To the best of our knowledge, we were the first to observe significant correlations between energy-adjusted dietary fiber and various serum aldehyde exposures. Dietary fiber was negatively associated with isopentanaldehyde and propanaldehyde but positively associated with benzaldehyde. Nonetheless, in the other 3 aldehydes, there was no significant trend toward an association with dietary fiber.
Previous studies have mainly focused on the involvement of aldehydes in human disease. Nonetheless, no experimental research has shown an effective approach to regulating these 6 aldehyde concentrations in the human body. According to a recent study, no dietary variables were associated with serum isopentanaldehyde and propanaldehyde concentrations (Silva et al. 2021). Our findings seem to contradict this. However, the study provided strict food consumption information from different sources, such as vegetable, fruit, legume, nut, and seed grain products. In contrast, our study provides a holistic analysis of dietary fiber and serum aldehyde. Therefore, this regulatory effect may be related to the increase in the diversity of dietary fiber intake sources, further suggesting a combination of dietary fiber from multiple sources to modulate serum aldehyde. In addition, our study supplemented the effect of dietary fiber on serum benzaldehyde that was not present in the abovementioned study.
The mechanisms by which dietary fiber affects the serum concentration of aldehydes may include the following aspects. First, dietary fiber reduces oxidative stress and inflammation by regulating beneficial metabolites and toxic molecules and increasing the metabolism of aldehyde substances. A previous study showed that dietary fiber reduces the accumulation of protein-bound uremia toxin (PBUT) in children with multiple chronic kidney diseases as renal function deteriorates, and a dose-effect relationship was observed between the potential benefit and dietary fiber (El Amouri et al. 2021). A randomized controlled trial also verified that dietary fiber increased the plasma concentrations of total alkylresorcinol (AR) (Donin et al. 2021), which has potential antioxidant effects, reduced the risk of hyperglycemia, and exerted neuroprotective effects on hippocampal neurons (Fan et al. 2020; Tryggvadottir et al. 2021). Previous studies have also shown that aldehydes are associated with tissue and cell damage caused by oxidative stress (Kuntic et al. 2020; Lin et al. 2021; Shafie et al. 2021; Vivarelli et al. 2021; Yusuf et al. 2020). Additionally, studies at the animal and human levels have shown that dietary fiber exerts an anti-inflammatory effect by presenting fiber-specific changes in their microbiomes, regulating the composition of the gut microbiome and increasing the production of microbiome-derived metabolites (Delannoy-Bruno et al. 2021; Tian et al. 2021).
Isopentanaldehyde is a methyl butyraldehyde formed by replacing the methyl group at the third position of butyraldehyde. According to the Human Metabolome Database, it has been shown to be associated with a substantial number of digestive disorders, such as ulcerative colitis, nonalcoholic fatty liver disease, and Crohn's disease (http://www.hmdb.ca/metabolites/HMDB0006478). These diseases have been shown to be ameliorated by adding dietary fiber intake (Ananthakrishnan et al. 2013; Zhao et al.2020). Therefore, this reflects that digestive disease conditions may be regulated by reduced dietary fiber, which may in turn reduce the absorption of isopentanaldehyde. Moreover, the specific mechanism has not been studied and is worth further exploration.
Propanaldehyde is commonly used in the manufacture of plastics and synthetic rubber chemicals and as a disinfectant and preservative. It can be absorbed into the body by inhaling its vapor and ingestion. Propanaldehyde, as a perinatal air poison, was positively associated with autism spectrum disorder (ASD) in high-risk families (Kalkbrenner et al. 2018). Environmental propanaldehyde is usually biodegraded to propionic acid, which is then further degraded to carbon dioxide and water (Urano and Kato 1986). Considering its unknown degradation pathway and harm in vivo, this study provides evidence for high-risk patients to increase dietary fiber intake.
The increased concentration of benzaldehyde with dietary fiber intake may be because benzaldehyde, as the second most useful flavoring agent in foods, is used in the postprocessing of dietary fiber (Duff and Murray 1989). The second reason is that the microbial biocatalyst can produce benzaldehyde (Jain et al.). Pichia pastoris can effectively convert benzyl alcohol to benzaldehyde through biotransformation (Duff and Murray 1989). Benzaldehyde can also be catalyzed to form phenylalanine in the presence of a cell extract of Lactobacillus plantarum (Nierop Groot and de Bont 1998); however, the conversion rate of benzaldehyde to benzyl alcohol is reduced in Escherichia coli expressing recombinant carboxylate reductase (Kunjapur et al. 2014). These possible paths may lead to a positive association between benzaldehyde and dietary fiber intake.
The limitations of this study include the following aspects. First, this cross-sectional study was designed to describe the association; hence, it cannot conclude and further explain causality. Second, given that this population is an American population, dietary habits exert regional differences. Moreover, due to the lack of potential bowel disturbances, we could not include it as one possible intermediate link. Further research is needed to determine whether these results can be extrapolated to other regions or other ethnicities.