Abstract
BackgroundBed-rest experiments are designed for investigation on catabolic effects of hypokinetic conditions and/or for microgravity simulation in on-ground aerospace research. Bed-rest effects include a reduction in fat-free mass and muscle mass. Urea and creatinine are catabolites of endogenous protein and of muscular energetic metabolism which are excreted mainly by the kidney. The study investigated on urea, creatinine, and kidney function during bed-rest.MethodsTwenty healthy young men underwent a 7-day adaptation period (day-6 to day-0) and a 35-day bed-rest experiment (day1 to day35) during normocaloric diet. Urine were collected from day-3 to day0 (baseline) and from day1 to day35. Blood samples and anthropometrical data were collected at day0 (baseline) and bed-rest days 7, 14, 21, 28, and 35.ResultsBed-rest reduced plasma volume, weight, fat-free mass, and muscle mass (P<0.001). During bed-rest there was a transient increase in plasma and urinary urea, a decrease in plasma creatinine, and no change in urinary creatinine. The overall integral of changes from day0 to day35 was on average +101.7 mg/dL for plasma urea (95%CI = +43.4/+159.9), +82.2 g/24 h for urinary urea (95%CI = +55.8/+108.7), −2.5 mg/dL for plasma creatinine (95%CI = −3.1/−1.9). Bed-rest reduced plasma cistatyn C also, which was used as mass-independent marker of glomerular filtration rate (−13.1%, P<0.05). Correlations with final reduction in fat-free mass and muscle mass were significant for the overall integral of changes in urinary urea from day0 to day35 (R = 0.706, P<0.001) and for early changes in urinary urea and plasma urea from day0 to day7 (R = 0.566, P = 0.009 and R = 0.715, P<0.001, respectively).ConclusionsStudy results shows that urea is a marker of catabolic conditions secondary to hypokinetic conditions.
Highlights
Experiments based on a rigorous regimen of continuous bedrest are performed for investigation about the effects of hypokinetic conditions during hospitalization and/or for simulation of microgravity on Earth in aerospace medical research [1,2]
Skeletal muscle mass is the main determinant of creatinine generation because creatinine is the final catabolite of muscular energetic metabolism [7]
None of these previous studies addressed the possible confounding of changes in kidney function during bedrest it is known that the kidneys account for about 70% of urea excretion [6] and for almost all of creatinine excretion [7]
Summary
Experiments based on a rigorous regimen of continuous bedrest (bedrest) are performed for investigation about the effects of hypokinetic conditions during hospitalization and/or for simulation of microgravity on Earth in aerospace medical research [1,2]. A muscle mass reduction could increase the urea generation because the muscular tissue has a high protein content and urea is the final catabolite of endogenous protein breakdown [6]. Skeletal muscle mass is the main determinant of creatinine generation because creatinine is the final catabolite of muscular energetic metabolism [7]. Previous bedrest studies reported for urinary urea either an increase [8,9,10,11] or no change [11,12,13] and no effect for urinary creatinine [9]. Bed-rest experiments are designed for investigation on catabolic effects of hypokinetic conditions and/or for microgravity simulation in on-ground aerospace research. The study investigated on urea, creatinine, and kidney function during bed-rest
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