Abstract Hydration status is a critical yet often underappreciated determinant of human health, influencing cognitive function, physical performance, renal health, and thermoregulation. Traditional assessment methods—such as serum osmolality, urine specific gravity, and body mass changes—offer only episodic snapshots and are confounded by renal function, diet, and diurnal variation. Over the past five years, however, the field has undergone a paradigm shift toward continuous, non-invasive, and multi-modal monitoring. This review synthesizes recent breakthroughs in wearable biosensors, artificial intelligence–driven predictive models, and molecular biomarkers such as copeptin and salivary microRNAs. We also highlight emerging technologies including microwave dielectric spectroscopy and optical coherence tomography for tissue hydration. Finally, we outline future directions, emphasizing the integration of hydration data into personalized digital health platforms and the need for validated reference standards across diverse populations.

1. Introduction Maintaining euhydration—an optimal balance of body water and solutes—is essential for cellular homeostasis. Even mild dehydration (1–2% body water loss) impairs attention, mood, and aerobic endurance, while chronic hypohydration is linked to urolithiasis, urinary tract infections, and possibly metabolic syndrome. Conversely, overhydration can precipitate hyponatremia, particularly in endurance athletes and elderly patients with polypharmacy. Despite this clinical significance, hydration status remains one of the least standardized vital signs. The 2023 consensus statement by the European Hydration Institute underscored that no single “gold standard” exists for field-based assessment, driving a surge of research into dynamic, context-aware solutions.

2. Advances in biomarker discovery: Beyond osmolality For decades, serum osmolality (Sosm) has been the reference method, with a threshold of >295 mOsm/kg indicating hypohydration. However, Sosm reflects only extracellular tonicity and is buffered by renal water reabsorption. A major breakthrough came from the validation of copeptin, the C-terminal fragment of provasopressin, as a stable surrogate for arginine vasopressin (AVP). A 2024 multicenter trial (n = 1,240) demonstrated that copeptin levels >10 pmol/L correlate with a 2.1-fold increased risk of hypo-osmolar dehydration in older adults, outperforming Sosm in detecting subclinical deficits. More recently, salivary microRNA panels—particularly miR-205 and miR-320a—have shown promise as non-invasive indicators of acute fluid loss. A pilot study by Tanaka et al. (2024) reported that after 3% body mass loss via exercise, salivary miR-320a increased 4.7-fold within 30 minutes, with a sensitivity of 88% and specificity of 91% against bioimpedance spectroscopy. These molecular markers, while not yet point-of-care, offer a window into intracellular dehydration that traditional markers miss.

3. Wearable and non-invasive sensor technologies The most transformative progress has occurred in wearable platforms. Three technologies stand out:

4. Artificial intelligence and predictive modeling Static measurements are limited by inter-individual variability. To address this, researchers have developed digital twins of fluid balance—personalized computational models that integrate heart rate variability, skin conductance, ambient temperature, and sweat rate. A landmark study by Chen et al. (2024) inNature Digital Medicineused a recurrent neural network trained on 2.3 million hours of wearable data from 1,900 athletes. The model predicted dehydration onset (defined as 2% body mass loss) with a lead time of 28 minutes, allowing real-time intervention. This predictive capability is particularly valuable in occupational settings (firefighters, miners) and in clinical wards where early detection of fluid overload can prevent heart failure exacerbation.

5. Challenges and unresolved questions Despite these advances, significant barriers remain. First, inter-individual variability in skin thickness, age, and body composition affects all optical and dielectric sensors. Second, hydration status is not a scalar—it has both volume and tonicity components. Most wearables measure extracellular volume, but they cannot distinguish hypovolemia (low volume with normal tonicity) from dehydration (low volume with high tonicity). Third, reference standards are population-specific. The deuterium dilution method is invasive and impractical for routine use, and there is no consensus on what constitutes “optimal” hydration in older adults with sarcopenia or in pregnant women. Fourth, regulatory approval for medical-grade wearables remains slow; the FDA has cleared only two hydration sensors to date, both for research use only.

6. Future outlook: Toward closed-loop hydration management The next decade will likely see the convergence of three trends:

7. Conclusion Hydration status assessment is transitioning from a retrospective laboratory measurement to a prospective, real-time physiological signal. The combination of molecular biomarkers (copeptin, microRNAs), dielectric and optical sensors, and AI-driven predictive algorithms offers a robust framework for personalized hydration management. However, translation into routine clinical practice will require rigorous validation in vulnerable populations, miniaturization of hardware, and harmonization of data standards. As we move toward a future of precision hydration, the ultimate goal is not merely to detect deficits but to anticipate and prevent them—thereby safeguarding human performance and health across the lifespan.

References

1. Tanaka, Y., et al. (2024). Salivary microRNA signatures of acute dehydration.Journal of Applied Physiology, 137(4), 891–902.

2. Chen, L., et al. (2024). Deep learning prediction of dehydration onset from wearable sensors.npj Digital Medicine, 7, 145. 3. Müller, S., et al. (2025). Microwave dielectric patch for continuous total body water monitoring.IEEE Transactions on Biomedical Engineering, 72(2), 512–523.

4. European Hydration Institute. (2023). Consensus statement on hydration assessment methods.Nutrition Reviews, 81(6), 701–715.

5. Patel, A., & Kim, J. (2023). Optical coherence tomography of buccal mucosa for rapid hydration screening.Biomedical Optics Express, 14(8), 3982–3995.

6. Hooper, L., et al. (2024). Copeptin as a marker of hypohydration in older adults: A prospective cohort.The Lancet Healthy Longevity, 5(3), e180–e190.