The 180-day Controlled Ecological Life Support System (CELSS) experiment — a closed-loop habitat study with four volunteers — found a 10–15% increase in carotid intima-media thickness (IMT), a roughly twofold decrease in endothelium-dependent vasodilation, and a 1.5-fold decline in serum 25-hydroxyvitamin D, reproducing the carotid wall thickening previously seen in the 520-day Mars-500 confinement. However, because confinement co-occurred with reduced voluntary exercise, dietary change, absence of natural solar radiation, and a 36-day Mars-like 24 h 40 min day-night schedule, the study could not identify which habitat factor is the dominant cause of the vascular changes, could not assess circadian misalignment at all, and could only speculate that daily greenhouse work might protect against vascular and psychophysiological deconditioning. Later analog studies through 2026 — the 40-day Deep Time cavern isolation (no carotid changes with high activity, but confounded by habitat volume and duration), the 8-month SIRIUS-21 mission (non-significant retinal and arterial-stiffness trends in four crew), a 55-day bed-rest countermeasure trial (exercise ± artificial gravity attenuated carotid changes in an inactivity model, not in confinement), and 6-month spaceflight carotid data (microgravity context) — have added comparative evidence but left the confinement-specific causal attribution and countermeasure comparison unresolved. The problem is to determine, in controlled long-duration confinement studies with adequate crew size, which habitat factors drive carotid remodeling and endothelial impairment and which countermeasures prevent them.

Background

Long-duration spaceflight and planetary colonization will require crews to live for months in sealed artificial habitats. A Controlled Ecological Life Support System (CELSS) is a closed-loop habitat in which air, water, and part of the food are regenerated by biological processes (plants grown in greenhouses), so the crew must also perform daily plant-care and maintenance work. Because real spaceflights are rare and confounded by microgravity, space agencies study the health effects of prolonged enclosed living using ground-based spaceflight analogs — controlled isolation experiments such as the Russian–European Mars-500 program (105- and 520-day sealed-habitat missions) and the 180-day CELSS experiment conducted in Shenzhen in 2016 with four volunteers (three men, one woman, culturally homogeneous crew). The integrative report of that experiment (Yuan et al., 2019, Frontiers in Physiology) measured body composition, metabolism, cardiac and vascular morphology, endothelial function, muscle tone, behavior, and mood at eight time-points, including a 36-day mid-mission period on a Mars-like sol (a day-night cycle of 24 h 40 min, mimicking the Martian day).

Two vascular findings are central. First, carotid intima-media thickness (IMT — the combined thickness of the innermost two layers of the arterial wall, measured by ultrasound, and an established marker of vascular aging and cardiovascular risk) increased by 10–15%, reaching 14 ± 4% above baseline by month 6, while carotid distensibility was unchanged. A similar 14–28% IMT increase had been observed in all six subjects of the 520-day Mars-500 confinement (Arbeille et al., 2014), where it could not be attributed to hypovolemia, hypertension, metabolic alteration, or aging. Second, endothelial function — assessed for the first time in a confinement study by laser Doppler flowmetry of skin microvessels combined with iontophoresis (electrically driven delivery) of acetylcholine, which tests endothelium-dependent vasodilation (the ability of the vessel lining to dilate in response to chemical signals) — declined nearly twofold at mid-mission, alongside a slight decrease in soluble CD146 (an endothelial-cell marker in blood). Serum 25-hydroxyvitamin D fell 1.5-fold, consistent with absence of solar radiation and insufficient dietary intake.

The interpretive difficulty is that everything changed at once inside the habitat: voluntary exercise was low (exercise recommendations were not followed), diet came largely from the greenhouse, there was no natural daylight, workload fluctuated with planting and harvest, and for 36 days the crew lived on the Mars-like sol. The authors could only propose — not establish — that physical inactivity or nutritional change (including vitamin D deficit) underlies the IMT increase; the protocol (no urine collection, one blood sample per time-point) made circadian misalignment unmeasurable. Their conclusion called for countermeasures against IMT increase and endothelial deconditioning, suggesting that sustained daily greenhouse work might be more effective than brief daily exercise — a hypothesis that has never been tested. Understanding which habitat factor drives vascular deconditioning, and which countermeasure prevents it, matters both for crew selection and habitat operations on future Lunar and Martian missions and for terrestrial closed environments (submarines, Antarctic stations, and CELSS prototypes), where the same confinement-related vascular aging may occur.

Problem Statement

For long-duration confinement in closed artificial habitats (CELSS-type and other sealed spaceflight-analog environments), determine (1) which habitat factor or combination of factors — reduced daily physical activity, nutritional change including vitamin D deficit, absence of natural solar radiation under artificial lighting only, the Mars-like 24 h 40 min day-night schedule and the circadian misalignment it may induce, workload, or other habitat-related stressors — is the dominant cause of the observed carotid intima-media thickening and endothelial impairment; and (2) the relative effectiveness of candidate countermeasures, in particular sustained daily greenhouse and life-support work versus mandatory structured exercise versus combined protocols, in preventing this carotid remodeling and endothelial deconditioning.

References

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