The cardiovascular benefits from optimal vitamin D supplementation strategies and exercise training

This 4-week non-drug intervention aims to compare the vascular and biochemical responses of outdoor isometric wall squat training combined with sunlight-induced vitamin D exposure, indoor isometric wall squat training without sunlight exposure, and an indoor control group. Arm 1 – Outdoor Isometric Training (Sunlight Exposure): Participants will perform isometric wall squat training outdoors under direct sunlight, based on the protocol of Edwards et al. (2024) and Wiles et al. (2018). The training will be delivered individually and supervised face-to-face by an exercise physiologist with more than five years of experience. Before intervention, participants will complete familiarization sessions and an incremental isometric wall squat test to determine their knee joint angle corresponding to 95% of peak heart rate (HRpeak). Each training session consists of four 2-minute bouts of wall squats at this angle, interspersed with 2-minute rest intervals. Sessions will occur three times per week, lasting approximately 30 minutes each, for a total duration of 4 weeks. During each outdoor session, participants will wear short-sleeved shirts and shorts, resulting in approximately 40% of total body surface area exposed, and will avoid the use of sunscreen. Sunlight exposure will be scheduled during periods when the local ultraviolet index is below seven, while excluding very high and extreme UV levels, to ensure both participant safety and experimental standardization. Based on Terushkin et al. (2010), each session is expected to generate approximately 1,000 IU of vitamin D, yielding an estimated total of approximately 30,000 IU over the 4-week intervention period. Participants will wear UV tracking devices (Sun-a-Wear, Switzerland) throughout the intervention to objectively monitor cumulative UV exposure. Arm 2 – Indoor Isometric Training: This group will perform the same isometric wall squat training protocol (same duration, intensity, frequency, and supervision) as the outdoor group but indoors in a controlled laboratory setting, without sunlight exposure. Training adherence will be ensured by supervision from the same exercise physiologist who records exercise attendance, heart rate data, and knee joint angles during each session. Intervention fidelity and adherence: To ensure consistent exercise execution, the heel-to-wall distance and coccyx-to-floor height will be recorded for each participant during the initial familiarization and replicated throughout the intervention. Heart rate will be continuously monitored using a Polar H10 device (Polar, Finland) to verify training intensity. Attendance will be documented for each session. UV exposure will be objectively tracked in all groups using UV sensors. Personalization: The training load is individually adjusted to each participant’s 95% HRpeak angle determined during the incremental wall squat test. Female participants will begin and end the intervention within the follicular phase of the menstrual cycle (days 2–5) to control for hormonal variation in vascular function. Location: All testing and training will be conducted at National Chung Cheng University, Taiwan. The outdoor intervention will occur in an open-field area within campus grounds, and the indoor interventions will take place in the university exercise physiology laboratory.