I am trying to find information about using fat as fuel for long duration activity - at high altitudes. All of my mountaineer friends insist that maintaining a sugar high by consuming primarily carbohydrates such as rice, potatoes, legumes etc at meals, and then simple sugars such as fruit, candy, etc. on the trail is the only way to be successful at altitude.
However, this flies in the face of what I have personally found to be useful in my normal diet at sea level. I am currently following a zone-paleo relatively low carb high protein diet with little to 0 simple sugar/starches.
Do you fuel your long duration activity primarily with carbohydrate or fat - even at altitude?
He had more questions but this is the gist.
The performance diet information in Extreme Alpinism is still valid. Find a copy.
But until then, a few strong positions:
All low-carb, so-called paleo, high protein, high fat eaters who do meaningful endurance activity are in a chronic state of glycogen depletion.
At genuinely high altitude the macronutrient composition of food will be a factor. Fat requires more O2 to produce a lower energy yield (yes, more calories but also less zing in terms of intensity). Fat slows gastric emptying. Fat (and other complex stuff, i.e. protein, fiber, etc) sitting in the gut monopolizes fluid for digestion. If you are going slow enough and at low enough intensity and you can afford to have compromised circulation (less blood/plasma circulating to deliver O2, etc to working muscles) then eating these nutrients while moving may be defensible. But probably not.
At genuinely high altitude caloric deficit is more of an issue than meal composition: most will eat whatever is appetizing and they can keep down.
At middle altitudes this (appetite) is less of an issue but a truly fit climber will be moving faster/ higher intensity at the lower altitudes so the gastric emptying issue is more important. Speaking of which, usually people who get into the weeds about pet issues like macronutrient composition for high altitude performance (or a similar subject) could get a much bigger bang for the buck by focusing on other areas of overall performance: fitness, recovery, technical ability, psychological training, etc.
If one thing is most important on this front I would say it is nutrient timing: eat in the most appropriate way to perform, eat in a different way to recover. Your sea level daily diet is one thing, and not transferable to high altitude (or even long endurance) performance. Cordain and Friel touch on the subject in their book. And stacks of research and experience show that using complex macronutrient profile fuels during performance yields gastric issues and a slower pace, invariably.
The better way to look at it may be this: what you eat and drink while you are doing the thing helps preserve and/or use efficiently the fuel you already have on board. Then, after you finish, what you eat and drink should replenish what you have spent.
Obviously, the definitions of high intensity, fast, slow, etc would have to be agreed upon before meaningful discussion can occur but the general lay of the land looks like this.
Start with Ex Alp, look at the Paleo For Athletes thesis, look into this, which has most of the research citations and those will lead down whatever rabbit hole you want to look: ????Sports Nutrition??????, (Maughan, Burke, IOC Medical Commission), Blackwell Publishing, 2002. Some will say 2002 is so old but most of the research cited here is still being used to support more modern fine-tuned lab and field work today. There are a lot of other resources, wading through them is your new job.
Along the way, experiment and record the actions/results for a while, and you will eventually find what you can a) tolerate and b) what improves/decreases performance.
After a few days of thinking about how to simplify this answer ... The better way to look at it may be this: what you eat and drink while you are doing the thing helps preserve and/or use efficiently the fuel you already have on board.