Study Reveals Critical Gravity Threshold for Human Health in Space

A recent study published in *Science Advances* has identified a crucial threshold of gravity necessary for maintaining muscle health in space, with implications for future human missions to the Moon and Mars. Conducted aboard the International Space Station (ISS), the research involved 24 mice exposed to varying levels of gravity: microgravity, 0.33 g, 0.67 g, and 1 g. The findings indicate that a gravity level of 0.67 g is essential for preventing muscle deterioration. Below this threshold, particularly at 0.33 g, the mice exhibited changes in muscle composition and reduced grip strength, although they did not experience complete muscle loss.

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The gravity levels on the Moon and Mars are significantly lower than the identified threshold, with the Moon at approximately 0.17 g and Mars at about 0.38 g. This raises concerns about the long-term health of astronauts on extended missions to these celestial bodies. Lori Ploutz-Snyder, a kinesiology expert, emphasized the importance of this research, noting that it provides a starting point for understanding how gravity affects human health in space. She stated, “You have to start somewhere, and this is an exciting development.”

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Mary Bouxsein, a co-author of the study, highlighted that the results are reassuring, as they suggest that mice can maintain muscular function without full Earth gravity. However, she cautioned that Mars' gravity alone would likely be insufficient to preserve muscle function in humans.

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Experts agree that further studies are needed to validate the 0.67 g threshold in humans and explore its implications for bone health and exercise countermeasures during space missions. Mark Shelhamer, a former chief scientist at NASA, noted the necessity of understanding whether the gravity levels on the Moon and Mars can prevent the deconditioning of astronauts' muscles and bones. He stated, “We have no idea if being on the Moon in one-sixth [Earth] gravity or being on Mars in three-eighths [Earth gravity] is enough to stop the deconditioning.”

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While the study provides valuable insights, it also raises questions about the applicability of mouse models to human physiology. Se-Jin Lee, a geneticist, pointed out that differences in muscle use and composition between mice and humans could affect the translation of these findings. Bouxsein acknowledged that while Mars gravity may not be sufficient for muscle preservation, the reduced gravity environment might alter the physical demands placed on astronauts.

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• “You have to start somewhere, and this is an exciting development,” — Lori Ploutz-Snyder, Dean of the University of Michigan’s School of Kinesiology

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• “Before I saw this study, I would [have said] we know nothing about how much gravity exposure is necessary to halt or slow down the deconditioning that goes on when you send people into space,” — Mark Shelhamer, Former Chief Scientist of NASA’s Human Research Program

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• “It does suggest that Mars gravity alone would not be enough to preserve muscle function,” — Mary Bouxsein, Professor of Orthopedic Surgery at Harvard Medical School

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The findings from this study are critical for NASA's plans to establish a lunar base through the Artemis program and for future manned missions to Mars, underscoring the need for effective countermeasures to ensure astronaut health in low-gravity environments.

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