributing. I love that there is conflict between low- and high-fat diet proposals here.
Someone asked what my level of pre-hike fitness will be. I won't be "gaining" fitness on the trek, and at day 0, will pretty much be in peak shape.
The trek will last around 20 days. There are no trails, no roads, no resupplies, no opportunities to refuel in town. I've done similar length hikes on long trails with the typical 5-day resupply period carrying 2-2.5 lb food/day and gorging at trail towns. Not the same, this time, that's a promise. Terrain = tundra, tussocks, gravel bars, mountain shale, scree, etc.
Fishing/foraging: no, not on this trip.
I'm monitoring my physiology closely and will have enough of the raw data to do a pretty good calorie balance when all is said and done. I've doing 4 metabolic tests (pre-training, mid-training, just before the trek, and just after) to watch changes in the body in response to all this.
My HR at VO2Max is currently around 196 bpm. This is not predicted by equations, but measured in a metabolic test. All my training is done with a Suunto T6, and I'm getting a very good picture - in the field (not gym) of what kind of metabolic response I'm getting in various terrain, altitude, trekking speed, etc. The responses being measured in the field are heart rate and respiration rate. From these, a model is calibrated with results from metabolic testing (lung capacity, max ventilation rate, VO2max, Max HR, Resting HR, Max Respiration Rate) to then predict altitude, distance, heart rate, and respiration rate-correlated ventilation rates, VO2, energy burned, and excess post-exercise oxygen capacity. It's all extremely geeky but very cool.
Doing it right with the proper calibration and using foods that are consumed efficiently, I've got my caloric balances (food intake + body fat lost = energy burned during exercise) dialed to within 5% on most 100 mile training hikes.
But those training hikes are predominantly a liquid diet. I don't know if I want to do that for 20 days.
I'm experimenting with the Accelerade-Endurox system as well as the Perpetuem-Recoverite (Hammer) system. They both taste fine, but I'm actually leaning towards the latter because it seems to keep my physiological parameters more stable while working at lower VO2 for long periods. Results are about the same when working at high VO2 levels. The main difference between the two systems is that the Hammer products use a complex carb base (maltodextrin), while the Accelerade products use simple sugars as the carb base.
So, for my long hikes to date, I've been starting the day with a big bowl of carb rich hot cereal, loaded with butter and dry whole milk. Then it's on to the liquid diet with some nuts, chocolate, dry fruit, and meat sticks. scattered throughout in small doses for variety, with a gel packet and a piece of Jolt Gum thrown in here and there when feeling iffy (no caffeine lectures please, it's used very sparingly ;)). Arrive camp, the first thing is a recovery drink (glycogen replacement and protein) with a few electrolyte capsules. Set up camp, get into bag, cook a really high carb dinner with MORE BUTTER (!) and fatty gravy plus fatty chocolate. Repeat.
What I'm trying to dial in here is a diet that (1) maintains very high energy levels throughout the day to maintain a brisk pace and (2) offer a solid recovery period so the whole thing can be repeated indefinitely without eating into the muscle mass that is desirable when you need it most in the mountains.
As for me being lean - no way - I want some reserves.
Fasting - great point. I can't tell you how valuable this exercise is. If nothing else, it gives you the data that without food but with some fat reserves, you'll be O-K ... for a little while...