Bioaccessibility and Bioavailability of Pigmented Cereal Polyphenols

Polyphenols from pigmented cereal grains are believed to exert various therapeutic effects, including antioxidant and anti-inflammatory properties. However, their role in promoting these health benefits is controversial due to reports of low bioavailability in the body. This thesis was aimed to provide novel insights into the bioavailability of pigmented cereal polyphenols by examining their release, stability, bioactivity, absorption and excretion following gastrointestinal digestion.
The study first investigated the release (bioaccessibility) and antioxidant activity of phenolic compounds subject to gastrointestinal digestion. Pigmented cereal samples used in this study included Oryza sativa (purple rice), Hordeum vulgare (purple barley), Triticum turgidum (purple wheat), and Triticum aestivum (blue wheat) flour. Digestion was simulated in vitro using commercially available gastric and intestinal enzymes. Phenolic profiling and antioxidant activity testing were performed using benchtop assays, mass spectrometry, and liquid chromatography. Purple rice was found to exhibit the highest level of total antioxidant activity, compared to pigmented barley and wheat. Simulated gastric digestion of flour samples resulted in an increase in the bioaccessibility of radical scavenging compounds in purple rice (anthocyanins) and purple barley (flavan-3-ols), however, these compounds were not detected post intestinal digestion. Flavones were the most bioaccessible, predominantly the ones found in wheat samples.
To evaluate the digestive stability and potential absorption of individual compounds during digestion, extracts were subjected to in vitro digestion and the human colon adenocarcinoma (Caco-2) cellular transport system. The study revealed that much of the antioxidant activity was retained after intestinal digestion, although some polyphenols appeared to have been broken down. Some of the phenolic acids and flavonoids extracted from the cereals were transported across Caco-2 cell membranes suggesting potential bioavailability.
Purple rice and purple barley extracts, which retained greater phenolic content and antioxidant activity after digestion, were selected to evaluate their potential to promote intestinal health and alleviate allergic inflammation. The study employed assays using Caco-2 and rat basophilic leukemia (RBL-2H3) as biological models of the intestinal epithelium and mast cells respectively. Both digested and undigested extracts attenuated oxidative stress-induced Caco-2 cell death and IL-8 secretion. Additionally, both digested and undigested extracts showed the potential to alleviate allergic inflammation by exhibiting time-dependent antihistamine effects and dose-dependent anti-inflammatory
effects in RBL-2H3 cells.
To assess the bioavailability of cereal polyphenols, a human crossover dietary intervention trial was conducted. Healthy volunteers consumed whole grain cereal porridges (purple barley, purple wheat, blue wheat and regular whole wheat) and the phenolic content of biological samples were analysed. The study identified phenolic compounds, including protocatechuic, caffeic acid and hippuric acid in the plasma and urine. Interpersonal variability and the type of grain consumed influenced the absorption and excretion of phenolic acids. Polyphenol bioavailability following pigmented grain consumption was found to be less than 6%, suggesting that a substantial fraction remained unabsorbed. The outcome of this study highlighted the complex dynamics of cereal polyphenol bioavailability.
Despite the limited in vivo bioavailability, this research highlights the potential of pigmented cereal grains as a source of bioaccessible, health-promoting polyphenols. These findings support the development of pigmented cereal varieties and functional food ingredients that can contribute to improved gut health and potentially offer other health benefits through their antioxidant and anti-inflammatory properties.