2.1 Design: the protocol (NCT04489043) has been recently published [31]. This is an intervention study in which renal transplant patients with prediabetes were treated with exercise for 12 months to revert this complication. Additionally, we evaluated: (a) adherence to exercise, and (b) improvements in metabolic risk factors: obesity, dyslipidemia and blood pressure.
2.2 Definitions:
2.2.1 Impaired fasting glucose (IFG):fasting levels of glucose between 100-125 mg/dl in more than 2-3 determinations using data from clinical records.
2.2.2 Impaired glucose tolerance (IGT): patients with glucose levels at 120 min during an Oral glucose tolerance test (OGTT) between 140-199 mg/dL [31-32].
2.3 Patients: inclusion criteria: (a) >18 years; (b) renal transplantation beyond 12 months after transplantation with stable renal function; (c) prediabetes: IFG and/or IGT and (d) ability to perform exercise. The exclusion criteria were: (a) PTDM; (b) diabetes before transplantation; (c) clinical conditions that limit the performance of exercise i.e., clinical instability: active infection, cancer, acute cardiovascular disease, advanced renal disease, pulmonary hypertension, limb amputations; (d) morbid obesity, body mass index (BMI) ≥40; (e) inability to understand the protocol; and (f) severe psychological disease.
2.3.1 Screening: to select patients with prediabetes we proceed as follows: patients with IFG were detected by the evaluation of clinical records. The diagnosis of IGT is more difficult: it requires an OGTT, since it frequently coincides with normal fasting glucose levels. Clearly, to perform an OGTT in all non-diabetic patients is not cost-effective. Based on a previous report [32] we selected those patients with metabolic syndrome, condition that is frequently associated with IGT as a proxy for prediabetes. We proceeded according to pre-specified steps: (a) selecting subjects with metabolic syndrome i.e., BMI > 27 kg/m2 and high triglyceride levels (> 150 mg/dL), (b) perform an OGTT in those patients. Finally, patients with IFG also underwent an OGTT to detect the presence of a concomitant IGT.
2.4 Procedures
2.4.1 OGTT: after a 10–12 h overnight fast, the patient drank a solution of 75g/200ml glucose. Samples were taken before (0 min) and after (120 min) to measure glucose. The test was performed at baseline (screening) and every 3 months to the end of the study (month 12) to evaluate the reversibility, relapse, or persistence of prediabetes. The OGTT was not performed in patients who developed PTDM.
To perform the OGTT as well as to prescribe exercise, patients had to be stable without conditions that could induce transient hyperglycaemia, insulin resistance or those that limit their capacity to perform exercise i.e.: infections, acute rejection, cardiovascular disease, and acute kidney injury [11]. The presence of any of these conditions postponed the test for 1-3 months after full recovery.
2.4.2 Pre-treatment evaluation: all patients underwent a personal interview to evaluate global physical activity or sedentary behaviour with the General Physical Activity Questionnaire (GPAQ) [33] described by the World Health Organization (WHO) covering: activity at work, travel to from places and recreational activities. Also, the importance of exercise, its relationship with PTDM and barriers to exercise (psychological, logistics and cognitive) were discussed.
2.4.3 Anthropometric measures: at baseline and every 3 months weight, waist circumference, hip circumference, body mass index and waist to hip ratio were measured [34-36].
2.4.4 Analytics: at pre-specified time points (baseline, 3, 6, 9 and 12 months), samples were taken to determine: hemogram, creatinine, glycated haemoglobin (HbA1c), total, LDL and HDL cholesterol, triglycerides, uric acid, and other parameters.
2.5 Exercise treatment: a stepped incremental protocol which includes two modalities of exercise, aerobic and strength training, was prescribed. The frequency, intensity, and duration of aerobic exercise, as well as the addition of strength training depended on the recovery, persistence, or recurrence of prediabetes.
2.5.1 Aerobic exercise: at baseline: all patients started with any form of aerobic exercise: brisk walking or cycling. The intensity was moderate, 30 min/day and 5 times per week. No more than two consecutive days between bouts were allowed [37-41].
Changes in exercise prescription: based on the persistence or recurrence of prediabetes during follow-up.
• At 3 months: in the case of persistent prediabetes, aerobic exercise was increased to 45 min/day and 5 times per week. When prediabetes reverted, prescription remained the same as in baseline.
• At 6 months:if prediabetes persisted, exercise increased to 60 min/day and 5 times per week. If prediabetes reverted, the prescription of exercise remained stable: 45 min/day and 5 times per week. In patients without prediabetes since month 3, baseline prescription was not changed.
• At 9 months: in patients without prediabetes since month 3 or 6, previous prescription was unchanged: 30 or 45 min/day and 5 times per week, respectively. In cases with relapsing prediabetes, exercise was increased. However, in patients with persistent prediabetes from month 3 to 9, aerobic exercise was practised 45 min/day and 3 times per week, adding strength exercise. A detailed information can be found in the published protocol [31].
2.5.2 Strength exercise: including chest press, lateral pull downs, shoulder press, arm curls, triceps extension, sit-ups, deadlift, leg press and squats, among others. Each type of exercise was monitored by a physiotherapist. The intensity was moderate: i.e., an exercise that can be repeated no more than 15 times to vigorous 60-80% of one repetition maximum (1 RM), i.e., repetitions of an exercise that can be repeated no more than 6–8 times [40].Patients practised at least 8-10 exercises of 1-3 sets near fatigue on every exercise [40] 2 times per week on non-consecutive days [41].
2.6 Evaluation of adherence: to evaluate and promote compliance, we designed a specific protocol, including a fixed and a flexible plan. The fixed plan included: the use of a bracelet activity tracker; telephone calls and a regular plan of visits to the centre. The flexible plan was pre-specified for patients with inadequate adherence and included both extra telephone calls and visits to the centre [42-43]. For the scope of this study, adherence was classified a priori in 3 groups depending on the accomplishment to training: (a) acceptable: ≥70%; (b) moderate: 40-70% and (c) bad: ≤40% [44] covering three major aspects:
a.- Digital monitoring: every patient received an activity tracker (Mi Global Home). The exercise performed was recorded in the device, which could be accessed and reviewed by the physiotherapist to evaluate that exercise was performed as prescribed and to have data on the amount of exercise, checking compliance. Also, with this device the patient could receive feedback and support from the physiotherapist. Finally, each session was recorded in a database.
b.- Phone calls: a scheduled plan of phone calls was designed to provide support to patients on different aspects: the performance of exercise, possible injuries, barriers to exercise, adaptation to training, improvements, limitations (physical and mental), doubts among others. The calls also provided an opportunity to identify a variety of obstacles to lifestyle changes and to discuss behavioural approaches to improve specific problems. In the fixed plan, patients were contacted once per week during the first 3 months, twice per month from 3 to 6 months, 1 every three weeks from 6-9 months, and 1 per month in the last 3 months. In the flexible plan, patients with bad-moderate adherence were contacted once per week, independently of the period. All telephone calls, those scheduled or extra, were recorded in a database.
c.- Visits: participants had to attend a face-to-face meeting every 3 months in the research unit of the hospital. Extra-visits were scheduled in patients with moderate-bad adherence. Visits were focused to evaluate aspects related either to exercise or prediabetes and PTDM after transplantation, such as: achievements i.e., weight and glucose levels reduction; behavioural/motivational aspects related to these achievements; possible injuries, adaptation to exercise and long-term consequences of prediabetes and diabetes after transplantation. Finally, general lifestyle advice including diet changes was given to patients. Sessions lasted from 25 to 40 minutes each. Visits, regular or extra, were also recorded in a database.
2.7 Outcome: the reversibility of prediabetes to normal glucose metabolism by means of exercise.
2.8 Statistical analysis and sample calculation: there is almost no evidence available on the effect of exercise on renal transplanted patients with prediabetes or PTDM. Moreover, a possible effect of exercise in preventing the evolution from prediabetes to PTDM has not been specifically investigated. This limits the possibility of calculating a sample size of a study.
Thus, we decided to test the capacity of physical exercise to revert prediabetes as step to design a clinical trial to prevent PTDM. In a previous study [11] we observed that prediabetes may spontaneously revert to normal glucose metabolism in 25-30% of the cases i.e., from month 24 to 36 after transplantation. Thus, studies aimed at the prevention of PTDM on patients with prediabetes, must consider this spontaneous reversibility in the calculation of the sample size. In consequence, to be considered as effective, any tested intervention designed to prevent PTDM in prediabetics must have an effect significantly larger than the spontaneous reversibility rate of prediabetes. We assumed that exercise could be considered effective when the reversibility rate doubles the spontaneous reversal, from 30% to 60%. Assuming that in 60 cases with prediabetes, the spontaneous reversibility could be of 30% (n= 18) and the reversibility induced by exercise will account for another 30% (n=18), the total reversibility will be 60%. Considering an expected drop out of 20%, the total number of cases necessary to include will be 72 patients. Ad interim analysis were performed during follow-up to test the certainty of this power calculation. Statistical analysis was carried out with SPSS (IBM Corp. Released 2017. IBM SPSS Statistics for Windows, Version 25.0. Armonk, NY), and sample size was calculated with GRAMM0 V. 7.12 (IMIM, Spain).
2.9 Sensitivity Analysis: we evaluated (a) gender differences in response to exercise training and (b) partial reversibility, defined as the persistence of prediabetes but with major reductions in glucose levels either fasting (about 20 mg/dL) or after an OGTT (about 40 mg/dL) i.e., from 125 to 101 or from 190 to 151 mg/dL.
2.10 Adverse events: all adverse events associated to exercise training were recorded.