The effect of 60 days 6° head-down-tilt bed rest on the metabolic physiology of young, healthy males

Abstract

Introduction. Six-degree head-down-tilt (HDT) bed rest is a valuable experimental model for examining the physiological adaptations of gravity deprivation (spaceflight, physical inactivity, sedentary behaviour, immobilisation and ageing) including muscle atrophy, a shift in myofiber type composition, reduced cardiovascular and functional capacity and metabolic dysfunction. Establishing an exercise prescription which conserves time but mitigates these deleterious physiological adaptations is of profound importance for life in space and life on Earth. The overall aim was to examine the changes in physical characteristics, metabolic characteristics and circulating novel biomarkers of insulin sensitivity and insulin resistance in healthy young males, pre- and post-60 days of 6° HDT bed rest, with and without reactive jump training (RJT), a novel low volume, high-intensity jump training protocol. Methodology. A total of 23 male subjects (29 ± 6 years, 181 ± 6 cm, 77 ± 7 kg) were randomised to a control (CTRL, n = 11) or RJT (JUMP, n = 12) group and exposed to 60 days of 6° HDT bed rest. RJT was performed 6 days per week and on average, each session consisted of 48 countermovement jumps and 30 hops, performed with maximal effort at a load equal to 80 – 90% body weight in a sledge jump system (≤ 4 minutes total exercise time). Body composition, V̇ O2peak, muscle strength were measured and an oral glucose tolerance test (OGTT) was performed to estimate insulin sensitivity pre- and post HDT bed rest. Circulating lipids, fetuin-A, retinol binding protein-4 (RBP4), irisin, adropin, adiponectin, acylation stimulating protein (ASP), apelin, apolipoprotein-J (apo J) and fibroblast growth factor-21 (FGF-21) were quantified in fasting serum. A subanalysis was performed a posteriori to investigate individual metabolic response post HDT bed rest. Results. Body weight, lean mass and V̇ O2peak decreased in both groups post-HDT bed rest, with greater reductions observed in CTRL (p < 0.05). Significant main effects of time were found for increases in triglycerides, LDL-cholesterol and fetuin-A and decreases were observed in HDL-cholesterol, whole-body insulin sensitivity (Matsuda index) and tissue-specific insulin sensitivity, irisin, adropin, adiponectin and FGF-21 post-HDT bed rest (p < 0.05). In the subgroup with decreased insulin sensitivity, fetuin A, RBP4, fasting insulin and glucose increased and irisin, adropin, adiponectin, FGF-21 and liver and adipose tissue insulin sensitivity decreased post-HDT bed rest (p < 0.05). In the subgroup with increased insulin sensitivity, adiponectin and FGF-21 decreased and liver insulin sensitivity increased post-HDT bed rest (p < 0.05). Discussion. RJT preserved muscle mass and function, but could not mitigate the decline in insulin sensitivity or induce favourable changes in circulating novel biomarkers following HDT bed rest. In the subgroup with decreased insulin sensitivity, blunted insulin action and impaired peripheral glucose uptake were identified post-HDT bed rest. In the opposing subgroup with increased insulin sensitivity, an improvement in liver insulin sensitivity was found post-HDT bed rest. Fetuin-A, RBP4, irisin and adropin are candidate biomarkers for examining changes in insulin sensitivity in response to intervention. Conclusion. This study provides insights into the physiological adaptation to HDT bed rest, including whole-body and tissue-specific insulin sensitivity, and highlights the importance of future studies to explore individual responses to obtain personalised information for the optimisation of exercise prescription to maintain health in all conditions of gravity deprivation in space and on Earth.