Calcium kinetics during bed rest with artificial gravity and exercise countermeasures

Abstract

We assessed the potential for countermeasures to lessen the loss of bone calcium during bed rest. Subjects ingested less calcium during bed rest, and with artificial gravity, they also absorbed less calcium. With exercise, they excreted less calcium. To retain bone during bed rest, calcium intake needs to be maintained.

Introduction: This study aims to assess the potential for artificial gravity (AG) and exercise (EX) to mitigate loss of bone calcium during space flight.

Methods: We performed two studies: (1) a 21-day bed rest (BR) study with subjects receiving 1 h/day AG (n = 8) or no AG (n = 7) and (2) a 28-day BR study with 1 h/day resistance EX (n = 10) or no EX (n = 3). In both studies, stable isotopes of Ca were administered orally and intravenously, at baseline and after 10 days of BR, and blood, urine, and feces were sampled for up to 14 days post dosing. Tracers were measured using thermal ionization mass spectrometry. Data were analyzed by compartmental modeling.

Results: Less Ca was absorbed during BR, resulting in lower Ca balance in BR+AG (-6.04 ± 3.38 mmol/day, P = 0.023). However, Ca balance did not change with BR+EX, even though absorbed Ca decreased and urinary Ca excretion increased, because endogenous excretion decreased, and there was a trend for increased bone deposition (P = 0.06). Urinary N-telopeptide excretion increased in controls during BR, but not in the EX group. Markers of bone formation were not different between treatment groups for either study. Ca intake decreased during BR (by 5.4 mmol/day in the AG study and 2.8 mmol/day in the EX study), resulting in lower absorbed Ca.

Conclusions: During BR (or space flight), Ca intake needs to be maintained or even increased with countermeasures such as exercise, to enable maintenance of bone Ca.

Fig. 1

Study design showing the length of baseline, bed rest, and recovery periods for the AG study (a) and EX study (b). The times of the three doses of tracer administration are indicated by the arrows above the timeline for each study

Fig. 2

A model of calcium metabolism in humans. Circles represent compartments, numbers in circles represent compartment numbers, arrows represent movement of calcium between compartments, and double-shafted arrows represent entry of calcium by way of the diet (V_i) or bone resorption (V_o−). The asterisks indicate entry of tracer, and triangles indicate sampled compartments. Compartment 1 contains blood, 2 extravascular space, and 3 exchangeable calcium on bone. Compartment 5 represents feces, and 6 urine, and 8 and 10 sites in the intestine. The box labeled 9 represents a delay in movement of calcium in the lower intestine. Fractional absorption was calculated as L(1,8)/(L(1,8)+L(10,8))

Fig. 3

Calcium kinetics (Ca absorption, bone deposition, bone resorption, and calcium balance) during and after bed rest with (EX) and without exercise [CON(EX)] and with (AG) and without artificial gravity [CON(AG)]. Asterisk indicates significant difference from pre-bed rest in all four groups. **P<0.05, significantly different from pre-bed rest in the AG group; ^#P<0.05, significantly different from pre-bed rest in the CON(AG), AG, and EX groups; ^##P<0.05, significantly different from pre-bed rest in the CON(EX) and AG groups

Fig. 4

Urinary N-telopeptide, serum bone-specific alkaline phosphatase (BSAP), and serum parathyroid hormone, before, during, and after bed rest. To simplify the figure, the x-axis time points were averaged into the groupings presented, but all statistical analyses were performed on the raw data that are presented in the online resources