Oxygen delivery and cardiac output during exercise following oral phosphate-glucose

Phosphate has been proposed as an ergogenic aid since it may enhance O₂ delivery and cardiac work efficiency by increasing plasma phosphate (P Pi), red blood cell phosphate (RBC Pi), 2,3-diphosphoglycerate (DPG), RBC adenosine triphosphate (ATP), and P50. In 10 normal, fasting males we measured cardiac output (Q) by CO₂ rebreathing, heart rate (HR), O₂ deficit (O₂DEF), and O₂ con-sumption (VO₂) during cycle ergometer exercise (60% of peak VO₂). Stroke volume (SV) and arteriovenous O₂ difference (A-VO₂) were calculated. Following a baseline blood sample (BASE) for P Pi, RBC Pi, DPG, RBC ATP, and P50 (3 h before exercise), a single oral dose of dicalcium phosphate (129 mmol) and glucose (500 ml/10% sol, PHOS), or placebo (PLA), was administered in a random, crossover, double-blind fashion. Blood sampling was repeated immediately before and after exercise (PRE-EX and POST-EX). PHOS induced increases in P Pi (3.87 to 4.35 mg · dl⁻¹, P < 0.05), RBC Pi (3.86 to 4.63 mg · dl⁻¹, P = 0.08), DPG (11.8 to 13.1 μmol · g⁻¹ Hb, P < 0.05), RBC ATP (4.2 to 4.4 μmol · g⁻¹ Hb, P < 0.05), and P50 (26.8 to 27.9 mm Hg, P < 0.05) from BASE to PRE-EX. All variables remained elevated through the exercise period, as evidenced by higher levels than BASE at POST-EX (P < 0.05). However, P50 was not different across conditions at PRE-EX (PHOS P50 = 27.9, PLA P50 = 28.3 mm Hg) or POST-EX (PHOS P50 = 28.0, PLA P50 = 28.1 mm Hg). No differences in Q, SV, HR, O₂DEF, VO₂, or A-VO₂ were noted across conditions. Thus, the acute alterations of the hemato-logical parameters resulting from PHOS do not affect O₂ delivery or cardiac output during submaximal exercise.