Non-linear relationship between Dietary Inflammatory Index and constipation: threshold identification and insights from NHANES [2005-2010] and Mendelian randomization analysis.

Background: With the acceleration of the pace of life and changes in dietary habits, functional gastrointestinal disorders, especially constipation, have become a significant public health issue affecting health and quality of life of people worldwide. Given the limitations of traditional treatments, adjusting dietary structure has become a more economical and convenient therapeutic approach. We aimed to explore the associations between the Dietary Inflammatory Index (DII) and constipation in this study.

Methods: This study was based on the National Health and Nutrition Examination Survey (NHANES) data from 2005-2010, utilizing bowel movement frequency and stool characteristics questionnaires to determine constipation status, and calculating the DII based on 24-hour dietary recall data. To assess the relationship between the DII and constipation, we employed three models, which were further explored through inverse probability of treatment weighting (IPTW), restricted cubic splines (RCS) analysis, and Mendelian randomization.

Results: Individuals with a higher DII exhibited a higher risk of constipation. In the unadjusted model, participants in the highest quartile (Q4) had a 2.85-fold increased risk of constipation compared to those in the lowest quartile (Q1) [odds ratio (OR): 2.85; 95% confidence interval (CI): 1.78-4.56; P<0.001], with similar results observed in various adjusted models and IPTW adjusted models. RCS analysis revealed a nonlinear relationship between the DII and constipation, with a threshold value (DII =0.974) beyond which the risk of constipation significantly increased. Subgroup analysis showed that gender, income level, and diabetes status affected the relationship between the DII and constipation. Mendelian randomization analysis did not find any significant causal relationships for components of the DII, except for energy intake.

Conclusions: There is a nonlinear relationship between the DII and the risk of constipation, with a threshold value of 0.974, and differences in the risk of constipation associated with the DII across different income, gender, and diabetes status groups. These findings provide a basis for using the DII as a strategy for the prevention and intervention of constipation.