Vitamin D metabolite ratio as a marker of nutritional bone health in adolescents.

Vitamin D deficiency is a widespread public health issue among Saudi adolescents, posing significant risks to bone health and long-term well-being. Traditional markers such as serum 25-hydroxyvitamin D [25(OH)D] may not fully capture functional vitamin D status, particularly during critical growth periods such as adolescence. This study aimed to evaluate vitamin D metabolite levels and their association with calcium intake and bone health markers in Saudi adolescents, with a focus on the vitamin D metabolite ratio (VMR) as a potential indicator of functional vitamin D sufficiency. A cross-sectional analysis was conducted involving 949 (54 % females) Saudi adolescents. Serum levels of vitamin D metabolites, including 25(OH)D; 24,25-dihydroxyvitamin D [24,25(OH)₂D], and 25(OH)D₂, were measured using liquid chromatography-tandem mass spectrometry (LC-MS/MS). VMR was calculated as the ratio of 24,25(OH)₂D to 25(OH)D. Dietary calcium intake was assessed via validated questionnaires. Parathyroid hormone (PTH) and serum calcium levels were analyzed in a subset of participants. Vitamin D deficiency was highly prevalent, with 86.5 % of participants showing insufficient 25(OH)D levels. Over 93 % had low 24,25(OH)₂D, 99.7 % had undetectable 25(OH)D₂, and 74.9 % exhibited low VMR. VMR was significantly associated with serum and dietary calcium intake among girls, but not boys, suggesting potential sex-specific metabolic differences. Additionally, only VMR showed a significant association with serum calcium. An inverse association between PTH and both total vitamin D (r = -0.28, p < 0.05) and serum calcium (r = -0.34, p < 0.01) was observed in the subset analysis. In conclusion, VMR may serve as a useful sex-specific biomarker of functional vitamin D status in adolescents. The findings highlight the importance of addressing both intake and metabolic processing of vitamin D to optimize bone health during adolescence.