Urinary oxalate excretion in astronauts during long‐duration spaceflight: an exploratory analysis

OBJECTIVES: To explore the relationship between diet, mission phase, urinary calcium, and urinary oxalate during long-duration spaceflight. SUBJECTS AND METHODS: Dietary intake of calcium and oxalate and 24-h urine parameters from 12 astronauts during long-duration spaceflight while consuming controlled diets were analysed. Mixed-effects models assessed differences in urinary calcium and oxalate levels, adjusting for daily intake of calcium and oxalate, and for in-flight status relative to pre-flight status. RESULTS: From pre-flight to in-flight, median urinary calcium increased from 178.9 to 270.3 mg/day (P < 0.01) and urinary oxalate from 37.9 to 42.3 mg/day (P < 0.05). Post-flight, urinary calcium decreased to 185.8 mg/day (P < 0.01) and urinary oxalate to 31.5 mg/day (P < 0.01), relative to in-flight values. Dietary oxalate remained similar pre-flight and in-flight (382.0 vs 339.4 mg/day; P ≥ 0.05) but decreased post-flight to 241.6 mg/day (P < 0.01). Adjusted in-flight urinary oxalate was 10.71 mg/day higher than pre-flight and increased by 0.050 mg/day per 1-mg/day increase in dietary oxalate (95% confidence interval 0.036-0.064; P < 0.001); dietary calcium was not associated with urinary oxalate (P ≥ 0.05). CONCLUSIONS: Hyperoxaluria was observed during long-duration spaceflights and persisted throughout the in-flight phase under controlled dietary conditions. Although high dietary oxalate was the strongest predictor, evidence of inter-individual susceptibility was observed, with some astronauts exhibiting elevated oxalate pre-flight levels, which increased further during flight. Assessment of oxalate content in current astronaut food systems, mitigation of dietary oxalate load through timed calcium intake, and further investigation of microgravity-related effects on oxalate absorption and metabolism are required.