Asian Journal of Pediatric Research

Background: Vitamin D is traditionally characterized as a nutrient essential for calcium homeostasis. However, emerging evidence suggests that the cutaneous CYP11A1-driven secosteroid pathway serves as a "chemical proxy" for sunlight, linking environmental UV radiation to the synchronization of the mammalian circadian clock and systemic endocrine axes.

Objective: This review delineates the role of CYP11A1-derived vitamin D metabolites [e.g., 20[OH]D₃ and 20,23[OH]₂D₃] as photoneuroendocrine signals that modulate core clock gene expression and regulate the hypothalamic-pituitary-adrenal [HPA], hypothalamic-pituitary-thyroid [HPT], and growth hormone [GH] axes.

Methods: A systematic synthesis was performed following PRISMA 2020 guidelines using PubMed and Scopus. We integrated enzymological data on CYP11A1 substrate flexibility with molecular evidence regarding nuclear receptor interactions [VDR, RORα/γ, AhR] and clinical observations of hormonal rhythmicity. Bioinformatic centralities were assessed via ClusterProfiler and cytoHubba.

Results: CYP11A1 initiates a non-canonical pathway in the skin, metabolizing vitamin D₃, lumisterol, and tachysterol into non-calcemic hydroxyderivatives. These metabolites function as inverse agonists of the orphan nuclear receptors RORα and RORγ, which directly suppress BMAL1 transcription and modulate the amplitude of the peripheral circadian oscillator. Mechanistically, this axis interacts with the NAMPT–NAD⁺ metabolic loop, coupling photic cues to mitochondrial oxidative metabolism. Clinical evidence indicates that while serum 25[OH]D₃ acts as a tonic nutritional marker, cutaneous secosteroids provide phasic signals that stabilize the pulsatile secretion of GH and the rhythmicity of TSH and cortisol. Vitamin D deficiency is thus re-characterized as a state that permits the attenuation of endocrine amplitude rather than causing primary hormone deficiency. The key mechanistic evidence is largely preclinical and that clinical implications remain investigational.

Conclusions: The CYP11A1-secosteroid axis represents a paradigm shift from nutrition to photoneuroendocrinology. These non-calcemic metabolites act as endogenous chronobiotics that align systemic physiology with the environmental light-dark cycle.

Clinical Implications: These findings support the use of non-calcemic vitamin D analogs as targeted chronotherapeutics for circadian misalignment, metabolic syndrome, and autoimmune disorders. In pediatric populations, maintaining adequate sunlight exposure and secosteroid signaling is critical for the stability of growth and stress-response axes.