Training in hot weather?

[Dave Holt]

I was recently told that the best way to train for altitude, if you don't have altitude, is to run in the heat.  Would anyone like to comment on that?

I did some research about this and found two things: first, that the rational I had for the same thought was incorrect; the only other thing I found was a quote by distance coach Brad Hudson stating the same thing (but with no scientific reasoning stated of why).  So...
I still think there is something to this, but I would like to know why as well.

[Bonnie]

I have seen articles that discuss this in terms of blood volume. Blood volume decreases in moderate exercise in the heat because the heart has to pump more blood to the skin and muscles as they try to stay cool.  Blood volume also decreases during altitude training but the number of red blood cells (and hemoglobin) increase (because of the lower levels of O2 at altitude - more red blood cells and hemoglobin mean more O2 is transported to cells).  Even though the initial decrease in blood volume are similar, the mechanism and ultimate physiologic changes between these two stressors appears to be different.  I haven't found any scientific studies that have demonstrated any similar long-term training benefit.  Altitude training seems to make substainative changes that last a while (I have read 3 months, but only in one article).  I think heat training makes minimal "longer term" changes, your heart just adapts to having to pump more to keep you cool.  Having trained in both, I have found a benefit from training at altitude ... I have never really found myself in any better shape training in the heat (although despite having to run more slowly, I don't think I have lost any training due to it either).