The authors performed a systematic review on this topic and found only 14 manuscripts matching the problem of the impact of physical exercise on kidney function during pregnancy. The methodology and results of this review are given in supplement 1.
2.2. Study group
In total 69 healthy pregnant Caucasian women in uncomplicated, singleton pregnancy (age 31 ± 4 years; gestational age 22 ± 5) were recruited to the study (Fig. 1). These subjects responded to our mass media invitation and volunteered for the study. All were healthy, ready to participate and all gave written consent. They had no contraindications to perform physical exercises. Women were all owed to participate in the exercise programs regardless of their level of fitness and exercise capacity. Inclusion criteria: 1. course of pregnancy allowing participation in physical activities adapted to pregnant women; 2. consent of the obstetric care provider to participate; 3. taking part in all diagnostic and control tests to assess selected biological and functional parameters; 4. availability to participate in classes three times a week.
Exclusion criteria: 1. contraindications to physical effort or to any of the diagnostic or control tests; 2. allergies to materials used during tests; 3. other conditions that, according to the researchers, will threaten the health or safety of the participants or will significantly affect the quality of the collected data.
The study was a randomized control trial. The participants were divided into two groups, according the time when they access to the study (Spring or Summer 2021).
The training group – (HIIT group). 35 women from the training group participated in 8-week on-line HIIT program, 3 times a week, in a volume of each session of 60 minutes.
The control (educational) group - (EDU group) was constituted of 34 pregnant women who attended educational sessions on educational sessions on a healthy lifestyle, physical activity in the perinatal period and selected aspects of pregnancy and motherhood.
2.3. Methods
The flow of participants through the study was presented in Fig. 1. The study program and procedures are described in details elsewhere [23,24,25]. The HIIT intervention was designed based on available data on prenatal HIIT published before [21,22,26,27].
The experiments were performed in standardized conditions in the Laboratory of Physical Effort and Genetics in Sport, at the Gdansk University of Physical Education and Sport in Poland in 2021, with supervision of the professional staff.
Experimental training interventions
In HIIT group the intervention consisted of attending three on-line training sessions a week for eight weeks, in a volume of each session of 60 minutes. The warm-up together with educational tips on how to perform exercises lasted 7–10 minutes. The main part (15–20 min) was conducted in the form of high intensity intervals, approximately 85–90% of maximum exercise capacity. The intervals consisted of performing exercises for 30–60 seconds, alternating with a 30–60 second rest break, in the ratio of exercise time to rest 1:2, 1:1 or 2:1, according to the individual possibilities of the participant and taking into account the training progression and stage of pregnancy. Women had the task to exceed, during the exercise intervals, their anaerobic thresholds (AT), individually set based on the exercise capacity test. Using a heart rate monitor (Polar RS400, Finland) women were supposed to exceed the value of anaerobic threshold (HR/AT) in workout intervals for as long as they felt comfortable. The exercise intensity was monitored with the use of the 0–10 Borg Rating of Perceived Exertion (RPE) [28] and the Talk Test [29]. On average, women participated in 18 training sessions, which accounted for 79% of the planned training volume. One subject did not finished 8 week program and was excluded from analysis. Eventually, 34 participants completed HIIT training program.
Educational interventions
Educational classes were conducted online in real time, once a week for 8 weeks. We encouraged women from the EDU group to individually undertake exercise and fulfill at least the recommended level of physical activity (minimum 150 minutes per week of moderate to vigorous intensity). Women were asked to keep a diary of all their physical activity (including both structured exercise sessions and daily activities lasting at least 10 minutes, such as cleaning the house, gardening, shopping). The educational group did not monitor the intensity with heart rate monitors, but used the RPE scale and Talk Test. On average, the women performed 21 exercise sessions on their own, with an average intensity of 5.5, assessed on a 1–10 RPE scale. Eventually, 21 pregnant women from control group completed educational program.
Anthropometric measurements
The body mass and body composition measurements were performed using the electrical bioimpedance method (Inbody 720). Height, weight, BMI, amount of body fat, skeletal muscle mass, lean body mass, percentage of fat, amount of intracellular and extracellular water were defined. Maternal systolic and diastolic blood pressure (mmHg) and heart rate were assessed using electronic blood pressure monitor (OMRON).
International Physical Activity Questionnaire (IPAQ).
The level of physical activity was measured by the short form of International Physical Activity Questionnaire [30,31,32].
Progressive maximal exercise test
Before and after the 8-week intervention programs, all women underwent a progressive maximal exercise test on a cycloergometer with electronically regulated load (Viasprint 150P) to measure maternal oxygen consumption. First test was performed before start of training/education programs. Second test was performed after the 8-week training/education programs. In order to establish the maximum oxygen uptake the stationary respiratory gas analyzer (Oxycon Pro, Erich JAEGER GmbH, Germany) was used. The highest oxygen uptake achieved during the maximum effort and maintained for 15 seconds was taken as maximal oxygen capacity (VO2 max). The anaerobic threshold (AT) values, such as oxygen uptake at AT (VO2/AT) and heart rate at AT (HR/AT) were established using the V-slope method [33]. On average, the HR/AT was set at 87% ± 5 of maximal heart rate.
Patient and Public Involvement.
To design this research procedure and the HIIT intervention, we surveyed participants from our previous projects [34,35] and pregnant women from this study. We also discussed the HIIT Mama project during local thematic events for pregnant and postpartum women.
Biochemical measurements
Before and after progressive maximal exercise test samples of blood and urine were collected. Venous blood samples were drawn from the antecubital vein in a sitting position by experienced nurses. Blood was collected into an SST gel separator tube (BD Vacutainer). At the same day measurements were performed in the commercial laboratory, which has accreditation from the Polish Center for Accreditation (no. AM002).
Assessment of kidney function parameters
Albuminuria was defined as urinary albumin to creatinine ratio (uACR). Kidney function was analyzed by measuring changes in sCr, sUrea and sUA. Estimated GFR (eGFR) was calculated using Cockcroft-Gault equation. Fractional excretions of UA and urea were calculated, as well as serum urea to creatinine (sUrea/Cr), serum UA to creatine (sUA/sCr) and urinary creatinine to serum creatinine (uCr/sCr) ratios. Changes in serum sodium and potassium levels were measured.
In total 12 parameters of kidney function were analyzed. 5 were measured in serum (Na, K, sCr, sUrea and sUA) and 7 were calculated. Calculations and equations are shown in Supplement 2.
To the final analysis the participants who completed training or educational programs and had normal kidney function were selected. The 36 participants (19 women from HIIT group and 17 from EDU group) with creatinine below 0.6mg/dl and uACR < 30mg/g in rest were analyzed (Fig. 1).