Chapman, Stray-Gundersen & Levine 1998, Individual variation in response to altitude training

J Appl Physiol 85:1448-1456. Splitting runners by 5,000 m time change after living at 2,500 m, “responders” showed a larger EPO rise, increased red cell volume and VO2max, while “non-responders” gained no red cell volume and trained slower at altitude. Iron availability gates the response. Later work questions whether responder status is a stable trait: McLean et al. 2013 (Br J Sports Med) found the individual haemoglobin-mass response correlated only R=0.21 across two camps, so the group mean is reproducible but the individual label is not. Moderate; pair with the reproducibility critique.