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Original Studies |
Laboratoire de Physiologie de l’Environnement, Faculté de Médecine Lyon Grange-Blanche (S.B., C.G., G.G.-K.), 69373 Lyon Cedex 08; Centre de Recherche en Nutrition Humaine de Lyon, Faculté de Médecine Laënnec (S.N., C.P.), 69372 Lyon Cedex 08; Laboratoire de Nutrition Humaine (P.R.), 63009 Clermont-Ferrand Cedex 1; Laboratoire de Biologie du Tissu Osseux, Faculté de Médecine Jacques Lisfranc (L.V.), 42023 St. Etienne; and Centre National d’Etudes Spatiales (G.G.-K.), 75039 Paris Cedex 1, France
Address all correspondence and requests for reprints to: Dr. Stéphane Blanc, Laboratoire de Physiologie de l’Environnement, Faculté de Médecine Lyon Grange-Blanche, 8 avenue Rockefeller, 69373 Lyon Cedex 08, France.
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Abstract |
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 Top Abstract IntroductionSubjects and MethodsResultsDiscussionReferences |
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Introduction |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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A limited amount of in-flight data are available, the samples are small, and the subjects few. Moreover, the extensive demands on crew time and, in most instances, the relatively short duration of missions further limit opportunities for scientific observations. These limitations make it essential to study changes in physiological functions in complementary in-flight and ground-based research. Head-down bed rest (HDBR) is the most common analogy of the human weightless environment, as it reproduces the cardiovascular, muscular, and bone adaptations that occur in microgravity. This model can also be used to understand illnesses associated with strict recumbency inherent in the treatment of trauma victims and patients with severe injury. Such studies may enable us to develop countermeasures to reduce or prevent the deleterious adaptations to microgravity and to help patients recover a normal quality of life.
Few studies have assessed the energy requirements of healthy subjects during simulated microgravity. Krebs et al. (5) showed that the body weight of subjects who consumed a constant diet for 6 weeks before and 5 weeks during horizontal bed rest did not change, although LBM decreased and body FM increased (both significantly). A short microgravity simulation (3 days) carried out by Acheson et al. (6) demonstrated that HDBR results in an 8% increase in the postabsorptive metabolic rate and that this is linked to decreased glucose oxidation and increased lipid oxidation. To our knowledge, only Gretebeck et al. (7) have used the DLW method to investigate TEE during a 10-day HDBR (-6°). TEE was 21% lower than under control conditions. Neither the basal metabolic rate nor the resting metabolic rate (REE) changed, but there was a significant increase in body FM.
These changes in energy metabolism and body composition during both actual and simulated microgravity suggest that this environment results in a decreased metabolic efficiency (8, 9), in which several factors seem to be involved: activity, stress, and oxidation foodstuffs. To dissociate these factors, the short term (poorly studied) and the long term (unknown) effects, it was necessary to determine the main components of energy metabolism and to evaluate the stress response, during long term simulated microgravity in humans. This study was therefore conducted to investigate for the first time, to our knowledge, the energy and water metabolism adaptations and their interrelationships with body composition and anabolic and catabolic hormones during prolonged HDBR (42 days).
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Subjects and Methods |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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A group of eight healthy men volunteered for this study. Their
initial anthropometric data are presented in Table 1
. Subjects were selected after physical
and psychological examinations and were in excellent health, with no
history of chronic or recent acute illness. They were thoroughly
briefed about the experimental procedures and signed a consent form
approved by the Comité Consultatif de Protection de Personnes
dans les Recherches Biomédicales Midi-Pyrénées I
(France). No medication, smoking, alcohol, or caffeinated drinks were
allowed during the study. The experimental conditions were well
tolerated by the volunteers who completed the study.
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The TEE was assessed by the multipoint DLW method (2H218O) (10) for two periods of 15 days. In the first period, subjects remained in the bed rest facility at Hôpital Purpan for a 15-day ambulatory control period (HDBR d-15 to HDBR d-1) before undergoing -6° HDBR. The subjects then underwent 42 days of -6° HDBR. The second period of TEE measurement was HDBR d+27 to HDBR d+42. During those two periods, the water turnover and the formation of metabolic water were determined from the isotope data. The subjects then remained in the hospital for a 15-day recovery period. Total body water (TBW) was determined on HDBR d-15 and HDBR d+27 by isotope dilution (18O). Respiratory gas exchanges were assessed by indirect calorimetry to measure REE and respiratory quotient (RQ) on HDBR d-15 and HDBR d+40. Body composition was assessed by dual x-ray absorptiometry on HDBR d-4, d+28, and d+42 and recovery d+30.
The subjects were fed weighed conventional foods ad libitum. A diet log was used to record the food consumed, the method of preparation, and the quantity. Any food remaining after meals was weighed, and the final weight of all foods consumed was recorded. A senior dietitian ensured the accuracy of the diet records. The food elemental composition was determined using PROFIL II software (version 2.1.5, ACIM, St. Doulchard, France). Dietary sodium was 3 g/day, and water intake was limited to 2.5 L/day. Water intake and urine volume were measured daily. Body weight was measured each morning using a special weighing system with the subjects remaining supine.
TEE
During the ambulatory period, the subjects provided baseline urine samples and then drank doubly labeled water composed of H218O 10% (Isotec, St. Quentin en Yvelines, France) mixed with 99.9% 2H2O (Isotec, St. Quentin en Yvelines, France). Each subject was given 0.05 g/kg 2H2O and 1.5 g/kg H218O. A urine sample was collected 4 h later, after equilibration of the isotopes with body fluids. Their second urine voids were collected each morning for 14 days. The subjects were given a second DLW dose (0.07 g/kg 2H2O and 1.7 g/kg H218O) on HDBR d+27 of the simulated microgravity. Daily urine samples were stored at -20 C in cryogenically stable tubes until analyzed by isotope ratio mass spectrometry.
Deuterium analysis (11)
Pyrex tubes were filled with 80 mg dry zinc (Indiana University,
Bloomington, IN) for the reduction reaction, and a 0.1-mL glass
conical insert was added. The tubes were attached to a 10-position
manifold and evacuated by a roughing pump while being heated to
approximately 250 C to drive off any residual moisture. The tubes were
backfilled with pure nitrogen and then opened to introduce 2 µL urine
sample. The valves above each sample were closed immediately, and the
tubes were immersed in liquid nitrogen for 5 min and evacuated. Tubes
were heated at 500 C for 30 min to reduce the water to hydrogen gas.
Ten samples were analyzed sequentially against a laboratory standard
gas, with a 
2H air-gas reference/V-SMOW = 0.00 
(SD = ±2 , V-SMOW, Vienna Standard Mean Ocean
Water). Deuterium analyses were performed in triplicate.
18-Oxygen analysis (11)
Bottles containing 2-mL samples were attached to the
equilibration manifold, evacuated, filled with pure CO2 to
400 mbar, and shaken for 4 h to ensure CO2/fluid
equilibration. The equilibrated CO2 was introduced into the
mass spectrometer via a trap (-80 C) to remove water and analyzed
against a laboratory standard prepared by digesting a carbonate with
acid and having a 18-o carbonate reference/V-SMOW =
35.68 (SD = ±0.2 ). The samples were analyzed
in an isotope ratio mass spectrometer (OPTIMA, Fisons, UK) at the
Centre de Recherche en Nutrition Humaine de Lyon (Lyon, France).
Calculations
The deuterium and oxygen-18 zero time intercepts and clearance
rates were calculated by least squares linear regression on the natural
logarithm of isotope enrichment as a function of elapsed time from dose
administration. The dilution spaces for 2H2O
and H218O (moles) were calculated from the
baseline and 4-h urine samples using the equation (12): N (mol) =
{[WA/(18.02)a] x [(a -
t)/(s - p)]}, where N
is the pool space, W is the amount of water used to dilute the dose, A
is the amount of dose administered, a is the dose diluted for analysis,
and is enrichment of dose (a), dilution water (t), postdose sample
(s), and predose baseline (p). The crude production of CO2
(r'CO2), assuming that losses of 2H and
18O occur at the same enrichment as that existing at the
same time in body water, was calculated by the fundamental equation
derived from the initial model of Lifson (13): rCO'2
(mol/day) = [(KoNo -
KdNd)/2] (Eq I), where No and
Nd are the dilution spaces of H218O
and 2H2O in moles, and Ko and
Kd are the turnover rates of H218O
and 2H2O in liters per day. The true
CO2 production (rCO2) is obtained taking into
account the allowances needed for the fractionation of 2H
leaving body water as water vapor (factor f1),
18O leaving by the same route (factor f2), and
the fractionation of 18O in its exchange between carbon
dioxide and water (factor f3). In that way, the equation
proposed by Coward (10), which further divides water loss into that
occurring as respiratory loss and those as skin losses, which are
fractionated, was used to assess the CO2 production rate
(moles per day). Values for respiratory loss were 1.1 mol water/mol
carbon dioxide (there is a constant relationship between respiratory
water loss and carbon dioxide loss) and 27.0 mol/day for fractionated
skin losses for normal adults in a temperate climate. Thus,
rCO2 (mol/day) = {[(KoNo -
KdNd - 27.3(f2 -
f1)]/[2f3 + 1.1(f2 -
f1)]} (Eq II), where f1 = 0.941,
f2 = 0.991, and f3 = 1.037 (10). The energy
equivalent of CO2 (EeqCO2) was calculated by
Wiers equation (14): EeqCO2 (KJ/L) = (15.457/RQ) + 5.573
(Eq III), where RQ is the respiratory quotient calculated for each
subject from the indirect calorimetry data. TEE (kJoules per
day) was then calculated as: TEE = rCO2 x
EeqCO2 x 22.4, where 22.4 is the conversion factor for
moles of CO2.
The precision of the CO2 production rate by the DLW multipoint method involves deriving the SE of Eq I, which, in turn, requires the variance and covariance of KoNo and KdNd. The precision is then estimated from the fractional error variance in r'CO2. The complex equation has been given in detail previously (15).
REE and RQ
On the day of the respiratory gas exchange measurements, each subject was awakened at 0630 h and transported to the room in which the test was to be performed. The subjects were tested in pairs. REE was measured by means of indirect calorimetry at 0° tilt during the control period (HDBR d-15) and at -6° tilt during the HDBR period (HDBR d+40). A ventilated hood connected to an open circuit indirect calorimeter (Deltatrac, Datex, Finland) was placed over the head of the subject. Measurement was performed for 1 h in the postabsorptive state. During the measurements, the volunteers were allowed to watch television, read, or listen to music under constant supervision by one of the investigators, but were not allowed to sleep. To prevent any stress associated with the respiratory exchange measurements, a simulation of the experiment was performed on each subject the evening preceding the real measurement. Standard calibration procedures of the gas analyzers and of the flow through the canopy were performed each day. Respiratory exchange measurements were recorded every minute and averaged over 15-min intervals. Only the last 45 min were considered. TEE minus REE was computed as an estimate of energy expended for physical activity plus diet induced-thermogenesis (DIT). The RQ was calculated as the ratio of VCO2/VO2.
TBW, water turnover, and metabolic water formation
TBW (liters) was deduced from the dilution space of oxygen-18 (No) after adjusting it by a factor of 1.01 to account for isotope exchange (12): TBW = (No/1.01) x 0.01802, where 0.01802 is the conversion factor for moles to liters.
Water turnover was calculated from the deuterium elimination rate (Kd, liters per day) using the fundamental equation derived by Lifson (13): water turnover (L/day) = TBW x [Kd/(xf1 + 1 - x)], where x is the true proportion of water loss that is fractionated and is assumed to be 23% of the total insensible water loss (16). The formation of metabolic water (moles per day) was determined from the product of RQ and the rCO2 of each individual (13).
Body composition
A Lunar energy x-ray absorptiometer (DPX-L, Lunar Corp., Madison, WI) was used to determine total and segmental FM and LBM. The hydration fraction of LBM, an indicator of the hydration state of the body, was calculated by dividing TBW measured by isotope dilution by LBM measured by absorptiometry.
Hormones and metabolites
Hormonal and metabolic concentrations were measured in 24-h urine samples. GH was assayed using a [125I]human GH U COATRIA kit (Laboratoires BioMérieux, France). The sensitivity was 0.5 pg/mL, and the intra- and interassay variabilities were 4.6% and 2.8%, respectively. Normetanephrine and metanephrine were measured by high performance liquid chromatography with electrochemical detection, and cortisol was determined by high performance liquid chromatography with fluorescence detection. Urea was assayed using a UREA-KIT S (Laboratoires BioMérieux, France). The sensitivity was 0.15 g/L. Lastly, creatinine was measured by the colorimetric method of Jaffe.
Statistical analysis
Each subject was his own control. The ambulatory period was taken as the control period. A nonparametric Wilcoxon paired test was used to identify differences among energy intake, dietary measures, and expenditure in the control and HDBR periods. The same test was used to detect differences in energy intake, total energy expenditure, and body composition between the control and HDBR periods.
The biological data for each week were pooled, and the data for the second ambulatory week were taken as the control values. One-way repeated measures ANOVA was used to identify differences in the urinary excretion of cortisol, urea, creatinine, GH, metanephrine, or normetanephrine between the control and HDBR periods. Fisher’s protected least significant differences test was used for post-hoc comparisons. To detect a possible link between TEE and LBM, an analysis of covariance was performed. All analyses were performed with StatView (Abacus Concepts, Inc., Berkeley, CA), and values are the mean ± SEM, with P < 0.05 considered statistically significant.
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Results |
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General variables
The results are presented in Fig. 1.
There was a significant reduction in body weight, which continued
throughout the HDBR (-0.6 ± 0.1% at week 3 to -2.1 ±
0.4% at week 8). Body weight increased during the recovery period, but
stayed significantly below that during the control period (-1.6
± 0.4% at week 9 and -1.4 ± 0.3% at week 10). Energy intake
gradually reduced from week 4 (10.5 ± 0.2 MJ/day) to week 8
(9.7 ± 0.2 MJ/day) from that during the control period (10.7
± 0.1 MJ/day). Urine output was significantly higher at weeks 5
(2.05 ± 0.12 L/day) and 6 (2.11 ± 0.19 L/day) than during
the control period (1.80 ± 0.13 L/day), and the water intake
during the HDBR period was, respectively, higher at weeks 6 and 9
(2.99 ± 0.99 and 3.13 ± 0.10 L/day) and lower at week 3
(2.60 ± 0.06 L/day) than during the control period (2.70 ±
0.07 L/day).
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Whole body composition changes (Table 2). Body weight measured on HDBR
d-4, d+28, d+42 and recovery d+30 showed a nonsignificant drop of
-2.8% between HDBR d0 and d+28. Significant decreases were noted
between HDBR d-4 and d+42 (-3.7%) and between HDBR d-4 and recovery
d+30 (-3%). LBM dropped significantly between HDBR d-4 and d+28
(-2.5%) and between HDBR d-4 and d+42 (-4.5%). Moreover, a decrease
was noted between HDBR d+28 and d+42 (-2%). FM, in contrast, did not
change significantly throughout the HDBR period compared to that during
the control period.
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).More subtle patterns were
observed when results of body composition were divided by the segments
(arms, legs, and trunk) and were expressed as the percent change in
variation of segment weights from HDBR d-4. These results showed that
FM significantly increased in both legs and trunk on HDBR d+28
(respectively, 13.5 ± 2.7% and 5.9 ± 3.4%) and HDBR d+42
(respectively, 18.3 ± 5.1% and 8.2 ± 4.9%), whereas no
changes were noted in the arms. The opposite pattern was shared for the
LBM, i.e. a significant decrease on HDBR d+28 (respectively,
-2.9 ± 0.5 and -1.4 ± 0.7%) and HDBR d+42 (respectively,
-3.4 ± 0.7 and -1.4 ± 0.8%), with no changes observed in
the arms.
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The results are listed in Table 3.
The constant elimination rate of both isotopes
(Kd and Ko) as well as the isotope
dilution spaces (Nd and No) were within the
range of values found in the literature for normal humans. The isotope
dilution spaces ratio Nd/No were constant
throughout the experiment and close to human values as well as the
crude and corrected CO2 production rates (r'CO2
and rCO2). The precision of the crude CO2
estimated from its fractional error variance was acceptable and lower
than 5%.
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The carbohydrate (-15%), fat (-14%), and protein (-9%)
intakes were lower during HDBR than in the control period, but nutrient
intakes and water intakes were unaltered during bed rest, when
expressed as percentage of daily energy (Table 4). The two 15-day experimental periods
were compared (Table 5), and the energy
intake during the second period was 11.6% lower than that in the first
period. The RQ showed an opposite pattern, with a 4.6% higher value
during the HDBR period than the control period. REE exhibited no
modifications from the control period. Conversely, the TEE during the
second period was 20% lower than that in the first. A significant
negative energy balance was observed during the control period
(-1.5 ± 0.5 MJ/day), whereas the subjects were in energy balance
during HDBR. The energy expended in physical activity plus DIT
decreased by 39.5% during the HDBR compared to that during the control
period. No significant covariance between TEE and LBM (analysis of
covariance = 0.379; P = 0.185) was noted.
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The results are listed in Table 6.
Total body water exhibited a 5.3% significant decrease by comparing
the days of DLW doses. Expressed as the hydration of the LBM, no
modifications of TBW were noted. As the subjects continued their normal
routines during the ambulatory period, and body weight was stable
during this period, a hydration coefficient for LBM was calculated by
dividing the TBW assessed on the days that DLW probes were given by the
LBM assessed by absorptiometry. The hydration coefficient for LBM was
unchanged by HDBR conditions. The water turnovers during the two
periods expressed either as liters per day (3.5 ± 0.2 and
3.2 ± 0.2) or as the percentage of TBW (8.4 ± 0.2 and
8.3 ± 0.2) were similar. Conversely, we observed a significant
17% reduction in the formation of metabolic water between the HDBR
period and the control period.
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The urinary excretion of metanephrine (Fig. 3a) was significantly lower during the 6
weeks of HDBR (week 3, 109 ± 6; week 8, 112 ± 6 µg/day)
than during the control week (133 ± 5 µg/day). Normetanephrine
excretion (Fig. 3a) was also lower during HDBR (week 3, 123 ± 6;
week 8, 134 ± 6 µg/day) than during the control week (165
± 7 µg/day). There was a significant increase during the recovery
period, at weeks 9 (214 ± 7 µg/day) and 10 (201 ± 11
µg/day). Significant increases were noted in urea excretion (Fig. 4) at weeks 5 (29.3 ± 0.7 g/day)
and 6 (29.7 ± 0.8 g/day) compared to the average control value at
week 2 (25.6 ± 0.9 g/day), but there was a significant reduction
at week 10 (20.7 ± 0.9 g/day). Urinary creatinine excretion (Fig. 4) did not change from the control value (2215.6 ± 39.1 mg/day)
during the HDBR experiment. The rates of urinary excretion of cortisol
(Fig. 3b) at weeks 4 (140.6 ± 17.4 ng/day) and 5 (143.3 ±
17.4 ng/day) were significantly above the control value (110.5 ±
9.3 ng/day; P < 0.05). Similarly, the urinary GH
excretion rate (Fig. 3b) was significantly above the control value
(1.83 ± 0.25 ng/day) at weeks 4, 5, and 6 (3.37 ± 0.53;
3.01 ± 0.46, and 3.25 ± 0.36 ng/day). There was another
significant increase in GH during recovery weeks 9 (2.97 ± 0.31
ng/day) and 10 (3.25 ± 0.43 ng/day).
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Discussion |
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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HDBR conditions and DLW method accuracy
One of the assumptions of the DLW method is that isotope dilution spaces and outflow rates of water and carbon dioxide production are constant throughout the experiment. This is not strictly correct for any living organism. In this study steady state is violated because of the loss of TBW and LBM. To investigate the putative error in CO2 productions due to these changes, we used the individual LBM differences between HDBR d+28 and HDBR d+42 (-1.19 ± 0.19 kg) and calculated an average water loss of 0.87 L (from the known hydration coefficient of LBM, 73.2%, unchanged during HDBR). The dilution spaces at the end of the protocol (HDBR d+42: n42) from the initial ones (HDBR d+27: n27) were determined by resolving the two equations: n42 = (No42/1.01 + Nd42/1.04) x 0.5 and Nd42 = No42 x 1.031. Then, the crude carbon dioxide production (r'CO2) is calculated from a modified equation described by Roberts et al. (17): r'CO2 (mol/day) = 0.5 x [(No x Ko - Qo) - (Nd x Kd - Qd)], where Qo and Qd are the changes in isotope dilution spaces over the course of the study in grams per day. In these conditions, the difference between the crude r'CO2 calculated by the equation of Roberts and calculated from Eq I led to a difference of less than 0.01%. The constancy of the CO2 productions is substantiated by the results of the precision calculated from their fractional error variances, which are below 5% of error during both control (2.8 ± 0.2%) and HDBR (3.6 ± 0.3%) periods, and so are highly correct. Another methodological problem arises from the fact that it is necessary to assume a value for a mean RQ over the measurement period to calculate EeqCO2 from Wier’s formula (Eq III). During this study, the RQ was only measured on HDBR d-15 for the control period and on HDBR d+40 for the bed rest period; unfortunately, we were not sure of the constancy of this variable throughout the two DLW studies. Because food intake was precisely recorded, and body composition was assessed, we determined the effect of a possible error in using a RQ measured at the beginning of each DLW period on the TEE measurements. For this purpose, we calculated the average food quotients (FQ) for the two 15-day periods using Black’s formula (18): FQ = [(710.71P) + (1377.06F) + (746C)]/[(879.06P) + (1948.34F) + (746C), where P, F, and C are protein, fat, and carbohydrate intakes expressed as grams per day. Because RQ will only be exactly equal to FQ when a subject’s protein, fat, and carbohydrate intakes over the double isotope measurement period precisely match the amounts oxidized over the same period, the numerator and denominator of this equation were modified to take into account the body composition changes. The difference was 0.2 ± 2.0% between the FQ and the QR, and led to -0.3 ± 1.5% of difference between the TEEs. Overall, the changes in the dilution spaces between HDBR d+27 and d+42 as well as the use of one session of RQ measurement had no significant or only a small effect on the TEE measurements, confirming the choice of our equations.
Energy and water metabolism
The 42-day HDBR conditions resulted in a reduction in body weight that seemed to stabilize at a new level on week 7 of the experiment, in parallel with energy intake. This change in food intake could be due to alteration of taste and adaptation of the subjects to reduced physical activity. Energy intake stabilized at a lower level than the preflight value after 2 days during the SLS-1 and SLS-2 missions (3). A similar reduction also occurred during an 8- to 10-day space flight (4). It is difficult to determine whether the losses that occur in space are due to microgravity per se, since astronauts frequently suffer from acute space motion sickness (with nausea) and loss of taste. These factors, combined with their heavy workload, contribute to the negative energy balance and to the use of body stores. This decrease in energy intake is different from that which occurs in HDBR, where there is a gradual reduction. The subjects were allowed to eat to satisfy their appetite as in space, conversely to the aforementioned bed rest studies on energy metabolism, where the subjects ate enough to maintain their body weight. As previously observed during a 28-day bed rest (19), the macronutrient composition of the diet remained stable. The intakes of proteins, lipids, and carbohydrates decreased along with energy intake, suggesting that the observed changes could have their origins in the metabolic effects of recumbency and not in a voluntary change in eating behavior.
There was a 20% reduction in TEE during HDBR and a 12% reduction in energy intake. The TEE was not correlated with LBM, impling that the reduction in LBM is not responsible for any reduction in TEE. In contrast to space flight (4), a positive energy balance was reported during HDBR (20). The results are less evident in this study, especially because no significant changes in body weight were noted during the control period despite a negative energy balance. Effectively during the control period, the subjects had a significant negative energy deficit (-1.5 ± 0.5 MJ/day), and there was a loss of weight (-0.28 ± 0.15 kg) during the first week that was offset during the second week (+0.25 ± 0.16 kg). Thus, it seems that energy expenditure increased during the first week, probably due to the stress of entering the hospital. This could be the main cause of energy deficit during the control period. Nevertheless, as loss of weight during the first week of the ambulatory period was offset during the second week, the subjects can be considered to be in a stable metabolic state before undergoing HDBR. On the other hand, significant reductions in body weight were observed during HDBR while the subjects were nearly in energy balance. Classically, it is the difference between energy intake and TEE that is the primary cause of an energy storage or release, and studies of energy restriction have shown that FM is primarily affected. A loss of body weight, without changes in FM, in a state of energy balance is observed during special metabolic circumstances such as metabolic stress (inflammatory responses, illness, etc.). During the SLS-1 and SLS-2 missions, reduced nitrogen retention, enhanced whole protein turnover, increased acute phase protein synthesis, increased cytokine secretion, the development of insulin resistance, and elevated urinary cortisol were observed throughout the flight, indicating that there is a typical metabolic stress response after entry into space (3). The results of this HDBR support the presence of metabolic stress. The pattern of urinary cortisol excretion was similar to that of urea, with a significant increase during weeks 4, 5, and 6 of HDBR and a return to the control values by week 7. The significant increase in GH occurred when the changes in cortisol, urea, body weight, and energy intake were greatest. GH has various anabolic effects, regulation of body composition, increasing energy expenditure, and enhanced protein synthesis, and thus drastically reduces the urinary excretion of urea. GH has also an anabolic action on bone and influences fluid, calcium, and sodium homeostasis. These metabolic actions did not occur during HDBR despite a significant increase in GH urinary excretion. In fact, LBM was lost as was TBW and urea. This secretion could limit the loss of tissue caused by the simulated microgravity, especially in bone and LBM, and thus can be considered an adaptive process. It could also be a direct consequence of the reduced energy intake via the action of insulin-like growth factor I on GH secretion. This excretion is also higher in the recovery period, when the muscle workload is increased, and protein synthesis is required. Taking these catabolic and anabolic hormones variations together with the body weight loss, in a context of energy balance, our results suggest that special metabolic and hormonal adaptations comparable to a metabolic stress could participate in the deleterious effects of recumbency. Moreover, these results demonstrate that the energy balance alone could not explain the body weight variation.
Another important question for the role of energy expenditure in day to day energy regulation is the relationship between changes in REE and TEE. The significant difference (39%) between TEE and REE is an estimate of the energy expended for physical activity plus DIT. During this HDBR, the DIT was shown to be unchanged (Ritz, P., personal communication). Taking into account the noncovariance of TEE and LBM, our results suggested that the reduction in TEE was mainly due to reduced physical activity, as reported by Gretebeck et al. (7) for short term HDBR (10 days). There could also be alterations in the thermoregulatory responses, as HDBR reduces thermoregulatory responses to heat stresses (21).
Water turnover was unchanged during bed rest, but it decreased significantly during 8- to 14-day space flights, undoubtedly because of the great physical activity inherent in astronaut training before flight (4). On the other hand, the formation of metabolic water was significantly decreased during the HDBR as a consequence of changes in both the production of CO2 and the composition of the substrate oxidation reflected by the RQ, but it was unchanged during space flight (4). These different responses during actual or simulated microgravity might principally be due to the different physical activity levels and/or the different stresses.
Whole and segmental body composition changes
Body weight is lost during space flight, principally due to a loss of body fluids, during the first few days. TBW decreased significantly (5.3%) after 27 days of HDBR. This loss includes a reduction in plasma volume (184 mL; P < 0.05) during the first 24 h of HDBR (22). Thus, most of the water lost is from the intracellular and interstitial fluid compartments, as occurred during the SLS-1 and -2 missions (1).
A recent study of TEE using the DLW method has provided a consistent picture of the relationship between energy expenditure for physical activity and body composition (23). The body fat content and the nonbasal energy expenditure (primarily resulting from physical activity) seem to be in equilibrium. Whole FM does not significantly change during HDBR despite the obvious decrease in physical activity. Nevertheless, segmental variations in FM showed a significant increase in both trunk and legs on HDBR d+27 and d+42; these two segments were clearly the most affected by inactivity. Conversely, no changes were noted in the arms, which conserved a consistent degree of activity during eating, reading, etc. In this way, a meta analysis of 53 studies reported by Ballor and Keesey (24) suggested that the relationship found in cross-sectional studies resulted from the influence of physical activity on body fat mass. The sympathetic nervous system must be involved in such effects because sympathetic activation enhances energy expenditure (25) and reduces body fat mass (26). Urinary excretion of normetanephrine and metanephrine, which are indirect indexes of sympathetic nervous system activity, were decreased and could account for a slight drop in sympathetic activity during HDBR. Urinary norepinephrine also decreased throughout the space missions, which is consistent with the results of SLS-1 and -2 (1), bed rest studies (27), and a decrease in sympathetic nervous activity. This is associated with enhanced activity (28) on landing and during recovery. Excretion of catecholamine metabolites during the recovery period was greater than that during HDBR, suggesting that the sympathoadrenal system is activated in response to blood pooling and physical activity. Greater sympathetic activity may be needed to keep blood pressure and cerebral perfusion normal during periods of orthostatic stress and to readjust the TEE to normal physical activity.
Space flights are known to result in a loss of mass and strength in locomotory and postural muscle groups (4, 29). There was a significant 3.7% decrease in LBM after 28 days of HDBR (-6°), as found by LeBlanc et al. (30) after 17 days of simulated microgravity. However, the drop on HDBR d+42 was still significant for both control d-4 and HDBR d+27, suggesting that the decrease in LBM is progressive throughout the bed rest but affects principally the disused part of the body, as a drop in LBM was noted in the trunk and legs, but not in the arms. These results are in accordance with the lower SNS activity and TEE. An important issue of this study is that the hydration coefficient for LBM was constant near the nominal value of 73.2%. This implies that it could be used for determining body composition from TBW during enforced inactivity with fluid shift. Moreover, the differences in body weight on HDBR d-4 and HDBR d+28 were not significant despite significant changes in body composition. This again indicates that body weight alone does not accurately reflect the energy state of the subjects.
The urinary urea and creatinine concentrations can only be accurately interpreted when subjects are consuming a meat-free diet. However, this was not done so as to give the subjects conventional foods during HDBR. Nevertheless, the protein intake was reduced by 9% during bed rest, making the results relevant. Urinary urea excretion is only an indirect index of protein breakdown and provides no insight into the underlying mechanisms, but the increased urea excretion during weeks 5 and 6 of HDBR is consistent with a decrease in the LBM. A recent study by Ku and Thomason (31) demonstrated that muscle continuously adapts its protein synthesis to the workload. Thus, the loss of LBM may be partly due to the reduced physical activity in the disused parts of the body. Our subjects had a significantly lower urea excretion in the second week of the recovery period that can be explained by the increased workload in the postural muscles, which are the first to be affected by disuse atrophy.
Conclusion
During this study we have demonstrated that energy balance could not explain the body weight reduction and that body weight alone does not reflect the energy state of the subjects. Taken together with the hormonal and body composition changes, the responses of human energy metabolism to long term HDBR conditions follow patterns consistent with transient metabolic stress as observed in space and that could partly explain the body weight loss. The reduction in TEE is not due to reduced active metabolic mass but to reduced physical activity, and the hormonal patterns confirm that this environment interferes with nutrient oxidation. LBM is principally lost in the disused parts of the body. No important modifications in water metabolism were triggered. The reported hydration coefficient for lean body weight is maintained at 73.2%, suggesting that body composition could be accurately determined by isotope dilution in recumbent patients. Further studies on the changes in nutrient use resulting from alterations in hormone balance demonstrated in this study and their interplay with others (T3, T4, insulin, leptin, insulin-like growth factor I, etc.) and on modifications in nutrient turnover (carbohydrates, lipids, and proteins) are now required to explore underlying mechanisms in the space deconditioning syndrome and illnesses associated with strict recumbency. This may allow us to develop nutritional countermeasures to reduce or prevent the loss of tissue due to microgravity and to facilitate the return of patients to normal life. Immobilization is often a necessary component of the intensive medical care of trauma victims and patients with severe illness.
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Footnotes |
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2 Fellow of the Centre National d’Etudes Spatiales.
Received May 18, 1998.
Revised August 27, 1998.
Accepted September 8, 1998.
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