Calcium loading and renal function in trained subjects during restriction of muscular activity and chronic hyperhydration.

It was suggested that negative calcium balance is not based on the shortage of calcium in the diet, but on the decreased tissular capacity of the body to retain calcium during hypokinesia (decreased muscular activity), and that chronic hyperhydration may be used to normalize calcium balance. To evaluate this hypothesis studies were performed on 30 long distance runners aged 23-26 yr, with an average maximum oxygen uptake 65 mL/kg/min during 364 d of hypokinesia (HK). All volunteers were divided into three equal groups: Ten volunteers were placed continuously under an average of 14.9 km/d (control subjects), ten volunteers were subjected continuously to HK (hypokinetic subjects), and ten volunteers were submitted continuously to HK with daily consumption of an additional amount of 26 mL water/kg body wt and 0.16 g sodium chloride (NaCl)/kg body wt (hyperhydrated subjects). For the simulation of the hypokinetic effect, the hypokinetic and hyperhydrated volunteers were kept under an average of 2.7 km/day for 364 d. During the prehypokinetic period and hypokinetic period calcium lactate loading tests (0.55 mEq/kg body wt) were performed. Urinary and blood electrolytes (sodium, ionized calcium, total calcium, magnesium, and phosphate) and blood parathyroid hormone (PTH) were determined. Urinary electrolytes and concentrations in blood thereof decreased in the hyperhydrated and increased significantly in the hypokinetic volunteers. Blood parathyroid hormone content increased in the hyperhydrated and decreased in the hypokinetic volunteers. After calcium lactate loading tests, the hypokinetic volunteers displayed a faster excretion of calcium and a decreased blood PTH content as compared to the control and hyperhydrated groups of volunteers. It was concluded that calcium deficiency during HK is associated with decreased tissular capacity of the body to retain calcium, whereas chronic hyperhydration may be used to prevent calcium deficiency in endurance trained volunteers during prolonged restriction of muscular activity.