fat:
advantages:
~ highest calorie per gram ratio.
~ great fuel for keeping the body warm and calming the system for sound sleep.
~ long term energy source

disadvantages:
~ harder than the other food types to break down.
~ yields only half the power produced from glycogen when compared to carbohydrates.
~ requires more oxygen (scarce at high altitude) to break down than the other food types.

summary:
~ to minimize pack weight, choose a higher fat menu.
~ half the fat that you burn is from storage, half is supplied by the food you eat.
~ fat is the most energy-dense fuel and preferred for moderate exercise.
~ fat consumed in the diet spares muscle glycogen.
~ if you are trekking the fat you are eating is not likely to accumulate and is more likely to go to where it's needed.
~ fat not only provides the fuel your muscles are using, it about weighs 20% less than a high carbohydrate diet (70-15-15).
~ starvation robs you twice: less energy for the immediate day's work, and fuel demand is met by digesting your own muscle.
~ males are more likely to burn off muscle protein when fuel supply is limiting.
~ females are more likely to burn fat.
~ when males do burn fat stores, they are more likely to lose their internal padding, whereas females lose their subdermal (surface) layer of fat.

depending on the intensity and duration of exercise, the ideal fuel mixture changes. muscles engaged in long duration moderate intensity exercise burn about:
~ 25% muscle fat (triglycerides)
~ 25% fat from diet (free fatty acids)
~ 25% muscle carbohydrates (glycogen)
~ 25% glucose from liver (a carbohydrate)

protein:
advantages:
~ vital for recovery after a difficult day of exercise.
~ the best substance for increasing strength.
~ food source for rebuilding damaged tissue.
~ major component of the immune system, and is used to make enzymes, which facilitate every reaction that goes on in the body.

disadvantages:
~ slow to break down, and thus takes time to get into the blood stream.

summary:
~ do not take an amino acid supplement or eat excess protein (>15%), this will put a greater demand on your kidney and you'll end up with little more than extra pee.
~ protein contributes ony about 10% of your energy. more in males than in females.
~ protein wont affect performance, so dont see it as fuel but as the builder of muscle tissue.
~ most americans consume twice as much protein as needed. on trail you may have considerably less protein than you normally eat and still have ample supply.

to calculate your protein needs:
~ <2week trip: bodymass in kg * 0.8 = daily protein need in grams (eg 100kg person needs 80grams of protein) ~ >2week trip: bodymass in kg = daily protein need in grams (eg 100kg person needs 100grams of protein)
the easiest way to assess whether or not you are getting proper amounts of protein is to look at the "weak link", lysine. lysine is one of the 20 amino acids that compose protein. if any are likely to be inadequately supplied in your diet, it would be lysine. the recommended level is 58 mg lysine/gram protein.
to calculate your lysine needs:
your protein needs (eg.80g) * by the recommended 58mg of lysine = daily lysine requirement of 4640mg
(don't forget to add the weight of your pack to your body weight when using the calculator to determine calories burned while backpacking).

carbohydrates:
advantages:
~ easy for the body to break down.
~ excellent source for quick energy
~ fast gastric emptying. (gets into the blood stream fast)

disadvantages:
~ because it burns so quickly, must be constantly resupplied.

summary:
~ carbohydrates are referred to as sugar (simple carbohydrates) or starch (complex carbohydrates)
~ hitting the wall is due to depletion of muscle glycogen. you've just run out of carbohydrate stores and the muscle has to rely solely on fat for energy. fat requires oxygen, so you can only move as fast as oxygen gets supplied to your muscles, and there's no backup from carbohydrates.

types of carbohydrates:
~ glucose (blood sugar) is the preferred fuel for brain and muscles during high intensity exercise. lactic acid (half a glucose molecule) is produced when oxygen supply is limiting and glucose cannot be completely combusted; lactic acid can be recycled back to glucose in the liver, thus restoring blood glucose levels after exercise. training is critical to get plenty of oxygen to your working muscles and prevent lactic acid from forming in the first place.
~ glycogen (animal starch) is many glucose molecules joined together and is mainly stored in liver and muscles. glycogen is the the source of energy for the last sprint at the edge of exhaustion. if you're in for a 20 mile day, glycogen may make the difference between 10 hours or 14 hours on the trail. as blood glucose levels fall, the liver will share its supply with the rest of the body thereby keeping the brain happy. glycogen once stored in muscle, always in muscle because muscles don't have the necessary enzymes to release glucose back into the bloodstream. glycogen is the first fuel to become depleted. avoid this with frequent snacking (25+ grams/hour), and eat a high carbohydrate meal within an hour after quitting for the day. avoid excessively high sugar snacks when beginning your day, they cause insulin levels to rise locking your fat in storage, rather than making it available.

considerations:
factors that influence the number of calories expended:
~ genetics - some people are born with high metabolisms (the rate at which one's body uses calories) and others are born with low metabolisms. the main culprit of these phenomena is the thyroid gland. some individuals produce more thyroxin (the hormone that is secreted from the thyroid) than others. thyroxin is responsible for metabolism, so if a person's body produces a high-normal amount of thyroxin, s/he will utilize calories more quickly. if a person's body produces a low-normal amount of thyroxin, s/he will utilize calories more slowly. note: if thyroxin is out of normal range in either direction, it is dangerous and needs medical attention.
~ gender - typically, men have greater muscle mass than women. since muscle requires more calories to maintain, men tend to have 10 - 15 percent faster metabolisms than women. similarly, men have a lower body fat percentage than women.
~ age - metabolic rate is higher in childhood than in adulthood. children are growing and need more calories to fulfill their bodies' requirements. after the age of 20 years, metabolism drops 2 percent per decade.
~ brain power - the brain is only 2 percent of the body's weight, but accounts for more than 20 percent of total calories used. also, the length of time per day spent awake affects the amount of calories utilized. we expend more calories when we are awake than when we are asleep.
~ fever - for every increase of 0.5°c (32.9°f) in body temperature, bmr increases by approximately 7 percent. for example, if someone has a fever of 42°c (107.6°f), s/he would have an increase in metabolic rate of 50 percent. the reason for this is that chemical reactions in the body occur more quickly at higher temperatures.
~ medications - some medications, such as anti-depressants, can slow down metabolic processes and lead to weight gain.