I have an idea which is so simple in one form, that if just stated, would be a truism. In another, it is so nebulous, complex and unwieldly that it's hard to organize the idea. I'm going to try to get from one to the other here and it's going to take a while, so get ready for some posts that don't seem to go anywhere, but when complete, might be worth the effort.
When one goes to altitude, there are some changes which take place almost immediately. First, one's heart rate and breathing rate increase in order to compensate for the lower oxygen tension. There are other less obvious changes which begin (eg. the levels of 2,3-bisphosphoglycerate shift, altering the cooperativity of oxygen binding to hemoglobin and other metabolic changes cause one's urine to become slightly alkaline), but these are minor at first.
After about three days, altitude sickness occurs (if it occurs), and is bad from days 3-5, when the changes taking place begin to be less drastic. The major change at this time is the increase in formation of red blood cells, which boost the blood's ability to carry oxygen. The average life span of a red blood cell is 120 days, but it appears that, upon decreasing altitude, the extra red blood cells are much more rapidly removed.
There are other changes that take place in adaptation to altitude that take place on a longer time scale. When one tries to find how long one should go to altitude to reap the benefits for racing at low altitude, there is no upper limit. Some changes take years (and some are not good; at very high altitude, clubbing of fingers and toes occurs after several years).
These changes seem analogous to me to those that take place when fasting. In the first day, one's liver glycogen store diminishes to keep blood glucose levels steady. After three days, one's brain is switching from using glucose solely to using glucose and keto acids derived from the breakdown of proteins and fat (muscle glycogen cannot be turned into glucose in the blood - except by burning as fuel, then the pyruvate being made into lactate and alanine, which can be exported to the liver and remade back into glucose); interesting - to me, anyway - is that hunger tends to go away after three days as well. If fasting continues, the body preserves its muscle by having the brain switch to using mostly keto acids for fuel; by 40 days, two-thirds of the brain's energy comes from keto acids (after 40 days, people tend to die, so it's hard to say what other changes take place).
When thinking of training, the same idea may hold: a one day change, a three (to five) day change, and more gradual changes over weeks and months.
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