Protein kinetics during and after long-duration spaceflight on MIR

T. P. Stein1, M. J. Leskiw1, M. D. Schluter1, M. R. Donaldson1, and I. Larina2

1 Department of Surgery, School of Osteopathic Medicine, University of Medicine and Dentistry of New Jersey, Stratford, New Jersey 08084; and 2 Institute for Biomedical Problems, Moscow 123007, Russia


    ABSTRACT
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Human spaceflight is associated with a loss of body protein. Bed rest studies suggest that the reduction in the whole body protein synthesis (PS) rate should be ~15%. The objectives of this experiment were to test two hypotheses on astronauts and cosmonauts during long-duration (>3 mo) flights on MIR: that 1) the whole body PS rate will be reduced and 2) dietary intake and the PS rate should be increased postflight because protein accretion is occurring. The 15N glycine method was used for measuring whole body PS rate before, during, and after long-duration spaceflight on the Russian space station MIR. Dietary intake was measured together with the protein kinetics. Results show that subjects lost weight during flight (4.64 ± 1.0 kg, P < 0.05). Energy intake was decreased inflight (2,854 ± 268 vs. 2,145 ± 190 kcal/day, n = 6, P < 0.05), as was the PS rate (226 ± 24 vs. 97 ± 11 g protein/day, n = 6, P < 0.01). The reduction in PS correlated with the reduction in energy intake (r2 = 0.86, P < 0.01, n = 6). Postflight energy intake and PS returned to, but were not increased over, the preflight levels. We conclude that the reduction in PS found was greater than predicted from ground-based bed rest experiments because of the shortfall in dietary intake. The expected postflight anabolic state with increases in dietary intake and PS did not occur during the first 2 wk after landing.

energy


    INTRODUCTION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

HUMAN SPACEFLIGHT is associated with a loss of body protein. Specific changes include a loss of lean body mass and decreased muscle strength (8, 13, 25, 32). The major sites of the protein loss are muscles with anti-gravity functions (9, 25). The weakness of astronauts after long-term spaceflight has been attributed to this loss of body protein. By analogy with ground-based bed rest studies in humans and hindlimb unloading models in rats (8, 15, 24), it is believed that once the initial adaptation process is over, muscle downsizing is largely accomplished by a reduction in the protein synthesis rate rather than a change in the degradation rate (6, 17, 18, 23).

To date, only data from short-term spaceflights on the shuttle are available, and they have shown an increase early in flight, with either a return to the preflight level or a possible trend toward a decrease by the end of the 2nd wk (20).

After spaceflight, astronauts and cosmonauts should be in an anabolic state, because there will be a need to regain the lost protein. By analogy with other anabolic states in which protein repletion is occurring, repletion should be associated with increases in dietary intake and protein synthesis (29). The results from the short-term flights on the space shuttle do not support this hypothesis. After the two Space Life Sciences 1 and 2 missions (SLS-1 and SLS-2), neither dietary intake nor the whole body protein synthesis rates were increased relative to preflight levels (20). The lack of an observable response could have been due to the protein losses not being large enough. After long-duration spaceflight the protein losses are much greater, thereby providing a better test of the hypothesis.

The objectives of this experiment were to test two hypotheses on astronauts and cosmonauts participating in the recent long-duration flights on the Russian space vehicle MIR. 1) If the sojourn in space is long enough, a reduction in the whole body protein synthesis rate will be found. 2) Dietary intake and the whole body protein synthesis rate should be increased postflight because protein accretion is occurring.

To test these hypotheses, the 15N glycine method for measuring whole body protein synthesis was used to measure the whole body protein synthesis rate before, during, and after long-duration spaceflight on the Russian space station MIR (3, 30). Dietary intake was measured concomitantly with the protein kinetics.


    METHODS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Methods of Procedure

The subjects for this study were two astronauts (US) and four cosmonauts (Russian). Informed consent forms were obtained in accordance with the policies of the US National Aeronautics and Space Administration (NASA), the Russian Academy of Sciences Experiment Board, The Russian Space Agency (RSA), and the University of Medicine and Dentistry of New Jersey.

Preflight and postflight. The preflight measurements were taken in two, three, or four sessions during the year before the mission. Each session lasted 2 days. Urine was collected on each day to give (at least) two 24-h pools. The actual number of preflight sessions depended on crew availability. Sessions were conducted at either NASA facilities in the US (the Johnson and Kennedy Space Centers) or the RSA facility at Star City, near Moscow, Russia. During each of the 2 days, the subjects kept a detailed record of their dietary intake.

On the 2nd day of the session, a whole body protein synthesis rate determination was made with the single-pulse 15N glycine method, with ammonia as the end product (3, 30). The 15N glycine was taken within an hour of waking up. At least one of the determinations was done within 3 wk of launch. The basic experimental procedure for the whole body protein synthesis rate determinations was for each subject to 1) void and 2) take by mouth 1.2 g of 99% 15N glycine (ICONS, Summit, NJ) in capsule form. The individual voids produced over the next 10 h (range 9.2-11.8 h) were collected and measured, and an aliquot was saved frozen for later analysis. Urine collection continued for the remainder of the 24-h period.

US astronauts landed on the shuttle in Florida. They were flown to Houston, and the postflight studies continued at the Johnson Space Center. Russian cosmonauts returned on a Soyuz space vehicle and spent the first 2 wk postflight at Star City. The postflight studies followed the same protocol as the preflight studies, with sessions on recovery day 1 (R+1) and R+2, R+6 and R+7, and R+14 and R+15. On occasion a session was displaced by a day because a crew member was unavailable. As for the preflight period, each session involved 2 days of dietary monitoring, with 24-h urine collections and a protein synthesis determination on the 2nd day. Protein kinetics, but not dietary information or 24-h urines, were obtained on R+32, R+40, and between R+63 and R+75.

Inflight. With two exceptions, a protocol similar to the preflight and postflight periods was performed inflight. First, while on MIR, the crews ate bar-coded food and recorded on a voice recorder the proportion of each food item consumed. Before opening a food package, the crew person used a hand-held bar code scanner to identify the food item and the data saved on the hard disc of an IBM PC 750 lap top computer (IBM, Armonk, NY). The data were copied from the hard drive onto a floppy disk during the next shuttle visit and returned to the Johnson Space Center for analysis. Second, because of the need to conserve water on MIR, only one 24-h urine collection could be done at a time. On MIR, urine water was recycled for future use. Studying one subject at a time decreased the potential water loss from the system if for some unexpected reason the urine water could not be returned to the system.

Inflight urine samples were collected in specially designed plastic bags, with a condom-like collection device at one end and a port for drawing off urine in a 10-ml syringe at the other end. Before a sample was withdrawn, the contents of the bag were thoroughly mixed to make sure that the lithium marker (for determination of the urine volume) was evenly distributed. After the samples were thoroughly mixed, a 10-ml aliquot was withdrawn and placed in the on-board -20°C freezer. The remainder of the void was returned to the waste disposal system for recycling.

Diet analysis. The dietary records were analyzed by NASA personnel from the Nutrition and Metabolism Laboratory of the Johnson Space Center by use of the Nutritionist 2 program (University of Minnesota, Minneapolis, MN), together with some data specially collected by NASA personnel on Russian foods.

Analytic methodology. On return, the urine samples were kept frozen at -70°C until analyzed. In some cases, when a 24-h pool was missing, the missing pool was supplied by NASA as part of a sample-sharing agreement between investigators; the remainder were made up by us. After thawing, 10-h and 24-h urine pools were made up by the fractional aliquot method. The 10-h pools covered the 10 h between dosing with 15N glycine and collecting the urine 10 h later. The urinary nitrogen content of the 10-h pools was determined by Kjeldahl reduction and that of the 24-h pools with an Antek automated nitrogen analyzer [Antek, Houston, TX (20)].

Ammonia was isolated from a 4-ml aliquot of the urine by addition of saturated K2CO3 (4 ml) and bubbling N2 gas through the solution. The resultant ammonia was trapped with 0.1 N H2SO4 (1 ml). The resultant (NH4)2SO4 was then concentrated on a hot plate to ~0.4 ml for isotopic enrichment analysis. The (NH4)2SO4 solutions were then converted to N2 gas by the Rittenberg hypobromite method, as previously described (20), and the 15N enrichment of the resultant N2 was determined by isotope ratio mass spectrometry by use of a VG-SIRA-II mass spectrometer (VG Instruments, Cheshire, UK).

Most of the urinary creatinine data were provided by NASA as part of the investigators' data-sharing agreements. For those few urines collected that were specific to this experiment, we determined the creatinine concentration using the picric acid method with a kit from Sigma (St. Louis, MO).

Urine cortisol was measured by use of an RIA kit marketed by Incstar (Stillwater, MN).

Methods of Calculation

Nitrogen balance. Nitrogen balance was estimated from the difference between protein intake and the urinary nitrogen excretion. No allowance was made for fecal nitrogen losses, and so the values are best described as estimates.

Whole body protein synthesis and breakdown. The whole body protein synthesis (S) and breakdown (B) rates were calculated from the urinary ammonia 15N enrichment (A), nitrogen intake (I), nitrogen excreted in the urine (Etau ), and the amount of isotope given (D). If Q is the flux, then Q = D/A, and S = Q - Etau (22, 30).

A comparable calculation for the protein breakdown rate cannot be made because actual food consumption times are not available. However, an estimate of protein breakdown rate can be obtained from the relationship B = Q - I, by normalizing the flux to 24 h and using the total 24-h dietary intake (30).

Statistics

Data were analyzed by a repeated-measures design analysis of variance, paired t-tests, linear regression, or the Pearson product-moment correlation, as appropriate. Statistical significance was accepted at P < 0.05. If the analysis of variance indicated significance at the P < 0.05 level or better, group differences were identified by the least significant differences test. The SAS Statistical System (SAS Institute, Cary, NC) was used for the statistical computations. Data in the text, figures, and tables are means ± SE, with either the number of subjects or the number of determinations in parentheses.


    RESULTS
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ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Body Mass

Preflight and postflight body weight measurements were obtained on six subjects. All of the astronauts lost weight during the flight period (Table 1). Unfortunately, in the early postflight period, there was some inconsistency in times at which postflight body weights were obtained. For two of the subjects, weights were obtained immediately after landing and on R+3. For the other four subjects, the first weight was obtained on R+1. When the R+1 and R+3 data are used, the average weight loss was 4.6 ± 1.1 kg (range 2.4-8.0 kg). There was a rapid recovery of body weight between R+1 and R+15. By the time that final weight determinations were made between R+63 and R+75, body weight had recovered to the preflight status for all subjects.

                              
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Table 1.   Summary of body weight data before and after spaceflight

Dietary Intake

Preflight, dietary records for between 4 and 12 days were collected for a total of 46 days (Tables 2 and 3). Inflight, dietary data were collected for 22 days, and for the postflight period between R+5 to R+14, data were collected for 27 days. All of the inflight data were collected after flight day (FD) 88. Intrasubject variability in dietary intake was small for all three phases of the study. The coefficients of variation were 8.4 ± 1.7% preflight, 11.5 ± 3.0% inflight, and 9.6 ± 1.4% postflight.

                              
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Table 2.   Summary of energy and nitrogen intake before, during, and after spaceflight


                              
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Table 3.   Summary of creatinine excretion and nitrogen balance before, during, and after spaceflight

Energy intake was significantly less inflight than either before flight or after flight (78 ± 9%, P < 0.05). This relationship was found even though one subject made a conscientious effort to eat during the mission and ate more inflight than preflight. All of the other subjects ate less inflight (70 ± 3% of preflight, n = 5). There was a trend for decrease in energy intake to correlate with the weight loss (r2 = 0.53, P = 0.10). The inflight fluid intake was similar to the preflight level (94 ± 11% of the preflight value).

The postflight data collection schedule called for three sessions of 2 days per session, beginning on R+0 or R+1, R+5 or R+6, and R+12 or R+13. Urine collections and dietary monitoring were incomplete for the sessions beginning on R+0 or R+1, and so these data have not been included in Table 2. At least two complete sessions were obtained for each subject during the time period R+5 to R+14. The sessions were combined to give a single data set for each subject. Energy intake was the same as preflight and greater than inflight (P < 0.05). A complete set of intake data (but not of nitrogen excretion) was obtained for R+1. These data are not included in Table 2, because they were for a single day and there were no accompanying urine data. However, like the data obtained 1 wk later, energy intake on R+1 (2,549 ± 275 kcal · kg-1 · day-1) was essentially indistinguishable from preflight (2,854 ± 268 kcal · kg-1 · day-1).

Nitrogen Intake and Balance

Unlike energy intake, nitrogen intake was not reduced inflight or increased postflight (Table 3). The nitrogen balance data in Table 3 are based on the excretion of nitrogen in the urine, because feces were not collected. The nitrogen balance data are reliable; there were no differences in urinary creatinine excretion during and after spaceflight (Table 3). There was a trend for nitrogen retention to be less inflight, but the trend was not significant because of the small data base. Postflight nitrogen retention was the same as before flight (Table 3).

                              
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Table 4.   Summary of urinary cortisol excretion before, during, and after spaceflight


                              
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Table 5.   Summary of whole body protein synthesis and breakdown rates before, during, and after spaceflight

Whole Body Protein Synthesis and Breakdown

Fifteen successful whole body protein synthesis (PS) determinations were obtained before flight, with at least two determinations for five of the six subjects. For the sixth subject, only a single determination was made because of incomplete urine collections on the first attempt. Eight successful PS determinations were made inflight. All data were obtained between 88 and 186 days in orbit (mean, 147 ± 8 days). There was good agreement between duplicate inflight measurements for the two subjects on whom two inflight measurements were made (Table 5). The inflight whole body PS rates were reduced to 45 ± 5% of the preflight value. Postflight PS returned to the preflight level (91 ± 17% of preflight) and was significantly greater than during flight (P < 0.05).

Because detailed dietary intake data during the 10-h 15N glycine period are not available, only an estimate of the protein breakdown (PB) rate can be given. The estimate of the whole body PB rate shows that PB paralleled PS, being decreased inflight (38 ± 8% of preflight) and unchanged after flight (84 ± 18% of preflight).

In this study three sets of measurements were made: weight, protein synthesis, and dietary intake. From these three sets of data we derived the changes associated with flight, namely %weight loss, %change in PS, and %reduction in intake during flight. %Weight loss was defined as the %difference between the means of the last two preflight weights and the weight on the 1st or the 3rd day after landing. The %change in whole body PS rate was derived from the difference between the mean of the preflight determinations and the mean of the inflight determinations, and the %change in energy intake was calculated from the difference between the mean of the preflight intake measurements and the mean of the inflight measurements.

A series of regression analyses was performed with the above parameters. The major finding was that, even though there were only six subjects, the inflight decrease in energy intake correlated with the decrease in PS (r2 = 0.86, P < 0.01, Fig. 1). The relationship is also statistically significant if the Pearson product-moment correlation test is used (P < 0.01). It might be argued that the correlations are primarily due to one subject (subject D) being an outlier. If this subject is dropped from the analysis, the values for the regression and Pearson tests become 0.23 (r2 = 0.53) and 0.166, respectively. The trend is still there, but it is no longer significant at the P < 0.05 level, in part because the subject was dropped and in part because of the reduced number of subjects.


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Fig. 1.   Percent change in energy intake with spaceflight vs. percent change in whole body protein synthesis rate. The regression is statistically significant (r2 = 0.86, P < 0.01).

Cortisol

Table 4 gives the urinary cortisol data for the 24-h urines. There were no differences between the preflight, inflight, and postflight periods.


    DISCUSSION
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES

Quality of the Data

Although there was considerable negative publicity in the lay press about the problems encountered with the Shuttle-MIR program and the data obtained were less than planned for, we were able to obtain good data. The principal impact on this study of the various technical mishaps on MIR was the loss of any measurements during the first 3 mo in orbit. The earliest inflight data point was obtained after 88 days. After that the opportunities for human experimentation were limited, mainly by the lack of crew time and facilities on MIR. Much of the crew's time during the day on MIR was taken up with routine maintenance of the spacecraft. Collecting urine on MIR was complicated by the need to conserve water for recycling. The water rationing was such that, even when urine could be collected, only one subject could be studied per day. Furthermore, it was technically difficult but necessary to recover the urine from the urine collection device for recycling.

There was actually very little time free for scientific endeavors. The combination of water problems and lack of time limited us to 11 inflight measurements, of which 8 were successful. Three inflight measurements were lost either because crew omitted to collect all urines between 0 and 10 h or because of problems in identifying urines after the samples had been returned to earth.

The actual days for the inflight measurements were selected by NASA and RSA mission managers. The days were selected to be free of mission-related problems and, to the best knowledge of mission managers on the ground, were representative of normal working days during the mission. When measurements were made, they were done properly. These were highly motivated and well-trained subjects; they understood the experiment, and two of the crew had previously performed the experiment on the space shuttle.

Body Weight

Body weights were obtained on the day of landing for two subjects and the day after landing for four subjects. Because of the uncertain tissue hydration status immediately after landing, because of the postflight fluid shifting of water from the upper body to the lower body, the R+0 body weights are uninterpretable. It is believed that most of the fluid shifts are over within a day of landing, and tissue hydration then is close to the preflight status (19). The difference between the preflight and the R+1 and/or R+3 weights provided an estimate of the true weight loss during flight. Recovery of body weight was complete by the time the final body weights were determined 2 mo after landing. Although not statistically significant, the decrease in energy intake correlated with the weight loss (r2 = 0.53, P = 0.10).

Nitrogen Balance

The preflight nitrogen balance values are similar to those found with Skylab (73.9 ± 4.0 mg N · kg-1 · day-1) and the shuttle (57.5 ± 9.1 mg N · kg-1 · day-1) (20). Inflight there was a weak trend toward a decrease, but the database is too small for much to be said. The interesting feature about the postflight nitrogen retention data is the lack of any clear evidence for a substantial increase in nitrogen retention. This lack of increase occurred despite the considerable inflight nitrogen losses. Dual-energy X-ray absorptiometry measurements by LeBlanc et al. (14) on the crew members of prior shuttle and/or MIR missions described muscle losses in the antigravity muscles of between 10 and 20%.

Dietary Data

For a given subject, dietary intake was reasonably constant within a period (before flight, inflight, and postflight from R+5 to R+14). For five of the six subjects, dietary inflight intake was less than preflight intake, and for four of the subjects it was reduced by 30% or more (Table 2). One subject ate more inflight than preflight (subject D, see Table 2). This subject was well aware from preflight briefings of the emphasis that one of us placed on the importance of maintaining intake and so made a deliberate effort to maintain intake. In fact, this subject's intake was actually increased inflight. This was a unique occurrence. None of the 9 Skylab astronauts showed greater intake during spaceflight, nor did any of the 11 astronauts on the SLS1 or 2 missions (16, 20, 32). Subject D also made a conscientious effort to follow the prescribed exercise regimen. Compliance by some of the other crew members was variable.

On the recent NASA Life and Microgravity Sciences (LMS) mission, on which there was a heavy exercise component, energy expenditure was 42.3 ± 1.3 kcal · kg-1 · day-1 (21). On the SLS-1 and SLS-2 missions, where there was no exercise period, the energy expenditure during flight was estimated as ~31 kcal · kg-1 · day-1 (20). The SLS-1 and SLS-2 crews worked long days and were very active. They did not have the time to exercise as a countermeasure.

When data for subject D are excluded, dietary intake inflight for the other five subjects was below the value of 23.4 ± 1.5 kcal · kg-1 · day-1, a value considerably less than that found for energy expenditure on the space shuttle (36.2 ± 1.4, n = 13) (10) and about the same as that found with bed rest (24.2 ± 0.8 kcal · kg-1 · day-1) (7). It is, therefore, highly likely that these five subjects were in negative energy balance. Had they been in balance, it is difficult to envisage how a correlation between the estimated energy deficit and decrease in protein synthesis could occur.

This is not the first time that low intakes have been found on missions with high exercise requirements. They occurred on the recent LMS mission, when daily energy intake averaged 24.1 ± 1.3 kcal · kg-1 · day-1, whereas energy expenditure as measured by the doubly labeled water method was 42.3 kcal · kg-1 · day-1 (21).

Why dietary intake is poor on missions with high energy needs is a mystery. Possible reasons include the absence of hedonistic aspects of eating, food tasting differently in space, the monotony of the limited choices of processed food, a cachexic effect in space, and boredom. Whatever the reasons, the consequences of a prolonged energy deficit are serious and, if allowed to persist, life threatening. Most of these subjects probably accommodated to the reduced energy intake by reducing the basal metabolic rate (BMR) and the amount of voluntary activity, namely exercise. Protein turnover by itself accounts for ~20% of the BMR (31). Compliance with the preplanned exercise program during flight is very variable. Thus the actual magnitude of the negative energy balance was probably somewhat less than that implied from the difference between the preflight and inflight energy intakes.

Protein Kinetics

Both protein synthesis and protein breakdown were decreased inflight. It follows that protein turnover was decreased during the flight period. The inflight values for protein synthesis are much less than would be expected from a simple bed rest type of response.

Bed rest studies have shown that the whole body protein synthesis rate is reduced by ~15% and that this can be accounted for by a 50% reduction in the protein synthesis by the antigravity muscles (4). The reduction in protein synthesis found during spaceflight was much greater, 45 ± 6% for the six subjects.

The reduction in protein synthesis is also larger than has been found in otherwise healthy adults placed on energy-restricted diets for an extended period. Ten days of total starvation results in a 28% reduction in the whole body protein synthesis rate (26). In another study, after 6 wk on a 50% reduction in energy intake, the whole body protein synthesis rate was reduced by 20% (22).

We suggest that the reason for the greater reduction with spaceflight is a synergism between the adaptive downsizing of the antigravity muscles and the energy deficit. We have assumed that the difference in energy intake between preflight and inflight reflects energy balance. The assumption here is that energy expenditure and activity are similar in space and on the ground. Lane et al. (10) measured energy expenditure on 13 shuttle astronauts by the doubly labeled water method and concluded that energy expenditure inflight was about the same as before flight (10).

A plot of the estimated deficit in energy expenditure, as defined by the difference between energy intake on the day of measurement and the mean energy intake for the preflight period, shows there to be a good correlation between the estimated energy deficit and the protein synthesis rate as determined by regression analysis (Fig. 1). As the estimated energy deficit increases, the protein synthesis rate decreases.

A decrease in whole body protein turnover is not a benign event. On the ground, the ability to respond to a challenge, be it a new environment, the need for a flight-or-fight response, invasion by a pathogen, or an injury, the response becomes progressively impaired as the energy and/or protein deficit progresses (33).

Cortisol

Urinary cortisol excretion was not increased during spaceflight (Table 4). However, there are a number of features about the cortisol data that merit further discussion. The preflight values are appreciably higher (94 ± 13 µg/day) than those from the Skylab astronauts (55 ± 6 µg/day) or from SLS-1 and -2 (54 ± 4 µg/day). The reason is that two of the subjects showed a single spike (subject A 154 µg/day and subject B 160 µg/day), and all three preflight values for subject C were more than double the means for the other five subjects (156 ± 21 µg/day). If these values are disregarded, the mean preflight value of MIR is 75 ± 7 µg/day. For the two US astronauts (who also flew on SLS1/2), the values on MIR were similar to the SLS1/2 values.

Inspection of the data in Table 4, when subject F is excluded, shows a decrease for three of the subjects (56 ± 4 µg/day) and an increase for the other two (117 ± 7 µg/day). The MIR data do not support there being a stress response after 3 mo in earth orbit for at least three of the six subjects. These findings are different from the 1974 Skylab data, in which an increase in cortisol excretion was found (preflight, 55 ± 6 µg/day vs. inflight, 90 ± 5 µg/day, P < 0.05). The increase is indicative of a stress response on Skylab (11, 12). A 42-day bed rest study by Blanc et al. (2) found cortisol to be elevated for the first 3 wk of bed rest, after which it declined to the preflight level. Interestingly, a 1-wk bed rest study by Vernikos et al. (28) showed cortisol to be increased only in males. No increase was found in females. One of the six subjects in this study was female, and her cortisol excretion was not increased.

The MIR missions were very different from Skylab in several ways. First, there is no overlap between the times of our measurements and the Skylab measurements. The mean of the inflight data collection days was FD 146 ± 8 days. The durations of the three Skylab missions were 28, 56, and 84 days. The closest approximation is the last (84-day) Skylab mission.

Second, the Skylab crews were told what to do every day. They had to maintain dietary intake because they were on a metabolic balance study. For the 84-day mission, the mean intake for days 56 through 84 was 45.1 ± 1.2 kcal · kg-1 · day-1. This is very much higher than on MIR (26.3 ± 1.9 kcal · kg-1 · day-1, P < 0.05).

Third, daily activities, including the prescribed exercise regimens, were compulsory on Skylab. In contrast, on MIR food was eaten ad libitum, and astronauts worked at their own pace, much as they would have on the ground. Although exercise was strongly recommended for the MIR crews, it was different from that on Skylab; it was not compulsory, and compliance was variable. The comparison confirms our previous suggestion that the human response to spaceflight is variable; individual mission characteristics appear to be a major factor in determining how some parameters, such as cortisol and dietary intake, change (20).

The Recovery Period

During the first 2 wk of the recovery phase, some repletion of lost protein occurs. Nitrogen retention increases, but the increase is not great. Unexpectedly, there was no increase in dietary intake during the 2 wk after landing, yet the astronauts and cosmonauts regained some of the lost weight. Another surprising feature of the postflight protein kinetics is that a consistent increase in protein synthesis was not found. We found the same result after SLS-1 and SLS-2 (20). The increase in nitrogen retention after SLS-1 and -2 was not particularly large either. In neither case was dietary intake increased postflight compared with preflight. Our data base is too fragmented for firm conclusions to be drawn, but there does appear to be a consistent finding that anabolism is attenuated in the early (first 2-wk) recovery period. A similar phenomenon has been reported in rodents. A rat hindlimb suspension study by Tucker et al. (27) found an early increase in protein synthesis, followed by a lag, and then a further increase during the recovery period from the unweighting.

Relevance to Countermeasures

The current focus is on exercise as the principal countermeasure. The findings in this study and the failure of the LMS astronauts to maintain the energy balance (21) suggest that a major focus of any countermeasures program should be on maintaining energy balance. The Skylab data shows that energy deficits can be minimized by requiring astronauts to eat strictly controlled diets. On LMS and MIR, astronauts and cosmonauts self-regulated dietary intake, and dietary intake was very much less on MIR than on Skylab.

Two recent bed rest studies showed that interspersing some exercise during bed rest attenuated the atrophic changes in muscle (1, 5). How relevant this finding is to spaceflight is not clear. Humans do not vegetate in space when they are not exercising; except when they are eating and sleeping, they are very active. The questions are, therefore, is this natural activity enough [as appeared to be the case on SLS-1 and SLS-2, (20)] or is there a need for supplemental exercise? If there is a need for exercise, can energy balance be maintained by encouraging or requiring astronauts to eat more?

If substantially increasing intake is not possible, consideration should be given to allowing the body to come to its natural equilibrium state. For long-duration missions there may be some benefit in embarking on a vigorous exercise program 2-3 wk before landing to rebuild the atrophied muscles. A short (2- to 3-wk) period of negative energy balance is of little consequence.

Conclusions

1) There was a problem in maintaining an adequate dietary intake on these long-duration missions. 2) The results of this experiment support our earlier suggestion that there are two components to the protein loss. There is the obligatory response to the decreased workload on the antigravity muscle, and then there is the variable mission specific attempt by the body to accommodate to the energy deficit by decreasing protein synthesis. 3) The greater the decrease in intake during flight, the greater is the decrease in protein synthesis.


    ACKNOWLEDGEMENTS

We thank our subjects for their participation. We would also like to thank numerous unnamed people at the Russian Space Agency and the National Aeronautics and Space Administration (NASA) who did much of the work in collecting data in Russia and the US. Special thanks are due to Robert Pietrzyk for acting as experiment manager, Barbara Rice for the dietary data, and Dr. Scott Smith for coordinating the project.


    FOOTNOTES

This study was supported by NASA contract no. NAS9-19409.

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.

Address for reprint requests and other correspondence: T. P. Stein, Dept. of Surgery, Univ. of Medicine and Dentistry of New Jersey, School of Osteopathic Medicine, 2 Medical Center Drive, Stratford, NJ, 08084 (E-mail: tpstein{at}umdnj.edu).

Received 19 October 1998; accepted in final form 22 February 1999.


    REFERENCES
TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
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Am J Physiol Endocrinol Metab 276(6):E1014-E1021
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