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

Abstract

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.