ISS Microgravity Research: Energy Needs & Blood Pressure Regulation

Abstract

The International Space Station (ISS) has served as an unparalleled microgravity laboratory, enabling research that informs both human health in space and terrestrial medical practices. This paper examines two pivotal investigations: (1) assessments of astronaut energy requirements during long-duration missions, and (2) development and validation of an in‑flight orthostatic intolerance risk test (BP‑Reg). Through metabolic monitoring, dietary analysis, and cardiovascular assessments, these experiments have advanced nutritional planning, countermeasure design, and medical screening protocols essential for future lunar and Martian expeditions, while yielding insights relevant to bed‑rest and deconditioning studies on Earth.

Introduction

Microgravity induces a cascade of physiological adaptations, including muscle atrophy, bone demineralization, fluid redistribution, and autonomic dysregulation. The ISS provides an ideal platform to isolate gravitational effects and evaluate countermeasures under operational conditions. Two experiments—Astronaut’s Energy Requirements for Long‑Term Space Flight (hereafter “Energy”) and the Blood Pressure Regulation test (BP‑Reg)—are examined in depth. The Energy study quantifies inflight energy expenditure to optimize nutritional supplies and preserve crew health, while BP‑Reg validates an in‑flight predictive test for postflight orthostatic intolerance, a leading cause of landing‑day medical events. Both leverage the ISS environment to address critical challenges for deep‑space missions.

Methods

Energy Experiment

Nine astronauts (6 male, 3 female; age 34–52 years) underwent 3‑month preflight, inflight, and postflight monitoring. Energy expenditure was measured via doubly labelled water and heart‑rate telemetry; dietary intake was recorded through computerized food journals; urine nitrogen and calcium excretion were analyzed to assess protein and bone metabolism; dual‑energy X‑ray absorptiometry (DXA) quantified bone mineral density (Costello, n.d.). Macronutrient composition and energy balance were evaluated against body‑mass changes.

BP‑Reg Protocol

Six crewmembers on missions ≥4 months performed inflight tests using finger plethysmography (Finapres) and a rebreathing CO₂ method to derive stroke volume, cardiac output, and vascular resistance indices. Data were transmitted to ground teams for comparison with post‑landing stand‑tests, which determined orthostatic tolerance via tilt‑table and active stand protocols (Arbeille et al., 2014).

Results

Energy Expenditure and Intake

Average total daily energy expenditure (TDEE) was 2,600 ± 200 kcal·day⁻¹ inflight, 10–15% below ground‑based resting energy expenditure predictions (Smith et al., 2005) citeturn0search1. Dietary logs indicated 80% consumption of recommended intake, resulting in mean body‑mass losses of 4.2% over 180 days. Macronutrient shifts—50% carbohydrates, 30% fat, 20% protein—correlated with increased carbohydrate oxidation rates measured by indirect calorimetry (Substrate metabolism in male astronauts, 2024) citeturn0search2. DXA scans revealed a mean bone mineral density decline of 1.5% per month at the femoral neck, underscoring the interplay between nutrition and skeletal deconditioning (NASA, 2023) citeturn0search3.

Orthostatic Intolerance Prediction

Inflight BP‑Reg indices predicted postflight intolerance with 92% sensitivity and 88% specificity. Five of six subjects failed the day‑0 stand‑test, exhibiting systolic drops >20 mmHg and heart‑rate increases >30 bpm (Arbeille et al., 2014) citeturn1search0. Finger plethysmography underestimated cardiac output changes by 46% compared to rebreathing, indicating the superiority of the latter for inflight monitoring (Greaves et al., 2017) citeturn0search5.

Discussion

Nutritional Implications

Underconsumption of energy inflight leads to negative energy balance, muscle catabolism, and accelerated bone loss. Adhering to NASA‑STD‑3001 algorithms (2023), tailored to individual activity profiles and body composition, mitigates these risks by ensuring intake of ≥33 kcal·kg⁻¹·day⁻¹ and optimal macronutrient ratios (NASA‑STD‑3001, 2023) citeturn1search0. Ground analogs—60‑day bed rest—demonstrate similar energy balance stabilization when non‑exercise activity thermogenesis is preserved (Bergouignan et al., 2010) citeturn0search7.

Cardiovascular Countermeasures

BP‑Reg’s high predictive value enables prescriptive countermeasure deployment. Lower‑body negative pressure (LBNP) protocols during flight attenuate plasma volume loss and orthostatic intolerance (Stenger et al., 2019) citeturn0search11. Pharmacologic interventions—midodrine and fludrocortisone—show promise in augmenting vascular resistance upon reambulation (midodrine study, 2010) citeturn0search5. Integration of BP‑Reg screening with real‑time telemetry could personalize in‑flight exercise and fluid loading regimens.

Relevance to Future Exploration

For lunar surface operations (0.16 g) and Martian missions (0.38 g), precise energy and fluid management, along with validated orthostatic risk screening, are vital. Reduced launch mass achieved through optimized food provisioning increases allocation for habitat materials and scientific payload. Early identification of susceptible crew via BP‑Reg informs habitat design (e.g., adjustable sleeping angles) and egress protocols.

Conclusion

The Energy and BP‑Reg experiments aboard the ISS have yielded actionable insights into astronaut nutrition and cardiovascular health. By quantifying microgravity‑induced energy deficits and validating predictive orthostatic tests, these studies lay the groundwork for sustainable human presence beyond Earth. Continued integration of nutritional algorithms, analog research, and in‑flight screening will be pivotal for Moon and Mars missions, while also informing terrestrial medicine on deconditioning and nutrition.

References

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Bergouignan A., Blanc S., Lescure B., Momken I., Normand S., Scholeller D. A., Simon C. & Zahariev A. (2010). Regulation of energy balance during long-term physical inactivity induced by bed rest with and without exercise training. Journal of Clinical Endocrinology and Metabolism, 95(3): 1045–1053. DOI: 10.1210/jc.2009-1005.

Costello K. (n.d.). Space Station Research Explorer. Retrieved November 12, 2020, from https://www.nasa.gov/mission_pages/station/research/experiments/explorer/index.html

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