Dose‐Dependent Effects of Dietary n‐3 Fatty Acids on Bowel Health: Plant‐Sourced <scp>ALA</scp> Modulates Diarrhea Risk While Marine‐Sourced <scp>DHA</scp> / <scp>EPA</scp> Prevent Constipation in <scp>NHANES</scp> 2005–2010

Although omega-3 polyunsaturated fatty acid (n-3PUFA) is generally considered to have positive effects on bowel health, the understanding of the impacts of n-3PUFA on bowel function is limited. This study aimed to deeply investigate the association between n-3PUFA, diarrhea, and constipation and discussed the role of different subclasses of n-3PUFA. 12,704 adults from the National Health and Nutrition Examination Survey (NHANES) 2005-2010 were collected in this study. Constipation and diarrhea were measured by bowel frequency and the Bristol Stool Form Scale (BSFS). Dietary n-3PUFA intake was collected by 24-h dietary interviews. Weighted logistic regression results showed that the highest quartile of n-3PUFA intake (OR: 0.70; 95% CI: 0.52-0.96) was associated with a reduced risk of constipation, and the third quartile of n-3PUFA (OR: 0.70; 95% CI: 0.56-0.87) was linked to a lower risk of diarrhea (compared to the lowest). According to restricted cubic spline (RCS) analysis, n-3PUFA showed a nonlinear association with diarrhea and a negative linear correlation with constipation. Moderate intake of n-3PUFA (1.38-2.25 g/day) was associated with a reduced risk of diarrhea. When the intake of n-3PUFA was more than 2.25 g per day, the risk of diarrhea increased. RCS-adjusted logistic regression indicated that the highest ALA quartile (vs. lowest) was associated with 31% lower odds of diarrhea (OR: 0.69; 95% CI: 0.57-0.85) with a U-shaped relationship. Conversely, higher DHA (OR: 0.60; 95% CI: 0.43-0.84) and EPA (OR: 0.70; 95% CI: 0.52-0.95) quartiles reduced constipation odds, showing negative linear (DHA) and nonlinear (EPA) associations. WQS regression quantified joint effects of n-3PUFA subclasses on constipation/diarrhea. For diarrhea, n-3PUFA contributions were: ALA 82.9%, EPA 16.5%, DHA 0.5%, DPA 0.1%; for constipation: DHA 75.8%, EPA 20.2%, ALA 3.0%, DPA 1.0%. Sensitivity analyses confirmed robust n-3PUFA and subclass associations with diarrhea/constipation. This large-scale study establishes differential impacts of n-3PUFA subclasses on bowel disorders, revealing ALA's U-shaped protection against diarrhea and DHA/EPA's linear constipation alleviation, providing evidence for targeted dietary interventions. However, the causal relationship still needs to be verified by prospective studies.