2.7. Vitamin D Measurement
Serum 25(OH)D levels were assessed annually, using radioimmunoassay (SRL Inc., Hachioji, Tokyo, Japan), starting from the initiation of supplementation and continuing every year thereafter during the same calendar month for up to 5 years. To ensure accuracy, blinded duplicate tests for 25(OH)D were conducted using 19 serum samples from a separate cohort, demonstrating a correlation coefficient of 0.92, as previously described [