Immersion Conditions

According to the conditions of the experiment, the subjects were immersed for 3 days. They were allowed to leave the immersion bath for some examinations and hygiene procedures. The average time spent by the subjects outside the immersion bath was 28.2 ± 2.8 min on the first day, 22.1 ± 0.9 and 18.1 ± 0.6 min on the second and third days, respectively (Supplementary Figure 1). Approximately 2/3 of the total time outside the immersion bath the subjects spent in the supine position. In the immersion bath, the subjects’ movements were moderately restricted; physical exercises were not performed. The water temperature was maintained at 32.5 ± 2°C. Every evening, the subjects were raised from the bath for an average of 15–20 min to carry out hygiene procedures, most of which were performed in the supine position. The subjects were under 24 h surveillance, including monitoring both main vital parameters and the working condition of immersion baths and other technical equipment. The day before the start of the immersion (B-1), every day of the immersion (DI1–DI3), as well as the day after its completion (R + 0), a comprehensive medical examination was performed three times a day (15:00, 21:00, and 8:00); the list of examinations is provided in Supplementary Table 2. During the time free from treatments and tests, the subjects had the opportunity to read, work on a laptop, watch TV, talk on the phone, etc.

To solve the problem of urine collection, subjects used a special urine collector (Lebedeva, 2020) which allows urination while lying in an immersion bath (Figure 2A). The device consists of a miniature urine collection funnel (Figure 2B) connected with a flexible silicone tubing to the apparatus (Figure 2C) which comprises a diaphragm pump, control boards, variable resistor and battery. Via a second silicone tubing, the pump was connected to a container for storing and transporting urine. This method of urine collection allows researchers and physicians to conduct adequate volumetric measurements and further clinical analysis, excluding contamination with vaginal secretions. Urination was possible in any position using this device. In this experiment in most cases, the device was used in support unloading conditions, but sometimes solid support was required, as described in section “Medical Traits and Risks.” A pillow under the back and shoulders was commonly used.

Scheme of a universal portable device for mechanical urine collection in women (A). Urine collection nozzle (B). Fully assembled device (C).

The day before the start of the experiment, during the entire immersion period and the day after its completion, the subjects were on a controlled diet with fluid intake ad libitum. A standard menu was developed with three meals a day, taking into account the taste preferences of the participants. The calorie content of the diet was calculated according to the formulas used for determining the energy value of the food rations for the ISS crew members. The recommended daily caloric intake was 2,315 ± 117 kcal/day for women under 30 years old, 2,224 ± 166 kcal/day for women over 30 years old, balanced in basic nutrients. Mineral composition of the diet was not calculated. During the day, a few snacks were allowed (fresh and dried fruits, cookies, candies). Indigestible, gas-forming, spicy and high fat food products were excluded from the diet. The use of alcohol, caffeine-containing products, nicotine, cannabinoids, and exogenous opiates was completely prohibited throughout the entire study period. To maintain a comfortable psychophysiological state during the immersion period, subjects were allowed some deviations from the proposed menu that were recorded in the food diary.