Effects of high-protein intake on bone turnover in long-term bed rest in women

Bed rest (BR) causes bone loss, even in otherwise healthy subjects. Several studies suggest that ambulatory subjects may benefit from high-protein intake to stimulate protein synthesis and to maintain muscle mass. However, increasing protein intake above the recommended daily intake without adequate calcium and potassium intake may increase bone resorption. We hypothesized that a regimen of high-protein intake (HiPROT), applied in an isocaloric manner during BR, with calcium and potassium intake meeting recommended values, would prevent any effect of BR on bone turnover. After a 20-day ambulatory adaptation to a controlled environment, 16 women participated in a 60-day, 6\ub0 head-down-tilt (HDT) BR and were assigned randomly to 1 of 2 groups. Control (CON) subjects (n = 8) received 1 g/(kg body mass\ub7day)-1 dietary protein. HiPROT subjects (n = 8) received 1.45 g protein/(kg body mass\ub7day)-1 plus an additional 0.72 g branched-chain amino acids per day during BR. All subjects received an individually tailored diet (before HDTBR: 1888 \ub1 98 kcal/day; during HDTBR: 1604 \ub1 125 kcal/day; after HDTBR: 1900 \ub1 262 kcal/day), with the CON group's diet being higher in fat and carbohydrate intake. High-protein intake exacerbated the BR-induced increase in bone resorption marker C-telopeptide (>30%) (p < 0.001) by the end of BR. Bone formation markers were unaffected by BR and high-protein intake. We conclude that high-protein intake in BR might increase bone loss. Further long-duration studies are mandatory to show how the positive effect of protein on muscle mass can be maintained without the risk of reducing bone mineral density

Calcium Isotopes in Human Urine as a Diagnostic Tool for Bone Loss: Additional Evidence for Time Delays in Bone Response to Experimental Bed Rest

The calcium (Ca) isotopic composition in urine during bed rest has been demonstrated to be systematically light, indicating a negative bone mineral balance (i.e., bone loss). Here we present new Ca isotope data on urine during the “nutritional countermeasures” (NUC) bed rest study. We analyzed the Ca isotopic composition of 24 h pooled urine samples from seven healthy male subjects during baseline data collection (BDC), head-down-tilt bed rest and recovery. Additionally, we analyzed urine from two follow-up examinations after the regeneration phase. We observed a change in Ca isotopic composition during the bed rest phase, indicative of bone loss with a time delay of 10 to 21 days. We also observe that the Ca isotopic composition of urine is strongly dependent on the individual Ca metabolism and varies between subjects. We relate this individuality in Ca metabolism to differences in the amounts of Ca being recycled in the kidneys. Previous studies have shown that the more Ca is reabsorbed in the kidneys the more enriched the urine becomes in heavy isotopes of calcium. The Ca isotopic composition of urine is thus modified by more than one process and cannot be used in a straightforward manner to monitor net bone mineral balance. To overcome this problem, we propose a new baseline approach for using Ca isotopes, which effectively cancels out the effects of individual renal Ca reabsorption. This allows us to detect bone loss in patients without ambiguity by combining measurements of the Ca isotopic composition of urine and daily Ca excretion rate and comparing these to data collected on healthy individuals with a normal steady-state bone balance

On the combined effects of normobaric hypoxia and bed rest upon bone and mineral metabolism: Results from the PlanHab study

AbstractBone losses are common as a consequence of unloading and also in patients with chronic obstructive pulmonary disease (COPD). Although hypoxia has been implicated as an important factor to drive bone loss, its interaction with unloading remains unresolved. The objective therefore was to assess whether human bone loss caused by unloading could be aggravated by chronic hypoxia.In a cross-over designed study, 14 healthy young men underwent 21-day interventions of bed rest in normoxia (NBR), bed rest in hypoxia (HBR), and hypoxic ambulatory confinement (HAmb). Hypoxic conditions were equivalent to 4000m altitude. Bone metabolism (NTX, P1NP, sclerostin, DKK1) and phospho-calcic homeostasis (calcium and phosphate serum levels and urinary excretion, PTH) were assessed from regular blood samples and 24-hour urine collections, and tibia and femur bone mineral content was assessed by peripheral quantitative computed tomography (pQCT).Urinary NTX excretion increased (P<0.001) to a similar extent in NBR and HBR (P=0.69) and P1NP serum levels decreased (P=0.0035) with likewise no difference between NBR and HBR (P=0.88). Serum total calcium was increased during bed rest by 0.059 (day D05, SE 0.05mM) to 0.091mM (day D21, P<0.001), with no additional effect by hypoxia during bed rest (P=0.199). HAmb led, at least temporally, to increased total serum calcium, to reduced serum phosphate, and to reduced phosphate and calcium excretion.In conclusion, hypoxia did not aggravate bed rest-induced bone resorption, but led to changes in phospho-calcic homeostasis likely caused by hyperventilation. Whether hyperventilation could have mitigated the effects of hypoxia in this study remains to be established

The impact of microgravity and gravitational countermeasures on the gut microbiome of humans enrolled in the AGBRESA study

The Artificial Gravity Bed Rest Study – AGBRESA – was the first joint study conducted by DLR, ESA and NASA to simulate the effects of microgravity on healthy subjects. Moreover, the study included the use of artificial gravity protocols in a short-arm human centrifuge as a measure to counteract the negative effects of weightlessness. The health of the gut translates into the overall wellbeing since the disruption of the gut symbiotic networks – dysbiosis – could be due to either diet, antibiotic ingestion, sleep disturbance, physical activity or psychological stresses. In recent times, the gut microbiome has changed from being a complementary addition to our digestive tract to a potentially life-changing role by directly being the source of stimuli which revealed to impact neurochemistry, behavior and overall physiological status. Combined, microbial fluctuations could alter the intestinal microbiota composition and bacterial metabolite production, or more severely, in the disruption of host intestinal barrier integrity and the immune system activity, triggering intestinal inflammation syndromes and making the gut a very relevant organ to be studied in the context of spaceflight. Thus, 12 subjects, 8 males, were subjected to bed rest at negative 6-degree inclination for a period of 60 days with a preceding baseline of 15 days and posterior recovery period of 14 days. In other to characterize the gut microenvironment of healthy humans in simulated microgravity, fecal samples were collected during the baseline stage (once), during the head-down tilt treatment (at days 10, 30, and 50) and during the recovery period (once), and the samples were then processed for 16S rRNA sequencing and taxonomic analysis of the gut microenvironment. The characterization of the prokaryote flora was conducted 1) throughout time in contrast to the baseline reference and 2) in the context of the gravitational countermeasure vs the bed-rest-only control. The analysis revealed the detection of commensal microorganisms described to positively impact the gut such as Bifidobacterium spp., Lactobacillus spp., Akkermansia spp. and Enterococus spp.. Interestingly, we were able to detect pathogens like Campylobacter hominis which has been linked to severe bowel diseases ulcerative colitis and Crohn's disease. Also, opportunistic microorganisms such as Fusobacterium spp., Prevotella spp., Pseudomonas spp., Staphylococcus and Streptococcus spp., could potentially indicate an imbalance of the microbial networks and be a good an indicator of dysbiosis. Additionally, we set aside samples to undergo proteomic and metabolite analysis to improve the characterization of the gut microenvironment under microgravity simulation and the extent of the gravitational countermeasure recovery on bowel condition. Overall, the microgravity simulation performed on the AGBRESA study did not impact dramatically the fitness of the participants. Nonetheless, the analysis of the gut provides important insights on the triggers that occur during the adaptation of human physiology to long term exposure to spaceflight conditions and whether these relate to the described complications associated with gut disease

The impact of bed rest on human skeletal muscle metabolism

Insulin sensitivity and metabolic flexibility decrease in response to bed rest, but the temporal and causal adaptations in human skeletal muscle metabolism are not fully defined. Here, we use an integrative approach to assess human skeletal muscle metabolism during bed rest and provide a multi-system analysis of how skeletal muscle and the circulatory system adapt to short- and long-term bed rest (German Clinical Trials: DRKS00015677). We uncover that intracellular glycogen accumulation after short-term bed rest accompanies a rapid reduction in systemic insulin sensitivity and less GLUT4 localization at the muscle cell membrane, preventing further intracellular glycogen deposition after long-term bed rest. We provide evidence of a temporal link between the accumulation of intracellular triglycerides, lipotoxic ceramides, and sphingomyelins and an altered skeletal muscle mitochondrial structure and function after long-term bed rest. An intracellular nutrient overload therefore represents a crucial determinant for rapid skeletal muscle insulin insensitivity and mitochondrial alterations after prolonged bed rest

Bone loss in microgravity

That bone is lost in space is now commonly known, but this recognition was quite a surprise when human spaceflight began. What is less well known, but no less true, is that the loss concentrates on the leg bones. Is it caused by fluid shifts, simple mechanics or space food? Loss of density in the leg bones can amount to a reduction of one-quarter within 6 months of spaceflight (Vico et al. 2000), a magnitude and rate that seem to outweigh the bone losses of 5–10% experienced by women after menopause. A substantial risk of fracture would thus arise for longterm missions were they done without adequate countermeasures. So, how could we try to prevent that kind of bone loss, or, as the physiologist would say, what is the cause of it

Einfluß einer hohen Kochsalzzufuhr auf die Natriumspeicherung, den Knochenstoffwechsel und den Säure-Basen Haushalt bei ambulanten sowie immobilisierten Probanden

Ziel der vorliegenden Arbeit war die Untersuchung der Auswirkungen einer hohen Kochsalzzufuhr (NaCl-Zufuhr) auf die Form der Natriumspeicherung, den Säure-Basen Haushalt und auf den Knochenstoffwechsel. In diesem Zusammenhang stellen Astronauten eine vulnerable Gruppe dar und wurden daher besonders betrachtet. Zum einen haben sie eine hohe NaCl-Zufuhr, die vermutlich auf einen reduzierten Geschmackssinn im All zurückzuführen ist, und zum anderen weisen sie, aufgrund fehlender mechanischer Belastung (Immobilisation) im All, einen erhöhten Knochenabbau auf. In welcher Form Natrium auch, entgegen der gängigen Lehrbuchmeinung, ohne gleichzeitige Wassereinlagerung (osmotisch inaktiv), gespeichert werden kann und in welchem Zusammenhang der Säure-Basen Haushalt zu einem kochsalzinduzierten Knochenabbau steht, wurde anhand zweier stationärer Studien an 9 (Studie 1) bzw. 8 (Studie 2) gesunden, jungen, männlichen Probanden untersucht. Beide Studien von je 28 (Studie 1) bzw. 2 x 21 Tagen (Studie 2) fanden im Stoffwechsellabor des Deutschen Zentrums für Luft- und Raumfahrt (DLR) statt. In Studie 1 erhielten die Probanden in 4 aufeinanderfolgenden Phasen eine unterschiedliche diätetische Kochsalzzufuhr (Phase 1 und 4 mit jeweils 6 Tagen: 0,7 mmol NaCl/kgKG/Tag; Phase 2 mit 6 Tagen: 2,8 mmol NaCl/kgKG/Tag; Phase 3 mit 10 Tagen: 7,7 mmol NaCl/kgKG/Tag). In Studie 2 wurden die Probanden nach einer 4-tägigen Adaptationsphase für 14 Tage im 6° Kopftieflagemodell immobilisiert und erhielten im cross-over design eine hohe (7,7 mmol NaCl/kgKG/Tag) und niedrige (0,7 mmol NaCl/kgKG/Tag) Kochsalzzufuhr. In beiden Studien wurde die Art der Natriumspeicherung durch die Erstellung täglicher metabolischer Bilanzen des Natrium-, Wasser- und Kaliumhaushaltes und durch Körpergewichtsveränderungen ermittelt. Der Einfluss auf den Knochenstoffwechsel wurde durch die Bestimmung von Knochenformations- (bAP, PINP, Osteocalcin) und Knochenresorptionsmarkern (CTX, NTX) bestimmt. Blutgasanalysen und pH Wert-Messungen im 24-h Urin gaben Aufschluss über gleichzeitige Veränderungen im Säure-Basen Haushalt. Bei hoher NaCl-Zufuhr wurde eine Natriumspeicherung zwischen 290 ± 226 mmol (Studie 1) und 335 ± 266 mmol (Studie 2) beobachtet, die in Studie 1, anhand der betrachteten Kalium- und Wasserbilanz, als vorwiegend osmotisch inaktiv einzustufen war. Bei immobilisierten Probanden hingegen ging die Natriumspeicherung mit einer negativen Kaliumbilanz einher, was die Folge einer osmotisch neutralen Natriumspeicherung mit einem intrazellulären Kaliumaustausch darstellen könnte. In Bezug auf den Knochenstoffwechsel zeigten beide Studien bei hoher NaCl-Zufuhr sowohl einen signifikanten Anstieg der Calcium- als auch der CTX- bzw. NTX-Exkretion (in beiden Studien p 1. Eine Natriumspeicherung bei hoher Kochsalzzufuhr von 7,7 mmol NaCl/kgKG/Tag erfolgt zum Teil ohne gleichzeitige Wassereinlagerung, die sich unter den gegebenen Studienbedingungen bei ambulanten Probanden als vorwiegend osmotisch inaktiv (Studie 1) und bei immobilisierten Probanden (Studie 2) als vorwiegend osmotisch neutral im Austausch mit Kalium äußerte. 2. Die Erzeugung einer milden metabolischen Azidose, als Folge einer hohen Kochsalzzufuhr, stellt einen potentiellen Mechanismus der kochsalzbedingten erhöhten Knochenresorption dar und eröffnet somit ein neues Feld an potentiellen Gegenmaßnahmen. 3. Eine Betrachtung individueller Reaktionen zeigt keinen offensichtlichen Zusammenhang zwischen einer Natriumspeicherung und einem kochsalzbedingten erhöhten Knochenabbau.Influence of a high salt intake on sodium retention, bone metabolism and acid-base balance in ambulatory and immobilised test subjects The present work aimed to examine the effect of a high salt (sodium chloride, NaCl) intake on different forms of sodium retention, acid-base balance and bone metabolism. Because of the fact that astronauts are a vulnerable group in this context, they were of special interest. Astronauts have a high salt intake, probably because of a reduced sense of taste, as well as an increased bone resorption resulting from the lowered mechanical load in space. In which forms sodium could be retained even without fluid retention (osmotically inactive)- contrary to the argumentation of physiological text books - and if the acid-base balance is connected to sodium chloride induced bone loss was examined in two stationary studies with 9 (study 1) and 8 (study 2) healthy, young, male test subjects. The studies consisting of 28 (study 1) and 2 x 21 days (study 2) were carried out at the German Aerospace Center (DLR). Test subjects of study 1 received consecutively 4 different dietary salt intake levels (phase 1 and 4 for a period of 6 days: 0.7 mmol NaCl/kgBW/d; phase 2 for a period of 6 days: 2.8 mmol NaCl/kgBW/d; phase 3 for a period of 10 days: 7.7 mmol NaCl/kgBW/d). In study 2, after an adaptation period of 4 days, test subjects were immobilised in 6° head-down tilt bed rest for 14 days during which they received a high (7.7 mmol NaCl/kgBW/d) and a low salt intake in cross-over design. The form of sodium retention was investigated by the calculation of daily metabolic sodium-, water- and potassium balances and by changes in body weight. The measurements of bone formation (bAP, PINP, Osteocalcin) markers as well as bone resorption markers (CTX, NTX) supplied insight into the influences of a high salt intake on bone metabolism. Blood gas analysis and ph values of 24-h urine were used to gather information about accompanying changes in the acid-base balance. High sodium chloride intake led to sodium retention of 290 ± 226 mmol in study 1 and 335 ± 266 mmol in study 2. Respectively potassium- and water balances show that the retained sodium in study 1 was stored in an almost osmotically inactive form. However, in immobilised subjects sodium retention was accompanied by a negative potassium balance which could be due to an intracellular exchange for potassium and therefore almost osmotic neutral sodium retention. Concerning bone metabolism, sodium chloride intake led to a significant increase in calcium-, CTX- and NTX-excretion (both studies p 1. Sodium retention because of a high salt intake (7.7 mmol NaCl/kgBW/d) is partially stored even without fluid retention. Under ambulatory conditions sodium is stored mainly in an osmotically inactive form, whereas in immobilisation the storage appears to be mainly osmotically neutral through an exchange for potassium. 2. Low-grade metabolic acidosis may be the cause for sodium chloride induced increased bone resorption. This leads us to think about a new field of countermeasures. 3. Considering individual reactions there is no apparent connection between sodium retention and salt induced bone loss

Knochenstoffwechsel in der Schwerelosigkeit - Mechanismen und Maßnahmen gegen den Verlust von Knochenmasse

Der Verlust von Knochenmasse im All stellt ein bekanntes Problem mit bedenklichem Ausmaß für Langzeitmissionen dar. Um den Verlust von Knochenmasse zu verhindern und geeignete Gegenmaßnahmen einzusetzen, ist das Verständnis der physiologischen Ursachen absolut unerlässlich. Die Flussigkeitsverschiebung als früheste Theorie, die mechanische Anpassung des Knochens an veränderte Umgebungsbedingungen sowie hormonelle und nutritive Einflüsse stellen wichtige Bereiche der Ursachenforschung dar. Der folgende Artikel gibt eine kurze Übersicht über die Inhalte dieser Theorien und betrachtet ihre aktuelle Bedeutung für die Entwicklung von Maßnahmen zur Verhinderung des Verlustes von Knochenmasse im All

Sclerostin and DKK1 levels during 14 and 21 days of bed rest in healthy young men

Objectives: Wnt signaling pathway may be crucial in the pathogenesis of disuse-induced bone loss. Sclerostin and DKK1, antagonists of the Wnt signaling pathway, were assessed during immobilization by bed rest in young, healthy people. Methods: Two bed rest studies were conducted at the German Aerospace Center in Cologne. 14 days of 6° head-down-tilt bed rest were applied to eight healthy young male test subjects in study 1 and 21 days of head-down-tilt bed rest to seven healthy male subjects in study 2. Results: Sclerostin levels increased in both studies during bed rest (study 1, 0.64±0.05 ng/ml to 0.69±0.04 ng/ml, P=0.014; study 2, 0.42±0.04 ng/ml to 0.47±0.04 ng/ml, P=0.008) and they declined at the end of the 14- and 21-day bed rest periods. DKK1 decreased during the bed rest period in study 1 (P<0.001) but increased during bed rest in study 2 (P=0.006). As expected, bone formation marker PINP decreased (study 1, P=0.013; study 2, P<0.001) and bone resorption marker NTX increased during bed rest (P<0.001). Conclusion: Data suggest that the Wnt signaling pathway is involved in disuse-induced bone loss in young, healthy humans