"But you still need the energy! A partial solution is to switch your diet to less complex carbohydrates. Avoid any fats at all, and eat sugars and simple carbos like biscuits. These digest and metabolise more easily."

+ 100!!! Absolutely critical. Simple carbs. 1 mole of glucose is oxidized by 1 mole of O2. C6H12O6 + 6 O2 = 6 CO2 + 6 H2O + energy. The oxygen requirements to oxidize a mole of a medium chain fatty acid or a simple protein are astronomically higher. Given the reduced availability of O2 at high altitude, the conclusion is pretty clear. The advantage of glucose is that it carries 1/2 of the oxygen required for its complete oxidation as part of the glucose molecule.

How many carbohydrates are in a mole?

"When you get up high, like 16,000+ feet, and if your appetite starts to wane, "

That night at 17,200', I was super hungry for some reason. The Chinese style soup noodles weren't that great -- but I ordered two bowls (double portion) anyway and wolved it all down. In other words, I completely ignored the guide's advice.

I felt no negative effect whatsoever, but posted the question because I could have just been lucky...

The suggestion to switch to simpler carbo makes sense!