Gents,
i`ll start with a question which will be answered at the end of this posting, with some reading matter inbetween:
Q: Air contains 21% oxygen, how much of that is used by the body with each breath? ie (what % do you exhale)...
Just a few other tips about altitude sickness relating to the question of being "too fit":
Firstly we need to remember a few things:
1) In a normal person, the urge to breath is not caused by the need for oxygen, but rather the need to get rid of carbon dioxide from the body , ..... (assuming you are not a long term smoke, in which case your breathing "trigger" starts assessing how much oxygen it can get ito the body and "forgets" about the co2 levels..... ie co2 levels gradually get higher and higher supplemeted by the additional co2 in the smoke)
2) when you are active your "krebbs cycle" (stay with me here agent...) speeds up and more c02 is produced by your cells..... when the levels of co2 elevate higher, the muscle burn starts.....
3) the process of respiration involves inhaling air with realitively high concentartions of o2 and low concentrations of co2 which is taken into the lungs.... a process of gasseous exchange takes place ie o2 diffuses from the lungs into the blood stream, and simultaneously the co2 in the blood stream diffuses into the lungs to be exhaled out......
4) Air, at whatever altitude, consists of the same gasses, in the same ratio`s.... basically 21% oxygen and 79% Nitrogen (with a small amount of other gasses eg co2)....... The ability to transfer these gasses between the air and the blood (inside the lungs) depends on the principle of "equalization"..... ie moving from a high concentration to a low concentration.... breathing in a breath of air creates a "pressure gradient " between the oxygen ie plenty on the lungs, not much in the blood..... so it "diffuses" into the blood stream...... the same principle of co2 applies but this time the high concentration is in the blood stream with the low concentration in the air that we have inhaled......
5) Altitude affects air pressure, and in turn pressure determines how effectively this process of gasseous exchange takes place.... from about 10000 ft asl, the partial pressure is too low for effective gasseous exchange of the gasses.... hence the need for pressurized aircraft.....
6) the complete process requires a few things to work simultaneously... on needs properly functioning lungs, a good heart, blood that has plenty of red blood cells and haemoglobin and then a good network of blood vessels to circulate the blood....... (simply put: air goes in and out, blood goes around and around.... any variation is a bad thing)
7) When permanently exposed to a particular environment, the body adapts as best it can to a particular situation........ take the person staying at the coast vs the typical gautenger living at 5500ft..... flying @ 6500ft the gautenger is out of his "norm" by about 1000ft........ the Durbanite or Captonian at the same altitude is out of his "norm" by 6500ft..... who`s body do you think is managing better? (all things being equal between both pilots)....... at constant lower ambient pressures, its normal for the body to start manufacturing more haemoglogin in the blood in oder to facilitate the transportation of more o2 and c02

This forced manufacturing process process is often used by athletes wanting to perform with better endurance...... they train their bodies at high altitudes and then compete at lower altitudes....... some use a technique called blood doping: they donate blood regularly and have the donations stored.... so the body produces more haemoglobin.... close to the competition, they have their own (previously donated) haemoglobin re-infused..... ie they overload their own blood with additional haemoglobin......
5) The term "fitness" is a bit of a broad term as there are many influencing factors so the original question about being "too fit to climb" is really inaccurate..... its not possibel to be too fit........ it is possible to be unaccustomed to the new partial pressure changes of the o2 and the co2... example: who is fitter: the brave mountineer sponsored by any and all of the best nutrition brands, who has spent years training his fitness levels, reaching the top of mount everest, sucking on his oxygen cycinder for dear life (without which he wouldnt last too long) with the typical hypoxic 1000 yard stare........ or his smiling sherpa in the back ground who has carried everything (tent, food, spare o2 cylinders, clthing, backpacks, etc etc ) on his back, without any spare o2 cylinders...... they are probably equally fit, just one more adapted to the lower partial pressures than the other......
A fine balance is necessary for the brain to function correctly in terms of the correct o2 and co2 levels........ mess it up and we get the onset of cerebral poor judgement, persistant errors in our discisions, mental lethargy etc etc etc (in a pilots world, thats when very bad things start to happen).... in conjunction with all of the clinical presentations that Bennie mentioned....
Bottom line is to stay within your bodys limits....
ANSWER: 5% is used..... (inhaled air containes 21% oxygen....... and 16% oxygen when the same breath is exhaled)

ZU-CPW..... t/bird mk2
Hoedspruit Civil Airfield
Hangar 8