Effects of consuming fish on vitamin D status and use of urinary vitamin D metabolite as a noninvasive biomarker

Physiological studies on the amount and duration of vitamin D intake are limited. Noninvasive biomarkers for vitamin D status also need to be explored. Thus, we conducted a dietary challenge study to assess vitamin D metabolism. This crossover study included 21 healthy Japanese individuals, with each test lasting 6 consecutive days. Participants consumed test meals during the study period for 5 days. The main dish was either fish (FD) or meat (MD), and the FD group was provided with at least 18 μg of vitamin D per day. Fasting blood samples and 24 h urine samples were collected on the first and last days. The mean serum 25(OH)D3 level were low before the intervention and changed only slightly after intervention, with higher levels in the FD group. Serum 25(OH)D3 demonstrated a strong positive correlation with serum 24,25(OH)2D3 but showed no correlation with urinary 25(OH)D3. Surprisingly, it had a relatively strong positive correlation with urinary 24,25(OH)2D3. In conclusion, in healthy Japanese, adequate intake of vitamin D from fish failed to increase serum 25(OH)D3 concentrations. On the other hand, noninvasive urinary24,25(OH)2D3 excretion may be used instead of serum 25(OH)D3 levels to assess vitamin D status. J. Med. Invest. 72 : 308-315, August, 2025.