Hemoglobin mass (Hbmass) is important for athletes because it helps determine maximal aerobic power. This study examined how lean mass, iron deficiency (ID), and sex influence Hbmass in athletic and nonathletic groups. NCAA Division I student athletes (21 men, 75 women; altitude: 1,625 m) were recruited from six athletic teams; 14 male and 12 female full-time students (non-varsity athletes) served as control subjects. Hbmass, body composition, and iron homeostasis parameters, including ferritin, soluble transferrin receptor (sTfR), hepcidin, erythroferrone, and 10 inflammatory cytokines, were measured two to four times across a competitive/training season. ID was defined as ferritin < 25 ng/mL. Hbmass was more closely related to lean mass (r² = 0.90) than body mass (r² = 0.69, P < 0.01). Compared with female subjects, male subjects had 19.9% higher Hbmass relative to body mass (Hbmass_BM) but only 7.5% higher Hbmass relative to lean mass (Hbmass_LEAN) (both P < 0.001). Prevalence of ID was higher in female than male subjects (47% vs. 9%, P < 0.01) but did not vary between groups. Hbmass_LEAN was 5% lower in ID vs. non-ID female subjects; Hbmass_BM was not different. ID was associated with lower hepcidin, elevated sTfR, and elevated erythroferrone but not with differences in inflammatory cytokines. Hbmass varied significantly between athletic groups and across sex, but the majority of these differences are explained by differences in lean mass. ID was common in female subjects and was associated with lower Hbmass_LEAN and hepcidin but not with differences in Hbmass_BM or inflammatory cytokines. Hbmass relative to lean mass seems advantageous when monitoring iron deficiency.NEW & NOTEWORTHY Differences in hemoglobin mass (Hbmass) between groups and across sex are primarily due to differences in lean mass. Iron deficiency (ID) independently decreases Hbmass; this effect is best characterized with Hbmass relative to lean mass. ID is common in females and is associated with lower hepcidin and elevated erythroferrone but not with differences in inflammatory cytokines. Hbmass relative to lean mass accurately quantifies hematological alterations secondary to iron deficiency.

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