Association between Vitamin D Deficiency, Obesity, and Menstrual Irregularities among Women of Reproductive Age: A Cross-Sectional Analysis of NHANES Data

Abstract

Background: Menstrual irregularities can reflect metabolic and endocrine dysfunction, yet the independent roles of vitamin D deficiency and obesity remain uncertain. This study examined their separate and joint associations with a severe menstrual-irregularity phenotype among U.S. women of reproductive age.
Methods: A cross-sectional analysis was conducted using August 2021-August 2023 National Health and Nutrition Examination Survey data. Women aged 18-49 years with valid reproductive-health, serum 25-hydroxyvitamin D [25(OH)D], body mass index (BMI), and survey-design data were included. The outcome identified women reporting no menstrual period during the previous 12 months for a non-pregnancy, non-lactation, nonsurgical, and non-menopausal reason. Vitamin D deficiency was defined as 25(OH)D <50 nmol/L and obesity as BMI >=30 kg/m². Survey-weighted logistic regression estimated crude and adjusted odds ratios (ORs).
Results: Among 980 women, 69 met the outcome definition; weighted prevalence was 6.1% (95% CI, 4.3%-8.0%). Obesity was associated with higher crude odds (OR, 1.61; 95% CI, 0.97-2.67), but the adjusted association was attenuated and imprecise (OR, 1.34; 95% CI, 0.79-2.27). Vitamin D deficiency was not associated with higher odds after adjustment (OR, 0.67; 95% CI, 0.26-1.73). Joint exposure to obesity and vitamin D deficiency did not increase odds, and no multiplicative interaction was detected. Women with obesity nevertheless had substantially lower mean 25(OH)D concentrations.
Conclusion: Vitamin D deficiency was not independently associated with this severe menstrual phenotype. Obesity showed a directionally positive but statistically inconclusive association, while the coexistence of obesity and vitamin D deficiency did not confer additional odds. Prospective studies using detailed menstrual-cycle phenotyping are needed.