Type 2 Diabetes Mellitus (T2DM) is a major chronic lifestyle-related disorder with a significant impact on quality and costs of care. As patients with T2DM often have insufficient knowledge about proper self-management and are insufficiently motivated for a lifestyle change, interventions with more motivational strategies and personalization are needed. The use of real-time monitoring of glucose values, nutrition and physical activity in combination with coaching aimed at lifestyle-related behavior change may improve patients' diabetes management. Therefore, ZGT, UT and RRD developed the Diameter app. The aim of phase 2 of this study is to investigate the Diameter as blended-care using a feasibility study. The primary objective of this feasibility study is to assess intervention usage and acceptability of the Diameter as a blended-care intervention in secondary care, of which some are also following a combined lifestyle intervention (GLI). Secondary objectives are to explore behavioral (e.g. physical activity), physiological (e.g. BMI), psychological (e.g. health-related quality of life) and clinical outcomes (e.g. glucose control, estimated HbA1c values).

This study has a mixed-method design with 3 (regular participants) or 4 (participants who decide to follow the Combined Lifestyle Intervention (GLI) COOL next to the Diameter during the study period) data collection points.

Patients will start with a two-week period of baseline measurements. Subsequently, patients will use the Diameter as a blended-care intervention for 10 weeks. The two-week measurement periods will be repeated twice (T1: week 13-14 and at T2: week 25-26). Between T1 and T2, patients will use a version of the Diameter without daily coaching messages. At T1 and T2, questionnaires will be administered, data on physical activity, food intake and glucose values will be logged, and blood and urine samples will be retrieved from regular care measurements. In addition, open-ended interviews will be performed with 10-15 patients at T1. For participants who also decided to follow the COOL program, some routinely collected measurements as part of the COOL program will be obtained from the patient record.

INTRODUCTION Type 2 Diabetes Mellitus (T2DM) is a major chronic lifestyle-related disorder with a significant impact on quality and costs of care. As patients with T2DM often have insufficient knowledge about proper self-management and are insufficiently motivated for lifestyle change, interventions with more motivational strategies and personalization are needed. The use of real-time monitoring of glucose values, nutrition and physical activity in combination with coaching aimed at lifestyle-related behavior change may improve patients' diabetes management.

It is hypothesized that a technology-supported lifestyle intervention is effective and a step forward in T2DM management. In the Twente region of the Netherlands, a personalized treatment strategy to improve lifestyle in T2DM patients is being developed by providing coaching and feedback in daily life through a mobile application, the Diameter. The development of the Diameter is a collaboration between ZGT hospital, University of Twente and Roessingh Research and Development Institute. The Diameter monitors food intake, physical activity and glucose values; it gives individual patients and healthcare professionals insight into lifestyle, blood glucose levels, as well as into the effect of lifestyle behavior on glucose values in daily life. Furthermore, the Diameter offers evidence-based coaching aimed at improving lifestyle, i.e. physical activity and nutrition. The coaching content offered in the Diameter comprises the e-supporter which is developed by the UT, ZGT and TNO within the E-Manager project. It consists of goal setting, daily lifestyle coaching via short messages and a weekly exercise that supports people to achieve selected goals for a period of ten weeks. An innovative and unique coaching strategy is the feedback on blood glucose levels through continuous glucose monitoring (i.e., FreeStyle Libre). This continuous feedback is considered to be a very powerful tool for behavior modification and offers great potential. The development of the Diameter is an iterative process. Since 2017, the monitoring and coaching elements of the Diameter have been developed, extended and validated in several studies. In a blended-care setting, the Diameter combines face-to-face counselling of usual T2DM care with the Diameter and continuous glucose monitoring through the FreeStyle Libre.

OBJECTIVE The Diameter as a blended-care intervention has only been evaluated in a small pilot study (phase 1) in ZGT hospital to test the proposed research protocol for phase 2 with T2DM patients treated at the outpatient clinic. Therefore, information about the intervention usage and acceptability are not yet available, as well as proper data for sample size calculation in future effectiveness testing. The next step is to implement and evaluate the Diameter in practice regarding intervention usage and acceptability (e.g. effort expectancy, pleasure), and to explore its effect on clinical outcomes (e.g. glycemic control), physiological (e.g. body composition), behavioural (e.g. physical activity), and psychological outcomes (e.g. health-related quality of life). Phase 2, which is registered in this trial system, concerns a feasibility study in ZGT hospital to evaluate intervention usage and acceptability of the Diameter as part of usual care. Usual care includes traditional diabetes care, possibly for a subgroup of participants, supplemented by the combined lifestyle intervention "Gecombineerde Leefstijl Interventie" (GLI) COOL which will be offered by ZGT to their T2DM patients.

STUDY DESIGN The Diameter will be assessed using a mixed-methods prospective longitudinal design among 80 patients with T2DM. Approximately 20-40 of them will also follow the COOL program as part of usual care. During the study, 3 rounds of measurement will be performed. Additionally, for participants who started the COOL program next to the use of the Diameter, an extra measurement moment after 8 months (T3) is included. The data for this measurement will be collected as part of usual care and will be extracted from the participants' health record with the patients' consent.

The measurements will be performed at the following time points:

• T0 (week 1-2): All participants will start with a two-week period of baseline measurements, including blinded glucose measurement (using the Freestyle Libre Pro IQ sensor), blinded nutrients intake measurement (using the Diameter app), a non-blinded physical activity measurement (using a Fitbit), other clinical and physiological outcomes and some questionnaires.
• Use of the Diameter (week 3-12): All study participants use the Diameter for 10 weeks to receive digital personalized coaching with a maximum of 2 coaching messages per day and one exercise per week about goal setting and achievement. Also, they use the Diameter to monitor their physical activity and nutrition. They receive 2 Freestyle Libre 2 sensors with which they can measure their glucose values continuously for in total 4 weeks, using the LibreLink app.
• T1 (week 13-14): All participants will perform a two-week period of measurements of all clinical, physiological, and behavioral outcomes, will fill in a number of questionnaires and 10-15 participants will be asked to participate in an interview about their acceptability of the Diameter.
• Use of the Diameter "light" (week 15-24): The participants will use a "light" version of the Diameter, consisting of all functionalities of the Diameter except the daily coaching messages as they will stop automatically after 10 weeks. They receive 2 Freestyle Libre 2 sensors with which they can measure their glucose values continuously for in total 4 weeks.
• T2 (week 25-26): All participants will again perform a two-week period of measurements of clinical, physiological and behavioral outcomes and will fill in some questionnaires. The follow-up measurement is performed after 6 months because it is assumed that there is a permanent change in behavior when the healthy behavior is maintained for a period of six months or longer.
• T3 (week 34): Only applicable for participants who are also following the COOL program. After 6 months of using the Diameter, participants who also follow COOL will continue their trajectory with the lifestyle coach in ZGT. In the context of the COOL program, after 8 months some measurements are taken that are part of regular care, consisting of medication use, HbA1c, BMI and waist circumference. These data will be obtained with consent of the participant from the patient record. No additional measurements will be conducted for this purpose. These data will be used as a final follow-up measure to assess the extent to which these participants can maintain any progress in clinical and physiological outcomes after 8 months which is the end of the basic part of the program that focuses on behavioral change.

STUDY POPULATION In phase 2, patients will be recruited in ZGT hospital. The patient population will consist of 80 T2DM patients (18+, male and female) visiting the outpatient clinic for T2DM at ZGT hospital. Patients who are already participating or participated in the DIALECT cohort will be recruited. The remaining number of patients will be recruited from the outpatient clinic in ZGT. Some of the T2DM patients recruited from the outpatient clinic also follow the COOL program within ZGT in addition to receiving regular diabetes care. People who will participate in the COOL program and want to participate in the study will start using the Diameter at the beginning of the COOL program. Patients who use blood glucose-lowering medication independent of their gender and socio-economic status will be included.

STUDY SETTING This research project will be conducted within the outpatient clinic for diabetic care in the ZGT hospital. Patients will participate in the pilot study for 5 weeks and in the feasibility study, phase 2, for half a year. Research appointments will be combined with regular outpatient clinic or COOL visits as much as possible. Currently, 17 healthcare professionals are working in this specific outpatient clinic. The team consists of 8 internists, 7 T2DM nurses, one nurse practitioner and one technical physician specialist together treating 2500 patients with T2DM a year. The current patient population consists of 450 T2DM patients, they are on average 63 years old with a mean duration of 11 years, and most of them are male (58%). Their average BMI is 32.9 ± 6.2; almost all patients (95%) have a BMI higher than 25.

In addition to traditional diabetes care, ZGT starts to offer the Combined Lifestyle Intervention (Gecombineerde Leefstijl Interventie; GLI) COOL to their T2DM patients. During the COOL program in ZGT, a professional lifestyle coach who is also a diabetes nurse will counsel adults with T2DM treated in ZGT who are obese or at high risk of obesity to achieve a healthier lifestyle. COOL consists of a basic part that focuses on behavioral change (8 months) and a maintenance part (16 months). In the first 8 months of the COOL program, participants follow 8 group sessions of 90 minutes and 4 individual sessions (2 sessions of 60 minutes, 2 sessions of 45 minutes).

Inclusion Criteria:

• Diagnosed with type 2 diabetes mellitus
• Being familiar with using a smartphone
• Being treated in ZGT hospital or primary care

Exclusion Criteria:

• Dependence on renal replacement therapy
• Severe general diseases or mental disorders make the participation in the study impossible
• Insufficient mastery of the Dutch language
• Other CGM device than Freestyle Libre.

The primary study aim is to assess intervention usage and acceptability (e.g. effort expectancy, pleasure) regarding the Diameter integrated in regular secondary care and how acceptability is associated with intervention usage. Therefore, the study does not make use of a double blind randomized controlled trial as this design does not fit the study's primary purpose. Consequently, there will be no randomization and treatment allocation. To minimize the treatment effect at baseline, baseline measures will be blinded as much as possible, i.e., participants will not be able to get insight in their glucose values and nutrient intake. The physical activity data collected with the Fitbit cannot be blinded, so that the participants do have insight into their physical activity during the baseline measurements