A study of metabolic balance in crewmembers of Skylab IV

A metabolic balance study was conducted on the three crewmembers of the 84-day Skylab IV earth orbital mission. Dietary intake was controlled, monitored, and kept very nearly constant for a period commencing 21 days prior to flight, throughout flight, and for a period of 18 days postflight. Within the first 30 days of flight urine calcium rose to a level approx. 100% above preflight levels and remained elevated for the remainder of the flight. Fecal calcium excretion increased more slowly but continued to accelerate throughout the flight and did not return to baseline levels during the postflight period. Urinary nitrogen increased to 25-30% above preflight levels within one month following launch and thereafter gradually subsided toward control values. The overall losses of calcium averaged approx. 200 mg per day throughout the mission while nitrogen losses averaged 590 mg. Various other indices of musculoskeletal deterioration are discussed and correlated. The parallelism between the effects of weightlessness and bed rest is reviewed. It is noted, that no evidence is yet available as to the identity of the initial biological response to the absence of gravity.     

Bone effects of space flight: analysis by quantum concept of bone remodelling.

During the manned Skylab flights mineral losses from the calcaneum and changes in external calcium balance were in the ranges found for healthy subjects at bedrest. Calcium balance reached a nadir of -200 mg/day by two months with no change thereafter; the negative balance was due to increased urinary excretion with no change in net absorption. The total calcium loss averaged 18 g in the longest flight of 84 days; the densitiometric data suggested that about two-thirds of this came from trabecular bone and about one-third from cortical bone. These data could represent reversible bone loss due to increased birth rate of normal osteoclasts and osteoblasts and consequent increase in bone turnover and in reversible mineral deficit, or irreversible bone loss due to overactive osteoclasts and/or underactive osteoblasts. If the former explanation is correct, significant bone loss is unlikely whatever the duration of future flights, except in older persons already losing bone; if the latter explanation is correct, space flights longer than six months may lead to a significant increase in fracture risk in later life. Neither terrestrial immobilization nor unwilling animals in orbit are ideal models for the effects of space flight on human bone. To choose between reversible and irreversible mechanisms of bone loss, and to determine the effects of space flight on lifelong fracture risk, future astronauts and cosmonauts must undergo adequate histologic study of bone after in vivo tetracycline labeling.     

Physiological effects of acute and ordinary bed rest conditions on endurance trained volunteers.

The aim of this study was to carry out a comparative study of water balance and water protein composition of the blood during exposure to acute (abrupt restriction of motor activity) and ordinary rigorous bed rest of 7 days. The studies were performed on 30 long distance runners aged 22-25 years old who had a VO2, max of 66 ml kg-1 min-1 on the average. The volunteers were divided into three equal groups: the volunteers in the 1st group were under a normal ambulatory life conditions (control subjects), the volunteers of the 2nd group subjected to an acute bed rest (abrupt restriction of motor activity) regime (acute bed rested subjects) and the volunteers of the 3rd group were submitted to ordinary and rigorous bed rest (rigorous bed rested subjects). All volunteers were on an average of 13.8 km day before taking part in this investigation. The 2nd and 3rd groups of volunteers were kept under a rigorous bed rest regime for 7 days. During the prebed rest period and actual bed rest period plasma volume (PV), total protein and protein fractions (albumins and globulins) and hematocrit were measured. Exposure to acute bed rest conditions induced a significant increase in hematocrit, hemoglobin concentration, protein fractions and marked decrease in (PV) and water balance which were significantly more pronounced than during exposure to ordinary rigorous bed rest. It was concluded that exposure to acute bed rest conditions induces significantly greater changes in water balance and water protein concentration of the blood of endurance trained volunteers than exposure to ordinary rigorous bed rest conditions.     

Ion regulatory function of the human kidney in prolonged space flights.

Ten cosmonauts, who performed 30-175-day space flights aboard Salyut-4 and Salyut-6, and over 60 test subjects who were exposed to bed rest of up to 182 days and immersion of up to 56 days, were examined. The renal excretion of potassium and calcium increased, reaching a maximum by the 4-6th weeks in prolonged space flights and simulation studies. During the load tests with potassium and calcium salt, excretion postflight was much higher than preflight. During potassium chloride load tests a positive correlation between the blood content of aldosterone and potassium excretion existed, whereas during calcium lactate load tests an increased calcium excretion was accompanied by a decrease in blood parathyroid hormone concentration. The most probable cause of the negative ion balance in weightlessness is the reduced capacity of tissues to retain electrolytes due to the decreased ion pool capacity. Different exercises have been shown to exert a beneficial effect on electrolyte metabolism.     

Mineral and nitrogen balance study: results of metabolic observations on Skylab II 28-day orbital mission

Prediction that the various stresses of flight, particularly weightlessness, would bring about significant derangements in the metabolism of the musculoskeletal system has been based on various observations of long-term immobilized or inactive bed rest. The only attempt at controlled measurement of metabolic changes in space prior to Skylab, a study during the 14-day Gemini VII flight, revealed rather modest losses of important elements. The three astronauts of Skylab II consumed a planned day-by-day, quite constant, dietary intake of major metabolic elements in mixed foods and beverages and provided virtually complete collections of excreta for 31 days preflight, during the 28 days inflight, and for 17 days postflight. Analyses showed that, in varying degree among the crewmen, urinary calcium increased gradually during flight in a pattern similar to that observed in bed-rest studies: the mean plateau peak of urinary calcium excretion in the latter part of flight was double preflight levels. Fecal calcium excretion did not change significantly, but calcium balance, owing to the urinary calcium rise, became either negative or less positive than in preflight measurement. Increased excretion and negative balance of nitrogen and phosphorus indicated appreciable loss of muscle tissue in all three crewmen. Significant losses also occurred inflight in potassium, sodium, and magnesium. Based on the similarity in pattern and degree between these observations and those in bed rest of the losses in calcium, phosphorus, and nitrogen, musculoskeletal integrity would not be threatened in space flights of up to at least 3 months. However, if similar changes occur, indicative of continuing losses of these elements, in the planned Skylab flights for considerably more than 28 days, concern for capable musculoskeletal function should be serious for flights of very many months' duration, and greater research attention will need to be given to development of protective counter-measures.     

Biochemical and hormonal changes in endurance trained volunteers during and after exposure to bed rest and chronic hyperhydration.

The objective of this investigation was to assess the effect of a daily intake of fluid and salt supplementation on biochemical and hormonal changes in endurance trained volunteers aged 19-24 yrs during 30-day bed rest and during 15 days of post bed rest period. The studies were performed on 30 long distance runners aged 19-24 yrs who had a peak oxygen uptake of 66 ml/kg/min and had taken 14.5 km/day on average prior to their participation in the study. The volunteers were divided into three groups: the volunteers in the first group were under normal ambulatory conditions (control subjects); the second group subjected to bed rest alone unsupplemented (bed rested volunteers); the third group was submitted to bed rest and consumed daily 30 ml water/kg bodyweight and 0.1 g of sodium chloride (NaCl)/kg body weight (supplemented bed rested volunteers). The second and third groups of volunteers were kept under a rigorous bed rest regime for 30 days. During the pre bed rest period of 15 days, during the bed rest period of 30 days and during the post bed rest period of 15 days cyclic adenosine monophosphate, cyclic guanosine monophosphate, prostaglandins of pressor, prostaglandins depressor groups, renin activity in plasma and aldosterone in plasma and in urine were determined. We found that in bed rested volunteers without fluid and salt supplementation intake plasma renin activity and aldosterone in plasma and urine continued to increase during the bed rest period as plasma volume decreased. Moreover, in this group, cyclic nucleotides measured as an indicator of adrenosympathetic system activity increased and prostaglandins as local vasoactive substances decreased during the bed rest period. These variables returned toward the baselines in the post bed rest period as plasma volume deficit was restituted. On the other hand, the hormonal levels in the other two groups remained rather constant during the experimental period. We concluded that daily intake of fluid and salt supplementation may minimize the biochemical and hormonal changes in endurance trained volunteers dorm their exposure to bed rest conditions.     

Fluid and salt supplementation effect on body hydration and electrolyte homeostasis during bed rest and ambulation

Bed rest (BR) induces significant urinary and blood electrolyte changes, but little is known about the effect of fluid and salt supplements (FSS) on catabolism, hydration and electrolytes. The aim was to measure the effect of FSS on catabolism, body hydration and electrolytes during BR.

Studies were done during 7 days of a pre-bed rest period and during 30 days of a rigorous bed rest period. Thirty male athletes aged, 24.6±7.6 years were chosen as subjects. They were divided into three groups: unsupplemented ambulatory control subjects (UACS), unsupplemented bed rested subjects (UBRS) and supplemented bed rested subjects (SBRS). The UBRS and SBRS groups were kept under a rigorous bed rest regime for 30 days. The SBRS daily took 30 ml water per kg body weight and 0.1 sodium chloride per kg body weight.

Plasma sodium (Na), potassium (K), calcium (Ca) and magnesium (Mg) levels, urinary Na, K, Ca and Mg excretion, plasma osmolality, plasma protein level, whole blood hemoglobin (Hb) and hematocrit (Hct) level increased significantly (p≤0.05), while plasma volume (PV), body weight, body fat, peak oxygen uptake, food and fluid intake decreased significantly (p≤0.05) in the UBRS group when compared with the SBRS and UACS groups. In contrast, plasma and urinary electrolytes, osmolality, protein level, whole blood Hct and Hb level decreased significantly (p≤0.05), while PV, fluid intake, body weight and peak oxygen uptake increased significantly (p≤0.05) in the SBRS group when compared with the UBRS group. The measured parameters did not change significantly in the UACS group when compared with their baseline control values.

The data indicate that FSS stabilizes electrolytes and body hydration during BR, while BR alone induces significant changes in electrolytes and body hydration. We conclude that FSS may be used to prevent catabolism and normalize body hydration status and electrolyte values during BR.     

Biochemical and hemodynamic changes in normal subjects during acute and rigorous bed rest and ambulation

Rigorous bed rest (RBR) induces significant biochemical and circulatory changes. However, little is known about acute rigorous bed rest (ARBR). Measuring biochemical and circulatory variables during ARBR and RBR the aim of this study was to establish the significance of ARBR effect. Studies were done during 3 days of a pre-bed rest (BR) period and during 7 days of ARBR and RBR period. Thirty normal male individuals aged, 24.1 +/- 6.3 years were chosen as subjects. They were divided equally into three groups: 10 subjects placed under active control conditions served as unrestricted ambulatory control subjects (UACS), 10 subjects submitted to an acute rigorous bed rest served as acute rigorous bed rested subjects (ARBRS) and 10 subjects submitted to a rigorous bed rest served as rigorous bed rested subjects (RBRS). The UACS were maintained under an average running distance of 9.7 km day-1. For the ARBR effect simulation, ARBRS were submitted abruptly to BR for 7 days. They did not have any prior knowledge of the exact date and time when they would be asked to confine to RBR. For the RBR effect simulation, RBRS were subjected to BR for 7 days on a predetermined date and time known to them right away from the start of the study. Plasma renin activity (PRA), plasma cortisol (PC), plasma aldosterone (PA), plasma and urinary sodium (Na) and potassium (K) levels, heart rate (HR), cardiac output (CO), and arterial blood pressure (ABP) increased significantly, and urinary aldosterone (UA), stroke volume (SV) and plasma volume (PV) decreased significantly (p<0.05) in ARBRS and RBRS as compared with their pre-BR values and the values in UACS. Electrolyte, hormonal and hemodynamic responses were significantly (p<0.05) greater and occurred significantly faster (p<0.05) during ARBR than RBR. Parameters change insignificantly (p>0.05) in UACS compared with pre-BR control values. It was concluded that, the more abruptly muscular activity is restricted in experimental subjects while they are very active, the greater hemodynamic and biochemical change there is and probably in individuals whose muscular activity is abruptly terminated after an accident or sudden illness.     

Effect of potassium and calcium loading on healthy subjects under hypokinesia and physical exercise with fluid and salt supplements

The objective of this investigation was to determine the acute responses to the electrolyte challenges under hypokinesia and physical exercise (PE) of different intensities with fluid and salt supplementation (FSS). The studies were performed on 12 physically healthy male volunteers aged 19-24 years under 364 days of hypokinesia (decreased number of steps per day) with a set of PE with FSS. The volunteers were divided into two equal groups. The first group was subjected to a set of intensive PE and the second group was submitted to a set of moderate PE. Both groups of subjects consumed daily water and salt supplements that aimed to increase the body hydration level. For simulation of the hypokinetic effect all subjects were kept under an average of 3000 steps per day. Functional tests with a potassium chloride (KCl) and calcium lactate (Cal) load were performed during the hypokinetic period of 364 days and the 60-day, prehypokinetic period that served as control, while both groups of subjects consumed daily calcium and potassium supplements. The concentration of electrolyte and hormone levels in the blood and their excretion rate in urine were determined. Renal excretion of calcium and potassium and the blood concentration thereof increased markedly in both groups of subjects. With the potassium chloride load tests the increased potassium excretion was accompanied by higher aldosterone and insulin blood levels, and with the calcium lactate load tests the increased calcium excretion was accompanied by a decreased parathyroid content in the blood. FSS and PE, regardless of intensity, failed to attenuate calcium and potassium losses. Additional intake of KCl and Cal also failed to normalize potassium and calcium abnormalities. It was concluded that during the KCl and Cal loading tests, the increased losses of potassium and calcium in the hypokinetic subjects were due to the inability of their bodies to retain these electrolytes, and that electrolyte abnormalities could not be reversed by PE or rehydration in individuals subjected to prolonged restriction of motor activity.     

Changes in prevalence of subjective fatigue during 14-day 6° head-down bed rest

The present study examines the prevalence of subjective fatigue in young healthy males during 14 days of 6° head-down bed rest (HDBR) by using a multidimensional questionnaire. Forty-one subjects completed the Subjective Fatigue Scale questionnaire to assess the fatigue-related complaints and symptoms. The questionnaire is composed of three sections, with 10 items each. The sections measured drowsiness and dullness (Section 1), difficulty in concentration (Section 2), and the projection of physical disintegration (Section 3). The subjects answered simple questions between 1400 and 1700 on 6 measurement days before and during the HDBR period. The prevalence rate of low back pain was markedly high (80.5%) on the second day and more than 50% in the first half of the HDBR period, and any complaints related to either a lack of sleep or a deterioration in the quality of sleep continued until the end of the HDBR period. Our findings may be useful in developing preventive strategies against physical and mental fatigue associated with prolonged HDBR, horizontal bed rest, and microgravity environments.     

Evaluation of spontaneous baroreflex response after 28 days head down tilt bedrest

The spontaneous baroreflex response was evaluated during supine rest and head up tilt (60 degrees) before and immediately after a 28 day 6 degrees HDT bedrest in 6 healthy adult men (age 30-42 years). Sequences of 3 or more beats where RR-interval and systolic blood pressure changed in the same direction were used to evaluate baroreflex response slope (BRS). Prior to bedrest, the mean BRS and RR-interval were 18.0 +/- 3.9 ms/mm Hg and 926 +/- 61 ms at rest and 10.5 +/- 2.5 ms/mm Hg and 772 +/- 63 ms during the first 10 min of 60 degrees tilt. Following bedrest, these values changed to 15.6 +/- 2.7 ms/mm Hg and 780 +/- 53 ms at rest, and to 6.5 +/- 1.2 ms/mm Hg and 636 +/- 44 ms during tilt. Thus, (1) the spontaneous baroreflex can be evaluated in human subjects during experiments of orthostatic stress; (2) the baroreflex slope was reduced on going from supine to the head up tilt position; and (3) 28 days of bedrest reduced the spontaneous baroreflex slope.     

Changes in mood status and neurotic levels during a 20-day bed rest

This study evaluated changes of mood status and depressive and neurotic levels in nine young male subjects during a 20-day 6 degrees head-down tilting bed rest and examined whether exercise training modified these changes. Participants were asked to complete psychometrical inventories on before, during, and after the bed rest experiment. Depressive and neurotic levels were enhanced during bed rest period according to the Japanese version of Zung's Self-rating Depression Scale and the Japanese version of the General Health Questionnaire. Mood state "vigor" was impaired and "confusion" was increased during bed rest and recumbent control periods compared to pre-bed rest and ambulatory control periods according to the Japanese version of Profiles of Mood State, whereas the mood "tension-anxiety", "depression-dejection", "anger-hostility" and "fatigue" were relatively stable during experiment. Isometric exercise training did not modify these results. Microgravity, along with confinement to bed and isolation from familiar environments, induced impairment of mental status.     

Plasma volume and biochemical changes in athletes during bed rest chronic hyperhydration

Daily fluid and salt supplements (FSS) may be used to reduce plasma biochemical changes during bed rest (BR). The aim of this study was to evaluate the effect of a daily intake of FSS on plasma volume (PV) and biochemical changes during BR. Studies were done during a pre BR period of 15 days and during a BR period of 30 days. Thirty male athletes aged 22-26 years were chosen as subjects. They were divided into three groups: unsupplemented ambulatory control subjects (UACS), unsupplemented bed rested subjects (UBRS) and supplemented bed rested subjects (SBRS). The UBRS and SBRS were kept under a rigorous bed rest regime for 30 days. The SBRS took 26 ml water/kg body weight and 0.1 g sodium chloride/kg body weight daily. PV, protein, albumin, sodium (Na), Chloride (Cl), potassium (K), osmolality, creatinine, glucose, and whole blood haematocrit (Hct) and haemoglobin (Hb) concentrations were measured. PV increased significantly (P < or = 0.01) while plasma protein, albumin. Na, Cl, K, glucose, creatinine, osmolality, and whole blood Hb and Hct concentration decreased significantly (P < or = 0.01) in the SBRS group when compared with the UBRS group. By contrast, PV decreased significantly (P < or = 0.01), while plasma protein, albumin, Na, Cl, K, glucose, creatinine, osmolality and whole blood Hct and Hb concentration increased significantly (P < or = 0.01) in the UBRS group when compared with the SBRS and UACS groups. The measured parameters did not change significantly in the UACS group when compared with the baseline control values. It was concluded that a daily intake of FSS may be used to attenuate PV losses and biochemical changes in endurance trained athletes during bed rest.     

Effect of hyperhydration on bone mineralization in physically healthy subjects after prolonged restriction of motor activity

The objective of this investigation was to evaluate the effect of a daily intake of fluid and salt supplementation (FSS) on bone mineralization in physically healthy male volunteers after exposure to hypokinesia (decreased number of steps taken/day) over a period of 364 days. The studies were performed after exposure to 364 days of hypokinesia (HK) on 18 physically healthy male volunteers who had an average VO2max of 65 ml/kg/min and were aged between 19 and 24 years. For the simulation of the hypokinetic effect the volunteers were kept under an average of 1000 steps/day. The subjects were divided into three equal groups of 6: 6 underwent a normal ambulatory life (control group), 6 were placed under HK (hypokinetic group) and the remaining 6 were subjected to HK and consumed a daily FSS (water 26 ml/kg body wt and NaCl 0.10 mg/kg body wt) (hyperhydrated group). The density of the ulnar, radius, tibia, fibular, lumbar vertebrae and calcenous was measured. Calcium and phosphorus changes, plasma volume, blood pressure and body weight were determined. Calcium content in the examined skeletal bones decreased more in the hypokinetic subjects than in the hyperhydrated subjects. Urinary calcium and phosphorus losses were more pronounced in hypokinetic than hyperhydrated subjects. Plasma volume and body weight increased in hyperhydrated subjects, while it decreased in hypokinetic subjects. It was concluded that a daily intake of FSS may be used to neutralize bone demineralization in physically healthy subjects during prolonged restriction of motor activity.     

Bone mineral measurement: Skylab experiment M-078

The observation that bone mineral is lost in patients who are either immobilized or remain in bed for extended periods of time formed the basis for the concern that large amounts of bone mineral may be lost during long periods of weightlessness. This concern was magnified when early X-ray densitometry studies suggested that rather large amounts of mineral could be lost during rather short periods of weightlessness (4-14 days). Even though these Gemini results have recently been modified, they still reflect substantial losses in the upper extremity. This led to a series of prolonged bed-rest studies (30-36 weeks) which, in addition to careful calcium balance, also employed a newer, more precise method of estimating bone mineral in the radius, ulna, and os calcis. It employed an essentially monoenergetic photon source (125I) and a scintillation detector operating in a rectilinear scanning mode to measure bone mineral by the absorptiometric technique. Bed-rest studies revealed variable mineral losses but suggested that little if any is lost during 4-6 weeks, with variable amounts being lost in 8 weeks. Losses up to 40% were noted in the os calcis after 9 months, with essentially none in the radius and ulna. When this technique was employed during the Apollo 14, 15, and 16 missions, only one crewman (CMP Apollo 15) showed significant losses in the os calcis and none in the radius or ulna. These results were, therefore, in concert with the bed-rest data but at variance with the earlier Gemini data. The variability observed during bed rest was reconciled when it was observed that the rate of loss could be correlated with the initial 24-hour urinary hydroxyproline excretion and the initial os calcis mineral content. Prediction terms were established. Measurements of the SL-II crew after 28 days of weightlessness revealed no significant bone mineral losses. The Skylab data lie within the predicted limits obtained from the bed-rest data. The relevance of the prediction terms to the Skylab and longer missions discussed.     

Effects of muscle electrostimulation during simulated weightlessness

In a 45-day experiment test subjects were exposed to bed rest with their heads down at -4 degrees C. Twice a day their muscles of the stomach, back, femur, and shin were stimulated with electric current for 25-30 min. The value of muscle tension was close to their maximum voluntary contraction. The main objective was to prevent muscle atrophy and to maintain their trophic and functional state. Physiological measurements were carried out together with morphological, cytochemical, and biometric evaluations. The tissue removed during biopsy from M. soleus 7 days before the test and on the 30th hypokinetic day was used as substrate. Electrostimulation affected favourably the tone and strength of muscles as well as their static and dynamic endurance. Morphological studies showed a positive effect of electrostimulation on the muscle tissue, preventing the development of atrophic processes. During the first post-hypokinetic day orthostatic tolerance increased.     

Effect on the cardiac function of repeated LBNP during a 1-month head down tilt

Cardiovascular assessment by ultrasound methods was performed during two long duration (1 month) Head Down Tilt (HDT) on 6 healthy volunteers. On a first 1 month HDT session, 3 of the 6 subjects (A, B, C) had daily several lower body negative pressure tests (LBNP), whereas the 3 subjects remaining (D, E, F) rested without LBNP. On a second 1 month HDT session subjects D, E, and F had daily LBNP tests and the A, B and C subjects did not. The cardiac function was assessed by Echocardiography, (B mode, TM mode). On all the "6 non LBNP" subjects the left ventricule diastolic volume (LVDV), the stroke volume (SV) and the cardiac output (CO) increase (+10%, -15%) after HDT then decrease and remain inferior (-5%, -5%) or equal to the basal value during the HDT. Immediately after the end of the HDT the heart rate (HR) increase (+10%, +30%) whereas the cardiac parameters decrease weakly (-5%, -10%) and normalize after 3 days of recovery. On the "6 LBNP" subjects the LVDV, SV and CO increase (+10%, 15%) after 1 h HDT as in the previous group then decrease but remain superior (+5%, +15%) or equal to the basal value. After the HDT session, the HR is markedly increased (+20%, +40%) the LVDV and SV decrease (-15%, -20%) whereas the CO increases or decreases depending on the amplitude of the HR variations. These parameters do not completely normalize after 3 day's recovery. Repeated LBNP sessions have a significant effect on the cardiovascular function as it maintains all cardiac parameters above the basal value. The LBNP manoeuvre can be considered as an efficient countermeasure to prevent cardiac disadaptation induced by HDT position and probably microgravity.     

Fluid electrolyte excretion during different hypokinetic body positions of trained subjects

The aim of this study was to evaluate the effect of different body positions on renal excretion of fluid and electrolytes after exposure to 364 days of decreased number of steps per day (hypokinesia, HK). The studies were performed on 18 endurance trained male volunteers aged 19-24 years who had an average of VO2max 67 ml/kg body/min. All volunteers were divided into three equal groups: the 1st group subjected to 12 h orthostatic position (OP) and 12 h clinostatic position (CP)/day, the 2nd group exposed to 8 h orthostatic position and 14 h clinostatic position/day, and the 3rd group submitted to 10 h orthostatic position and 16 h clinostatic position/day for 364 days. For the simulation of the hypokinetic effect all volunteers were kept under an average of 3000 steps/day for 364 days. Diuresis and the concentrations of sodium, potassium, chloride, calcium and magnesium as well as excretion of creatine were determined in 24-h urine samples. By the end of the hypokinetic period all volunteers, regardless of their body position during HK, manifested a significant increase in renal excretion of fluid and electrolytes as compared to prehypokinetic period values. It was concluded that prolonged restriction of motor activity induced a significant increase in renal excretion of fluid and electrolytes in endurance trained subjects regardless to their body position and duration thereof per day.     

Increased urinary excretion rates of serotonin and metabolites during bedrest

Astronauts are often on a voluntarily reduced energy intake during space missions, possibly caused by a metabolic or emotional stress response with involvement of the central serotonergic system (SES). We investigated 24 h urinary excretion (24 h-E) of serotonin (5-HT) and 5-hydroxyindol acidic acid as indicators of the SES in healthy males under two different normocaloric conditions: normal physical activity (NPA) and -6 degree head-down-tilt (HDT). HDT or NPA were randomly arranged with a recovery period of 6 months in between. 24 h-E of hormones varied widely among individuals. Values were higher in HDT compared to NPA. Assuming that the 24 h-E values are, beside being indicators for alterations in the number and metabolism of platelets. Also indicators of central SES, HDT condition seems to activate central SES in a higher degree compared to NPA. Therefore, changes in central SES might be involved in the mechanisms associated with space flight or microgravity, including possible maladaptations such as voluntary undernutrition.