**Background:** Osteoporosis is a global health problem with a reported worldwide prevalence of 18.3%. In Iran, approximately 17% of the general population over 30 years have osteoporosis and about 35% have osteopenia. While biochemical factors are known to affect fracture risk, the association between serum 25(OH)D levels, dietary vitamin D intake, sun exposure, and bone mineral density (BMD) remains controversial. This study aimed to evaluate the association and correlation between these vitamin D-related factors and BMD.
**Methods:** A total of 186 individuals from Sirjan Gol Gohar Company staff were included in winter 2020. Exclusion criteria included pregnancy, lactation, diseases interfering with vitamin D metabolism (chronic pancreatitis, IBD, intestinal resection, hyperparathyroidism, renal failure, advanced liver failure, rheumatoid arthritis), calcium or vitamin D supplementation within specified periods, smoking >10 cigarettes/day, alcohol consumption >5 years and >1 glass/day, and drug addiction. BMD was measured at the lumbar spine (L1–4), femoral neck, and total hip using Dual-energy X-ray absorptiometry (Hologic Horizon WI, USA). Serum 25(OH)D, calcium, phosphorus, parathyroid hormone (PTH), and calcitonin were measured using standard laboratory methods (ELISA for 25(OH)D, photometry for calcium/phosphorus, chemiluminescence for PTH/calcitonin). Dietary intake was assessed using a semi-quantitative, validated Food Frequency Questionnaire completed by a nutritionist. Sun exposure was estimated using a validated questionnaire covering outdoor sunlight exposure duration, sunscreen use, and body parts exposed. Statistical analyses included independent sample t-tests, chi-square tests, bivariate and partial correlations (controlled for BMI, age, PTH, and calcitonin), and logistic regression models (crude and adjusted). Benjamini–Hochberg correction was applied to all p-values.
**Key Results:** There was no significant difference between the normal BMD group and the osteopenia/osteoporosis group in baseline anthropometric, socioeconomic, or serum indicators. Participants with osteopenia/osteoporosis consumed significantly higher amounts of soluble fiber than the normal BMD group (0.26 ± 0.18 g vs. 0.16 ± 0.09 g; p = 0.043). In partial correlation models controlled for BMI, age, PTH, and calcitonin, there was a significant moderate correlation between serum 25(OH)D and spine measurements: Spine BMD (Pearson r = 0.302, p = 0.046), Spine T-score (r = 0.322, p = 0.033), and Spine Z-score (r = 0.328, p = 0.030). No significant correlation was found between serum 25(OH)D and femoral neck or total hip BMD. There was no significant correlation between dietary vitamin D intake and any BMD measurement. In bivariate models, sun exposure showed a significant negative correlation with Spine BMD (r = -0.355, p = 0.017) and BMD diagnosis (r = -0.326, p = 0.029). In logistic regression crude models, sun exposure was significantly associated with a protective effect for spine BMD (OR = 0.69, 95% CI: 0.50–0.94; p = 0.023) and BMD diagnosis (OR = 0.69, 95% CI: 0.49–0.87; p = 0.036). In adjusted models, serum 25(OH)D was significantly associated with spine BMD (OR = 0.92, 95% CI: 0.86–0.99; p = 0.025), and sun exposure remained significantly associated with BMD diagnosis (OR = 0.51, 95% CI: 0.24–0.98; p = 0.049). No significant association was found between dietary vitamin D intake and BMD in any model.
**Clinical Implications:** This study provides evidence that serum 25(OH)D levels and sun exposure are correlated with spine BMD, while dietary vitamin D intake alone showed no significant association. The findings suggest that clinical assessment of vitamin D status for bone health should consider both serum levels and sunlight exposure rather than relying solely on dietary intake estimates. The protective association between sun exposure and BMD diagnosis (OR 0.51–0.69) highlights the potential importance of adequate sunlight exposure for osteoporosis prevention. However, the study's limitations include a small sample size, non-representative sampling (company staff, predominantly male), and the retrospective design with potential recall bias. The COVID-19 pandemic also affected sample size due to quarantine restrictions. Prospective studies with larger, more representative samples are needed to confirm these findings and elucidate the mechanisms linking vitamin D status components to bone health.