Un Ménage à Trois, Understanding the Multifaceted Interactions Between Polysaccharides, Phenolics and Digestive Enzymes

Abstract

The subject of this doctoral thesis presents an exploratory study into the complex intricate and multifaceted interactions between bioactive compounds specifically, phenolic compounds and polysaccharides (comprising dietary fibres and starches) in conjunction with digestive enzymes. An enduring paradox exists between the outcomes of in-vitro experimentation and clinical evidence regarding the consumption of nutritionally rich bioactive compounds. The principal motivation of this research is to bridge this gap by emphasising the critical importance of considering three-way interactions between phenolics, polysaccharides, and digestive enzymes, instead of focusing solely on pairwise relationships, as has been done in prior research.

The investigation spans five chapters, concluding in the sixth chapter. With there being a significant lack of literature in this field of study, this thesis explored multiple new ways to study three-way interactions between phenolics, polysaccharides, and digestive enzymes, by means of observing multiple potential consequences rather than focusing on any one very specific aspect (on fibre viscosity, on starch viscosity, on enzyme activity, on complexation, etc). These chapters collectively furnish a comprehensive understanding of how these bioactive components could potentially intermingle within the dynamic context of the human digestive system, spanning the entire spectrum from the preparation of dietary constituents to their eventual consumption and subsequent digestion. The findings emerging from this study are anticipated to serve as a stepping stone in the reconciliation of divergent in-vitro and in-vivo observations, further furnishing invaluable insights into the intricate web of dietary interactions, their impact on the digestion process, and the broader implications they hold for human nutrition. Chapter 2 provides an in-depth analysis of the interactions between phenolic compounds and digestive enzymes, particularly α-amylases and starch, using tea phenolics as a representative model. The study focuses on understanding binding kinetics and the intricate mechanisms of phenolic amylase inhibition. The outcomes elucidate the distinct binding kinetics exhibited by various phenolic compounds, with structural dissimilarities emerging as a decisive factor in these interactions (phenolics with terminal galloyl moieties showing greater inhibitory capacity). Additionally, this chapter highlights the pivotal role of incubation times when considering the interactions between phenolics and digestive enzymes, as prolonged incubation periods are shown to result in heightened enzyme inhibition levels, an important observation to consider when dealing with less potent α-amylase inhibitors. Notably, the chapter underscores the introduction of a competitive dimension by the presence of starch, necessitating a nuanced approach to understanding these intricate dynamics.

Chapter 3 explores the interplay between potato starch, phenolic compounds from potatoes, and pancreatic lipase. The research delves into an alternative and simplified perspective to consider a static system in which the digestive enzyme doesn't act on one of the components (starches) that is both its substrate and a sequester for phenolic compounds. Lipase is a suitable example in this context, as it primarily works on lipids and not starches, thus starch would maintain its supramolecular structure throughout the assay acting as a dietary fibre. The study further uncovers distinct inhibitory strengths among various phenolic compounds derived from potato starch, shedding light on the influence of both structural differences and solubility. Moreover, the chapter highlights the influence of starch on phenolic inhibition of lipase activity, emphasising the importance of context-specific assessments for understanding these complex interactions.

Chapter 4 focuses on interactions between gallic acid and different starch types, including potato, maize, wheat, and rice, delving into their effects on α-amylase activity and starch digestion. The experimental design modulates the timing of gallic acid introduction, thereby promoting either the formation of starch-GA complexes or competition between starch and enzymes for phenolic binding. The findings indicate that physical effects, such as diffusion kinetics and entrapment, play a role in gallic acid's inhibitory capacity, underscoring the significance of the microenvironment.

The final research chapter, Chapter 5, provides insights into the interactions between phenolics, polysaccharides (including dietary fibres and starches), and digestive enzymes, using oat bran as a food model. The chapter explores the evolution of digestive viscosity development and reveals a significant reduction in viscosity during digestion in the presence of phenolics. This reduction is attributed to both the inhibition of digestive enzymes and the formation of aggregates between phenolics and β-glucans. Moreover, molecular weight disparities among phenolic compounds emerge as a pivotal determinant in shaping these outcomes, further underscoring their profound relevance in this intricate narrative.

In summary, this doctoral thesis enriches our understanding of the intricate interplay between phenolic compounds, polysaccharides, and digestive enzymes, emphasising the necessity of context-specific assessments, considering structural characteristics, incubation times, pH levels, and the immediate microenvironment. The work done here is an intentional step towards a broader, more integrative perspective that recognises the value of multi-way interactions. These findings offer a more comprehensive perspective on the dynamics governing nutritional outcomes in diverse dietary scenarios and underscore the significance of considering three-way interactions to enhance the reliability and reproducibility of experimental results and help in bridging the gap between in-vitro experimentations and in-vivo observations.