On a somewhat related note which might be of interest to some (if not, don't bother to read):
Here are some Muscle/Exercise Phys. "numbers" and info. I'd appreciate feedback from those with more up to date info on whether this info is still considered valid based upon more up to date research.
Depending upon the level of physical exertion and your body's related heart rate, your muscles will burn "fuels" in different ratios - subject to the availability of oxygen being transported into muscle cells. More specifically the hiker is engaged in a long duration moderate intensity form of exercise. So, a "ballpark"/approximate breakdown of the "fuels" utilized is normally as follows:
25% each from:
-- fat stored as triglycerides within the muscle
-- carbohydrate stored as the starch glycogen within the muscle
-- glucose released into the bloodstream from the liver.
-- fat coming from diet or released from storage in adipose tissue
if at all possible, our bodies don't like to exclusively use just energy sources stored in the muscles, thereby depleting/exhausting them completely, just in case some sort of "surpreme" exertion is attempted/required. our bodies don't want to let us down if at all possible.
I think that these percents are a bit off in this simplified example, and that often more fat is burned due to the lower levels of exertion by the hiker on level terrain. Do those with more up to date info disagree? Is any of this still valid? I think it's a simplification, in terms of the percentages given, of what actually happens.
as exercise intensity increases beyond a certain level, approx. the same amount (in an absolute sense) of fat is "burned" (absolute amount depends upon "cardio" conditioning which is related to the amount of oxygen available in the red corpuscles in the blood and intracellular training benefits to utilize oxygen, fats, and carbs in the little cellular "engines"), and any increased energy demand is met by burning more carbohydrate, so the ratio of carbohydrate to fat increases.
a typical value one often reads, for a well conditioned long distance hiker, for depletion of stored glycogen reserves, is approx. 6hrs. i've never been able to determine a continuous heart rate associated with this 6hr figure. my guess might be ~60% of the so-called age-adjusted-maximal-heart-rate, but this is just a guess. frequent "snacking" while hiking is a good strategy for preventing depletion and replenishing these energy stores.
the talk is mainly about carbs and fats as proteins are not burned as fuel, per se. they undergo a catabolic pathway beginning with deamination, IIRC, and are converted to simpler carbon compounds via different pathways to produce glucose or ATP - my memory is not real clear on this, however. as far as storage of amino acids/protein i don't recall this occurring. i do remember that when an amino acid deficiency does occur, the body in order to utilize the other amino acids to allow protein synthesis to continue will catabolize labile body proteins, including plasma albumin, and muscle tissue in order to permit protein synthesis to continue. to the best of my recollection, amino acids are not stored. any unused protein/amino acids are deaminated and then oxidized via both gluocse or fat metabolic pathways and stored as glycogen or fat, respectively. the excess nitrogenous waste produced by these pathways is excreted in the urine as either ammonia or urea. once stored as glycogen or fat, they are later used as fuel/energy, not building blocks for protein. when protein intake is inadequate or specific essential amino acids lacking (those that cannot be metabolized by the human body from other sources, but must be ingested) the body will then have to catabolize ("break down") certain other protein sources already present in the human body to supply the missing amino acids. depending upon the degree to which this needs to be done, there can be serious side effects from this. an increase in urinary nitrogen can be caused by insufficient protein intake and is looked for as an indication of this condition. (at least that's how i recall it, but it's been quite a while, so please verify this).
Also, I'm feeling like perhaps, due to all involved maintaining some semblence of brevity in their posts on a complex subject (this one being an exception), that, at times, different points are being emphasized and the precise topic is not covered in each post (e.g. the example of Lance, and my response - maybe we weren't on the exact wavelength, hence some minor discrepancies).
This sure is a very interesting subject though - to me at least. Sorry, I can't contribute better info, it's been 30-35yrs since I've had to think about this stuff. Hope our Phys. Prof. can shed better light and correct any errors I may have introduced. I think at this point, other than asking questions, I'm all "Posted-out".