RESUMEN
BACKGROUND AND AIMS: A common practice in the assessment of vitamin D is to measure its blood level in the morning after overnight fasting. The aim of this study was to measure vitamin D during different times of the day and night for a random sample of healthy individuals, to see if there are significant changes throughout the day and night. METHODS: A cross sectional study was carried out on a total of 52 randomly selected Jordanian healthy volunteers (26 women and 26 men) aged between 18 and 45 years. Six blood samples were taken from each participant on the same study day; 7:30 AM (at fasting), 10:30 AM, 1:30 PM, 4:30 PM, 7:30 PM and at 10:30 PM. An extra 7th blood sample was taken next morning at 7:30 AM (after fasting overnight). RESULTS: There was a significant difference (P < 0.05) between mean serum vitamin D level at 7:30 AM (19.64 ± 0.26 ng/ml) and 1:30 PM (20.60 ± 0.26 ng/ml), and 4:30 PM (20.61 ± 0.26 ng/ml), P-value 0.0096 and 0.0090, respectively. When taken into consideration the effect of exposure to the sun and the time, there was a significant difference (P Ë 0.05) between the two groups (group 1: sun protected group, and group 2: sun exposure group) at different times on the same day and the morning of the second day (P-value = 0.0113). CONCLUSION: The serum vitamin D concentrations vary during different times of the day, night and next morning, and that its concentration at fasting does not represent its true value.