To our knowledge, our study is the first to explore the associations between energy-adjusted fiber intake and AA hemoglobin biomarkers among the general population. Our results indicated that dietary fiber intake displayed an inverse and nonlinear relationship with the blood concentration levels of AA hemoglobin biomarkers.
We have observed J-shaped relationship between dietary fiber intake and AA hemoglobin biomarkers. The regression splines suggested a possible threshold effect for dietary fiber intake of approximately 20g/day on AA hemoglobin biomarkers. On the left side of threshold effect point, dietary fiber intake showed a linear and inverse relationship with the concentrations of AA hemoglobin biomarkers and the effect became flat on the right side of threshold effect point. In addition, our results showed a significant interaction between dietary fiber intake and obesity status in relation to the concentrations of AA hemoglobin biomarkers. The associations between dietary fiber intake and AA hemoglobin biomarkers were less significant among obese adults compared with non-obese adults.
There have been no previous studies investigating dietary fiber intake that modifies blood concentration levels of AA hemoglobin biomarkers in general populations. Our results are consistent with previous data reporting the inverse association of fiber intake with environmental pollutants such as perfluoroalkyl acids (Dzierlenga et al. 2021) and isopentanaldehyde (Shi et al. 2022). In the study conducted by Dzierlenga et al., dietary fiber was associated with lower serum concentrations of perfluoroalkyl substances due to increased gastrointestinal excretion. Additionally, in the study conducted by Shi et al., dietary fiber intake was inversely associated with blood levels of isopentanaldehyde and propionaldehyde. Our results add information of interest to the available research on this topic.
The underlying mechanism of dietary fiber intake-induced decreases in blood concentration levels of AA hemoglobin biomarkers remains unknown. Possible mechanisms are as follows. First, diet was considered the major source of exposure to AA, though insoluble dietary fiber helped to reduce the absorption of AA because it increased the volume of feces and decreased the transit time in the intestine. Second, epidemiological study showed that acrylamide exposure was positively associated with systemic inflammatory mediator plasma C-reactive protein. Animal experiments demonstrated that dietary fiber mediated oxidation by activating the Acly/Nrf2/NF-κB signaling pathway, which exerted anti-inflammatory effects by inhibiting the differentiation of macrophages into pro-inflammatory M1 macrophages (Shao et al. 2022). Thus, it is plausible to consider that the pathogenesis of dietary fiber may be related to decreased concentrations of AA hemoglobin biomarkers’s through its anti-inflammatory effects. Third, the liver is in the first line to help our body defense against harmful substances outside including environmental pollutants. A recent study clarified the inverse associations of dietary fiber intake with nonalcoholic fatty liver disease (NAFLD) (Zhao et al. 2020). NAFLD patients are likely to have decreased dietary fiber intake and impaired liver function, while the liver plays a crucial role in AA metabolism. Finally, intestinal microorganisms are fermented by dietary fiber to produce short-chain fatty acids, which can improve liver metabolism (Lundin et al. 2004), thus increasing AA metabolism.
There are some limitations in our study. First, we cannot infer causal relationships directly and which factor came first because of the cross-sectional nature of this study. Second, dietary fiber intakes were obtained from two 24 h dietary recall interviews at an interval of 3 to 10 days, while AA hemoglobin biomarkers can reflect the exposure level within 120 days (Bergmark et al. 1993). Dietary fiber intake evaluated over 2 days may not be long enough to estimate the cumulative effect of fiber intake on AA exposure. Third, the dietary data of fiber intake were obtained from two 24 h dietary recall interviews, and ineluctable recall bias may exist. Finally, although we try to control a series of confounding factors, unmeasured confounding factors may also play a role.
4.Conclusion
Dietary fiber intake was inversely and nonlinearly associated with environmental exposure to AA. The possible obesity-specific effect of dietary fiber intake on AA hemoglobin biomarkers was suggested in our study. Our results also suggested that high-fiber diets are recommended for populations exposed to elevated AA levels. However, further studies should be employed to elucidate these relationships.