Association Between Tobacco Exposure and Serum 25-Hydroxyvitamin D Concentrations in Adults Aged 20–40 Years: A Cross-Sectional Study

The vitamin D status depends on a variety of behavioral, environmental, and biological factors. The observational studies suggested that cigarette smoking is associated with reduced circulating 25-hydroxyvitamin D [25(OH)D]. Objective: This study aimed to determine the relationship between cigarette smoking status and serum 25(OH)D concentrations, vitamin D status categories among adults aged 20-40 years. Methods: Cross-sectional comparative study was conducted between 1 February to 1 July 2026 at Tikrit Teaching Hospital, Iraq. One hundred participants were recruited consisting of 50 current smokers and 50 non-smokers. About 3 ml venous blood sample was taken, allowed to clot and centrifuged at 3000 rpm for 10 minutes. 25(OH)D concentration was determined in hospital laboratory using Roche Cobas e 411 analyzer with final results were provided to the research team. Frequency distribution of categorical variables was reported. Due to the absence of individual 25(OH)D levels data in the revised dataset, no continuous data analysis between groups was done. Results: Study population included 82 (82%) males and 18 (18%) females. Among smokers, 29 (58%) were aged 30-40 years and 21 (42%) were aged 20-29 years. Low vitamin D status was found in 87 participants (87%): all 50 smokers and 37 (74%) non-smokers. Among non-smokers, 9 (18%) had normal and 4 (8%) had high vitamin D status; there was no smokers having normal or high vitamin D status. Reported Pearson Chi-square statistic value for smoking status and vitamin D category was χ²=14.943 (p=0.001); exact test based on aggregated data showed p≈0.00010. Conclusion: Current smoking status in our population was related to increased prevalence of low vitamin D status. Limitations of the study include cross-sectional design, small sample size, different age distribution, insufficient description of smoking exposure, and unadjusted confounders.