This study is a randomized controlled trial. The objective of this study is to investigate:

1. the effects of 1-year evidence-based interventions incorporating wearable device functions alone on device-measured movement behaviours and cardiometabolic risk factors among adults with central obesity; and
2. the effects of 1-year evidence-based interventions incorporating wearable device functions combined with health coaching on device-measured movement behaviours and cardiometabolic risk factors among adults with central obesity.

We hypothesise that the use of wearable device functions in combination with health coaching will lead to significantly greater improvements in device-measured movement behaviours and cardiometabolic risk factors compared with no intervention and wearable device functions alone, both after the 12-month intervention and at the 6-month follow-up.

This study will implement a parallel-group, open-label, 1-year randomized controlled trial involving 133 adults aged 40 years or older with central obesity. The trial will examine the effects of a cutting-edge intervention using evidence-based wearable device functions alone or in combination with evidence-based health coaching on movement behaviours and cardiometabolic risk factors.

All participants will provide written informed consent prior to participation and will be informed that they may withdraw from the study at any time during the study period. A staff member without access to participant information will generate a computer-based randomization list using permuted blocks of six, with two participants allocated to each of the three groups (two intervention groups and one control group) per block, in a 2:2:2 allocation ratio.

The control group will not receive a Fitbit device and will be asked to continue their usual lifestyle. The two intervention groups will use four evidence-based Fitbit functions: step goal setting, sleep goal setting, activity prompts, and community functions. The key difference between the two intervention groups is that one group will additionally receive a structured health coaching program adapted from the U.S. Centers for Disease Control and Prevention (CDC) National Diabetes Prevention Program (NDPP), as used in our prior trial.

The CDC NDPP recommends that at least 22 health coaching sessions be delivered over a 1-year period, depending on participants' needs, and provides corresponding curricula and handouts outlining session content. Coaching sessions will be delivered by trained research staff who have received appropriate professional training in health coaching delivery.

The lifestyle change program will include, but not be limited to:

1. education on cardiometabolic disease risk factors (e.g., type 2 diabetes, coronary heart disease, and stroke);
2. goal planning and goal setting;
3. strategies for self-monitoring physical activity, sleep, diet, and stress;
4. strategies for initiating and maintaining favourable levels of physical activity, diet, and sleep;
5. strategies for obtaining support from family, friends, and co-workers; and
6. review of achievements and challenges during the intervention period, with goal setting and planning for the subsequent 6-month follow-up period.

Participants will attend the research laboratory on three occasions: at baseline, immediately after the 12-month intervention, and at the 6-month follow-up. Assessments will include demographic information, standing height, body weight, body mass index, waist circumference, hip circumference, waist-to-hip ratio, systolic and diastolic blood pressure, and questionnaires.

At each laboratory visit, one EDTA blood sample and one clotted blood sample will be collected by a trained phlebotomist and analysed by an accredited medical diagnostic centre to assess cardiometabolic risk markers, including total cholesterol, HDL cholesterol, LDL cholesterol, and triglycerides.

An Axivity AX6 device will be used as a research-grade accelerometer to assess movement behaviours, including sleep, sedentary time, light physical activity, moderate-to-vigorous physical activity (MVPA), steps, energy expenditure, and resting heart rate. Participants will also be asked to wear the Axivity device on the dominant wrist for seven consecutive days at baseline and post-intervention.

Research staff will access participants' Fitbit web accounts to extract Fitbit data, activate Fitbit functions for the intervention groups, and monitor device battery life. Reminders to wear and charge the Fitbit and research-grade accelerometer will be sent every seven days via WhatsApp to promote protocol compliance. To further support adherence, participants who complete all study assessments will receive a HKD 500 supermarket voucher.

Inclusion Criteria:

• Age 40 years or older
• Central obesity, defined as waist circumference: ≥90 cm for men; ≥80 cm for women
• Insufficient physical activity based on World Health Organization (WHO) guidelines
• Physical activity assessed using the International Physical Activity Questionnaire (IPAQ) - Short Form
• Ownership of a smartphone
• Able to read and understand English or Chinese

Exclusion Criteria:

• unable to perform daily-life activities, based on the Physical Activity Readiness-Questionnaire (PAR-Q) (24) and/or participation in another lifestyle-change intervention study.

The datasets analysed during the current study and statistical code are available from the corresponding author on reasonable request, as is the full protocol.

The datasets analysed during the current study and statistical code will be available after the completion of the study

IPD data can be accessed based on reasonable request by contacting the corresponding author.

Group allocation will be concealed from study staff until the 7-day pre-intervention period begins (for preparation of corresponding e-leaflets, lifestyle coaching courses and baseline Fitbit step goal calculations), and from participants until the interventions are delivered. Given the nature of the interventions delivered, it will be impossible for participants to be blinded to the specific intervention they receive once the initial interventions are provided; however, study staff analyzing participants' de-identified data will remain blinded to participant randomization assignment.