Space flights : hormone treatments can be tailored to minimize muscle loss for individual space travelers

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A new study from The University of Texas Medical Branch at Galveston has further documented how muscles are affected by reduced gravity conditions during space flight missions and uncovered how exercise and hormone treatments can be tailored to minimize muscle loss for individual space travelers.

The findings are available in PLOS One.

NASA has recently announced that it will allow private citizens to visit the International Space Station.

The growing number of space travelers underscores the need to understand the impact of reduced gravity on the human body.

“The study has given us the ability to identify biomarkers that predict how susceptible each individual is to muscle function decline and how effectively different exercise and hormone treatments can combat the atrophy,” said senior author Randall Urban, UTMB chief research officer and professor in the department of internal medicine.

Senior author Melinda Sheffield-Moore, professor in the Texas T&M department of health and kinesiology and UTMB department of internal medicine, said, “This new ability may allow scientists to personalize space medicine by designing specific exercise and/or hormone intervention programs for each astronaut on Earth before they embark on a long-term mission to space.”

Space flight-related losses in muscle mass and strength are a key concern for long space exploration missions.

The muscle loss during space flight largely stems from fact that weight bearing muscles don’t work as hard in reduced gravity conditions.

While in space, people exercise in an effort to counter this muscle loss, but it cannot completely prevent muscle atrophy.

So, researchers are searching for additional interventions that compliment inflight exercise.

The effects of long-term muscle inactivity can be investigated with extended bed rest.

In the study, 24 healthy male participants were placed on bed rest for 70 days.

During the bed rest period, some of the men followed an exercise regimen and blindly received either testosterone supplements or a placebo while a control group remained in the bed without any exercise training or supplements.

Throughout the study, the researchers collected muscle biopsies to analyze the proteins within the muscle tissue.

The researchers uncovered several changes to the men’s muscle proteins during the bed rest period that were blunted or reversed with exercise, which appeared to drive a healthier protein organization within the muscle fibers.

The testosterone supplements prompted further protein changes that promoted muscle growth beyond that of exercise alone.

“The unique insights we’ve gained on muscle proteins during extended bed rest could someday be applied to predict changes to muscle mass/strength in various situations and then develop a personalized program of exercises and hormonal countermeasures,” said senior author E. Lichar Dillon, UTMB assistant professor in the department of internal medicine.


There have been many reviews over the last 30 years of the human metabolic response to space flight.

For the most part, the reviews focused on the biochemical/physiological aspects of the muscle and bone losses, assessing their potential for adverse effects on crew health and performance and the ‘limited’ success of various proposed counter-measures for the muscle and bone losses.

Some of the reviews have been commissioned by government agencies to assist in program development.

There is a potential conflict of interest in these reviews because often the reviewers are content experts, tend to emphasize the importance of their areas of interest and sometimes are the beneficiary of future grants. (The same might be said of this review).

That said, this review will argue that there is a lack of balance in the literature; the published reviews are unduly pessimistic.

They focus almost exclusively on the numer- ous biochemical changes and physiological decrements found with the musculo-skeletal system found during space flight and its ground-based analogs.

Even though the problems have been known for many years, progress over the last 40 years is questionable.

Are some of the problems manageable with current protocols?

Which problems require further work before humans can safely venture into space long-term?

Reviewing a problem from a different perspective might provide new insights. In an era of limited resources, prioritization is necessary.

Identifying a problem provides the rationale for further research.

But to get a funding agency to pay for the research requires making a convincing case that there is a serious risk to the astronauts if the problem is not better understood and a counter-measure developed.

There is a natural tendency by investigators to over-exaggerate the importance of their particular area of research. Thus, the literature is replete with warnings about the dire consequences of allowing ‘problems’ to continue unresolved.

These concerns might have been justified 40 years ago, but are they today?

An unintended consequence has been to delay further human exploration of space beyond the Space Station era.

Planning for new manned missions have moved ever further into the future for both financial and ‘physiological’ reasons. Are the ‘physiological’ concerns really valid?

The basic fact is that more than 500 people have now flown in space for up to 1 year and have done remarkably well. Humans adapt if not perfectly, rather well to life without gravity.

The observed biochemical and physiological changes reflect this accommodative process.

There have been no life-threatening events.

(In contrast there have been catastrophic engineering failures with both the US and Russian programs resulting in the death of 14 US and 4 Russian astronauts).

Traditionally, there have been two ways of studying the human response to space flight. (i) From actual measure- ments, preferably inflight but for relatively invariant parameters such as body composition, immediately post- flight data are acceptable. (ii) By the use of ground-based models, principally bed rest for humans and hind limb unloading for rats.

There is now a third.

The use of NASA’s longitudinal study of astronaut health (LSAH) data base.

The LSAH includes data from the earliest space missions (Apollo, Skylab) through the latest ISS missions.

It is likely that use of the LSAH will assume increasing prominence as the data base increases and interest focuses on long-duration missions.

The longitudinal study of astronaut health (LSAH) data base

Nearly all reported studies on the human response to space flight, especially those pertaining to the muscle loss problem have been based on measurements made on a few astronauts on one mission.

Because no two missions are alike, extrapolating the results of one mission to space flight in general is problematic.

The ever-expanding LSAH data base makes comprehensive meta-analyses feasible and a better assessment of long-term risks as more data are accumulated on the ISS.

Few measurements of changes in lean body mass or body protein content with space flight are available, so body weight is often used as a proxy for lean body mass.

A number of investigators have used this data base and important findings have resulted. Three analyses are rele- vant to the topic of this review. (1)

Factors associated with weight loss. Weight loss is highly variable. Figure 2 shows recent data from the ISS (Matsumoto et al. 2011; Smith     et al. 2004). Does analysis of the LSAH contribute new information? (2) The type, incidence and severity of inju- ries to the musculoskeletal system incurred during space flight and (3) Are there long-term health-related problems post-flight?

Risk from the weight/muscles loss

The two major direct effects of the muscle loss that have been observed are weakness post-flight and the increased incidence of low back pain during and after flight. Most of the muscle loss occurs early in flight but continues at a lower rate, once the initial  response is  over (Matsumoto  et al. 2011; Smith et al. 2004, 2005). After several months

between modules, exercise and EVA suit injuries were the most common causes. The rate of exercise-related muscle injuries was estimated to be 0.003 per flight day (Scheuring et al. 2009). The rate did not increase with increased flight duration. For a 1-year mission with six astronauts, this translates into a probability of *7 injuries (Scheuring et al. 2009).

The adaptation to weightlessness leaves astronauts ill- equipped for life with gravity when they return to earth. Astronauts returning from even short duration space flights of 1–2 weeks often experience muscle fatigue, weakness, a lack of coordination in movement and muscle soreness (Edgerton and Roy 1994; Riley et al. 1995; Stauber et al. 1990). Isometric, concentric and eccentric force develop- ment declines by as much as 30 %.

The loss of muscle mass is responsible at least in part for the decrease in muscle strength and increased fatigability observed after space flight (Fitts et al. 2000; Grigorev et al. 1996; Leonard et al. 1983; Nicogossian 1994; Vorobyov et al. 1981).

The post-flight muscle weakness has been a major focus of counter-measure programs.

The available flight data show that these problems do not seem to impair inflight performance or post-flight health status for missions lasting up to 1 year (Smith et al. 2005; Zwart et al. 2009).

Three factors have been identified as contributing to the inflight muscle loss. Firstly, the reductive remodeling, secondly the level of pre-flight physical fitness and thirdly an inability to maintain energy balance inflight.

Table 1 The frequency of inflight injury has declined with space vehicle development (Scheuring et al. 2009)

Program Total flight hours Incidence
Mercury 6 3.005
Gemini 1,940 0.049
Apollo 7,506 0.010
Skylab 12,352 0.002
Shuttle 1299,467 0.033
Apollo/Soyuz 652 0
NASA/MIR 22,693 0.004
ISS 56,581 0.008


Journal information: PLoS ONE
Provided by University of Texas Medical Branch at Galveston

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