It has been observed that astronauts lose substantial quantity of calcium through urine during space flight. This is due to

Astronauts experience significant calcium loss through urine during spaceflight primarily due to microgravity. In a weightless environment, the skeletal system is 'unloaded' as it no longer needs to support the body's weight against Earth's gravity [t2][t4]. This lack of mechanical stress leads to bone demineralization, where calcium leaves the bones and enters the bloodstream [t2]. Consequently, the kidneys filter this excess calcium, resulting in hypercalciuria (increased urinary calcium excretion) and an elevated risk of renal stones [t1][t2]. Research from missions like Skylab and Mir confirms that urinary calcium levels can increase by 60-70% within days of entering microgravity [t2]. While other factors like diet and lighting play minor roles, the removal of the gravity vector is the fundamental cause of this physiological shift [t2][t5].

Sources

1. [1] https://ntrs.nasa.gov/api/citations/20100030546/downloads/20100030546.pdf
2. [2] https://www.nature.com/articles/s41526-022-00194-8
3. [3] https://www.ncbi.nlm.nih.gov/books/NBK217835/