Carsten Lundby

Blood volume (BV) is an important clinical parameter and is usually reported per kg of body mass (BM). When fat mass is elevated, this underestimates BV/BM. One aim was to study if differences in BV/BM related to sex, age, and fitness would decrease if normalized to lean body mass (LBM). The analysis included 263 women and 319 men (age: 10–93 years, body mass index: 14–41 kg/m 2 ) and 107 athletes who underwent assessment of BV and hemoglobin mass (Hb mass ), body composition, and cardiorespiratory fitness. BV/BM was 25% lower (70.3 ± 11.3 and 80.3 ± 10.8 mL/kg BM ) in women than men, respectively, whereas BV/LBM was 6% higher in women (110.9 ± 12.5 and 105.3 ± 11.2 mL/kg LBM ). Hb mass /BM was 34% lower (8.9 ± 1.4 and 11.5 ± 11.2 g/kg BM ) in women than in men, respectively, but only 6% lower (14.0 ± 1.5 and 14.9 ± 1.5 g/kg LBM )/LBM. Age did not affect BV. Athlete's BV/BM was 17.2% higher than non‐athletes, but decreased to only 2.5% when normalized to LBM. Of the variables analyzed, LBM was the strongest predictor for BV ( R 2 = .72, p < .001) and Hb mass ( R 2 = .81, p < .001). These data may only be valid for BV/Hb mass when assessed by CO re‐breathing. Hb mass /LBM could be considered a valuable clinical matrix in medical care aiming to normalize blood homeostasis.