Longitudinal Training-related Hematological Changes in Boys and Girls from Ages 12 to 15 yr.

Purpose: Natural growth and maturation cause hemoglobin mass (Hbmass) and blood volume (BV) to increase during childhood and adolescence. Whether endurance training during the same period can cause further increases in these variables is not clear. Also, body composition develops differently in girls and boys during puberty, and the effect of these differences on hematological variables has not been previously studied.

Methods: Forty-two endurance athletes (End group; 24 boys) and 34 athletes from other sports (non-End group; 23 boys) were tested at ages 12, 13, and 15 yr for Hbmass, BV, plasma volume (PV), red cell volume (RCV), hematological variables, and anthropometrics.

Results: At age 12 yr, Hbmass and BV showed no difference between sexes and training groups in absolute values or relative to fat-free mass (FFM). Relative to FFM, Hbmass and BV averaged 11.7 (0.8) g·kg and 95 (6.8) mL·kg. Increases in FFM from ages 12 to 15 yr primarily determined the increased Hbmass and BV for both sexes with no differences between training groups. At age 15 yr, Hbmass relative to FFM was higher in boys than girls, 13.1 (0.8) g·kg and 12.1 (0.9) g·kg (P < 0.001), whereas BV relative to FFM was not significantly different between sexes and training groups at any ages, averaged 100 (6.7) mL·kg at age 15 yr. Relative to FFM, PV was higher in the End group at all ages, and RCV was, on average, lower and increased less compared with non-End group.

Conclusion: Our results indicate that increases in Hbmass during puberty are mainly associated with increased FFM and independent of sex or volume of endurance training.However, the FFM-relative PV was higher and the FFM-relative RCV was lower in the End group compared with the non-End group.