Caloric restriction decreases orthostatic tolerance independently from 6° head-down bedrest.
Astronauts consume fewer calories during spaceflight and return to earth with an increased risk of orthostatic intolerance. Whether a caloric deficiency modifies orthostatic responses is not understood. Thus, we determined the effects of a hypocaloric diet (25% caloric restriction) during 6° head down bedrest (an analog of spaceflight) on autonomic neural control during lower body negative pressure (LBNP). Nine healthy young men completed a randomized crossover bedrest study, consisting of four (2 weeks each) interventions (normocaloric bedrest, normocaloric ambulatory, hypocaloric bedrest, hypocaloric ambulatory), each separated by 5 months. Muscle sympathetic nerve activity (MSNA) was recorded at baseline following normocaloric and hypocaloric interventions. Heart rate (HR) and arterial pressure were recorded before, during, and after 3 consecutive stages (7 min each) of LBNP (-15, -30, -45 mmHg). Caloric and posture effects during LBNP were compared using two-way ANOVA with repeated measures. There was a strong trend toward reduced basal MSNA following caloric restriction alone (normcaloric vs. hypocaloric: 22±3 vs. 14±4 burst/min, p = 0.06). Compared to the normocaloric ambulatory, both bedrest and caloric restriction were associated with lower systolic blood pressure during LBNP (p<0.01); however, HR responses were directionally opposite (i.e., increase with bedrest, decrease with caloric restriction). Survival analysis revealed a significant reduction in orthostatic tolerance following caloric restriction (normocaloric finishers: 12/16; hypocaloric finishers: 6/16; χ2, p = 0.03). Caloric restriction modifies autonomic responses to LBNP, which may decrease orthostatic tolerance after spaceflight.
Navy Experimental Diving Unit, Panama City, Florida, United States of America.
John P
JP
Florian
DLR-Institute of Aerospace Medicine, Cologne, Germany.
Friedhelm J
FJ
Baisch
DLR-Institute of Aerospace Medicine, Cologne, Germany.
Martina
M
Heer
Department of Kinesiology, Noll Laboratory, The Pennsylvania State University, University Park, Pennsylvania, United States of America.
James A
JA
Pawelczyk
D000328:Adult; D062186:Arterial Pressure; D018480:Astronauts; D001510:Bed Rest / Q000009:adverse effects; D031204:Caloric Restriction / Q000009:adverse effects*; D018592:Cross-Over Studies; D004038:Diet, Reducing / Q000009:adverse effects*; D006339:Heart Rate; D006801:Humans; D008150:Lower Body Negative Pressure; D008297:Male; D018482:Muscle, Skeletal / Q000098:blood supply* / Q000294:innervation; D054971:Orthostatic Intolerance / Q000209:etiology* / Q000503:physiopathology; D011187:Posture; D013026:Space Flight; D013564:Sympathetic Nervous System / Q000098:blood supply*; D018474:Weightlessness Simulation