2.6. Sample Size
We calculated sample sizes based on the hypotheses of achieving (1) higher serum 25OHD; and (2) lower LDL levels in the intervention group compared to the control group. Assuming medium effect sizes of 0.4 standard deviation (SD) differences in serum 25OHD (with a SD of 15 nmol/L) and LDL levels (with a SD of 0.7 mmol/L) between the intervention and control groups, we required a sample size of 113 per group, ensuring at least 80% power and with two-sided 5% type 1 error rate. Considering 25% dropout rate, we aimed to recruit a total of 300 participants, with 150 per group.