What are the physiological effects on the body from training at altitude?
Altitude is a physiological "stressor" due to the lower barometric pressure, and your body adapts to this stress. Lower barometric pressure means there is "less" oxygen around. Although the perecentage of oxygen in the air is the same as at sea level, the lower pressure gradient means the oxygen available is functionaly less compared to sea leavel. Being at altitude on a constant basis has similar physiological effects on the body as running (to some degree, I am oversimpfying it here). The limited oxygen causes the body to adapt via an increase in blood volume and an increase in red blood cell count.
What is the minimal time for benefitting from altitude training? Does one ever “fully adjust” to the point where it stops giving an advantage?
Depending on which physiologist you talk to you will get a different answer. In my experience, if you have an athlete that has been to altitude before, then 18 days is enough time to see an effect. The "fully adjust" is an interesting question, but I would look at it more from a "stress/adaptation" perspective versus an "advantage" perspective. I think there is a point where the body normalizes to being at altitude and maybe the slope of the physiological effect curve gets a little less step. But, when does that happen and how does that play out on the track...I do not know.
Are there any drawbacks to training at altitude?
One draw back is seen in race specific sessions. The lack of Oxygen is compensated by running at slower paces--this is not ideal for creating muscle memory at a specific pace. Another draw back is seen in day to day recovery. Oxygen is a limiting factor in recovery, thus having less oxygen slows recovery. This has to be compensated by decreasing the overall training load or increasing recovery times between workout sessions. A third drawback is seen in hydration levels. The ambient air at altitude is dry. This dry climate causes moisture to be lost through respiration. It is hard to maintain hydration levels.
Is there an “elevation sweet spot” or is higher always better?
The drawbacks at too high of altitudes can outweigh the benefits. Recovery is important! I think 6,000-8,000 feet is a good zone because you can get a bump in red blood cells while maintaining a decent quality work.
How is the initial stages of altitude training different from the latter part of the altitude training cycle?
Initially training errs on the easier side while the body acclimates to the new stressors. Then the training is very similar to sea level training with slight variations. The easy days are a little slower, and there is more recovery time between workout sessions.
How quickly do the benefits of altitude wear off?
This is debateable and difficult to answer. The body attempts to decrease red blood cell production immediately upon drop in altitude. Red blood cells live for 100-120 days, therefore so you could assume that the physiological benefit lasts for a few months. Most blood panels start to show a drop in hematocrit after just a few days at sea level. Therefore, the body starts to adapt to sea level immediatly, and fully adapts after the 100-120 days. The timing of when "benefits" wear off depends on the individual and the training stimulus. Although the physiological benefits subside, the jump in fitness gained from these benefits allows for a higher level of training. The benefits of this higher level training last longer than the physiological benefits reaped at altitude.
What is your opinion on altitude tents?
Even more tricky than altitude! Altitude tents are hypoxic, with a normal barometric pressure--meaning that the oxygen levels simulate altitude, but the pressure simulates sea level. There have been studies indicating the barometric pressure change can be a key factor in physiological adaptations. Another huge factor for tents is you have to be in them 12-14 hours because once you leave the tent, there goes the body's response to the hypoxic condition and it stops the red blood cell production. There are responders and non-responders to the tent. Personally, when I was an athlete I suffered using a tent. I was chronically faitgued, and my anabolic hormones were low. Currently, none of my athletes use the tent, although I am not opposed to it. I think with proper monitoring they can be a great training tool.