Background <p>Soluble Alpha-Klotho (S-αklotho) protein and 25-Hydroxyvitamin D (25-OH-D) have emerged as potential modulators for activating and recruiting Brown Adipose Tissue (BAT). The present study aimed to investigate whether circulating S-αklotho and 25-OH-D levels are related to BAT volume, ¹⁸Fluorine-Fluorodeoxyglucose (¹⁸F-FDG) uptake, and BAT radiodensity in young healthy adults.</p> Methods <p>A total of 128 participants (68% women) aged 18–25&#xa0;years old participated in this cross-sectional study. Serum levels of S-αklotho were determined by a solid-phase sandwich enzyme-linked immunosorbent assay kit and 25-OH-D serum levels were analyzed using a competitive chemiluminescence immunoassay, both in blood samples collected after an overnight fast. All participants underwent a personalized cold exposure to determine their BAT volume, ¹⁸F-FDG uptake, and radiodensity, using a static positron emission tomography combined with computed tomography scan.</p> Results <p>After adjusting for multiple covariates, serum levels of S-αklotho (all R² ≤ 0.228 and P ≥ 0.364), 25-OH-D as continuous (all R² ≤ 0.242 and P ≥ 0.088) or by vitamin D status (all R² ≤ 0.767 and P ≥ 0.061) were not associated with either BAT volume and ¹⁸F-FDG uptake, or BAT radiodensity.</p> Conclusion <p>Serum S-αklotho and 25-OH-D levels within the physiological range are not related to BAT-related variables in young healthy adults. Further studies are needed to fully understand the underlying mechanisms involved in BAT metabolism in humans. (ACTIBATE; ClinicalTrials.gov identifier:&#xa0;not applicable).</p>