Individual variations in pre‐altitude hemoglobin mass influence hemoglobin mass responses to repeated altitude sojourns

Introduction Previous studies have shown variable within‐subject hemoglobin mass (Hb mass ) responses to altitude training. We investigated whether Hb mass responses depend on individual variations in pre‐altitude Hb mass during repeated altitude sojourns. Methods Nine elite endurance athletes carried out 3–5 altitude sojourns over 17 ± 10 months (mean ± 95% confidence interval), at an altitude of 1976 ± 62 m, for 21 ± 1 days, and a total hypoxic dose of 989 ± 46 km·h, with Hb mass assessed before and after each sojourn (carbon monoxide rebreathing). The individual mean baseline was calculated as the mean of all pre‐altitude Hb mass values for an athlete, and it was investigated whether the percent deviation from the individual mean baseline affected the altitude‐induced Hb mass response. Results On average, Hb mass increased by 3.4 ± 1.1% ( p < 0.001) from pre‐ to post‐altitude. The intra‐individual changes in Hb mass were highly inconsistent (coefficient of variation, CV: 88%), and we found no relationship between Hb mass changes in successive altitude sojourns ( r = 0.01; p = 0.735). However, the percent increase in Hb mass was highly correlated with the pre‐altitude Hb mass , expressed as the percent deviation from the individual mean baseline (y = −0.7x + 3.4; r = 0.75; p < 0.001). Linear mixed‐model analysis confirmed a −0.6 ± 0.2% smaller increase in Hb mass for each 1% higher pre‐altitude Hb mass than the individual mean baseline ( p < 0.001) after adjusting for the covariates hypoxic dose ( p = 0.032) and the relative Hb mass (g·kg −1 body weight; p = 0.031). Conclusion Individual variations in pre‐altitude Hb mass significantly influence the athletes' Hb mass responses to repeated altitude sojourns, with a potentiated response after traveling to altitude with a low pre‐altitude Hb mass .