"Makes sense that very low fat reserves could make your body think it's in danger of starvation, so it will protect itself by ramping down your metabolic rate."
It doesn't really work like that. Body fat stores are a response to starvation + gorging. Eating smaller meals more frequently leads your body to adapt by storing less fat.
"Instead of increasing your body-fat %, would increasingyour lean muscle mass help insulate ? "
It doesn't offer much insulation, so adding muscle wouldn't make much of a difference to warmth.
"I've had a similar problem and in my case I've attributed it to having a low resting pulse. Do you have a particularly low resting pulse?"
A low resting heart rate is just a sign of an efficient and healthy circulatory system combined with a strong heart. What it really means is that during exertion, you can crank your metabolism higher than most people can.
"Since our ancestors didn't eat a lot of carbs, that couldn't have been what kept up their body temperature. Since they couldn't have moved about constantly all the time, how did they stay warm when resting?"
1) Wear the same thing their prey did
2) Stay active
3) Build more body fat stores than we do (read the first answer above)
4) Share warmth
Miguel, you raised a lot good points. You also reminded me about the mountain guides on the Kilimanjaro trip; our guide in particular ate a meal that consisted mostly of a grain-based dish that looked remarkably like mashed potatoes (obviously, it wasn't potatoes, since it was made from grains, IIRC corn or something similar) after dinner before our summit day. He also said that he wasn't going to eat anything else until the following evening.
Good Luck isn't a big guy. Quite lean, not unusually muscular. I suspect that part of the reason that he is able to do that trip with the apparent ease with which he does it is that he is very efficient -- he doesn't waste any effort. He was carrying quite a bit of stuff, also -- probably more than I was, even with my 35-pound pack. (Mostly due to photo gear.)
BTW, to clarify -- glycogen is an anaerobic source of energy. It's used for fast-twitch movement rather than for endurance; due to being anaerobic, burning it produces lactic acid. We need oxygen to get rid of lactic acid. Fat on the other hand burns aerobically, and the body uses it to make ATP which consumes oxygen and emits carbon dioxide and water.
Most mammals use fat to provide both insulation and energy storage. It's what enables whales to consume vast amounts of krill, and then swim north to give birth, and not eat again for six months. It's what enables bears to sleep through the winters, and what keeps critters like walruses warm in the cold Arctic waters, penguins to go for months at a time without food.
"Is it forcing a lack of one major nutrient that causes weight loss? Or do all these diets have in common, even if it's just incidental, that they force the eater to consume less sugar and refined/processed foods?"
Part of it is that when we eat overly processed foods, we basically trip up our metabolism; we're evolved to eat complex foods, not raw sugars and junk like high fructose corn syrup. Our bodies metabolize that stuff into fat because it metabolizes so easily.
The weight loss from *most* diets is due to starvation -- most diets lead to thin, flabby people. Not fit people. And certainly not healthy people...
Diets that combine exercise with proper nutrition achieve weight loss by balancing caloric intake with activity levels, as well as balancing where those calories are coming from, so that the body can use that energy like it's evolved to use it.
This has been an interesting discussion so far. :)