The effect of tea polyphenols (TPLs), specifically tea catechins, on the postprandial glycemic response to cooked starches differing in amylose contents was investigated. The in vivo test using a mouse model showed a moderate reduction of the postprandial glycemic response to co-cooked normal (containing 27.8% amylose) or waxy corn starch with 10% TPLs (dry weight of starch), while an augmented glycemic response with a delayed blood glucose peak was observed when high amylose corn starch (HAC, containing 79.4% amylose) was used as the starch component. Enzyme kinetics results demonstrated that TPLs noncompetitively inhibit the digestion of waxy or normal corn starch, while the digestion rate of HAC starch was increased in the presence of TPLs, which supports the observed postprandial glycemic responses. Further studies using X-ray powder diffraction showed that the diffraction intensity (area under the diffraction curves) of normal and HAC starch was increased by 45% and 74%, respectively, whereas no change was observed for waxy corn starch. Consistently, dynamic laser light scattering studies using a solution of pure amylose showed an increased hydrodynamic radius of amylose molecules from ∼54 nm to ∼112 nm in the presence of TPLs. These experimental results indicate that there might exist an interaction between TPLs and amylose, which facilitates the association of amylose molecules to form a special nonordered structure that can produce a high and sustained postprandial glycemic response. Thus, a combination of tea polyphenols and specific starches could be used to manipulate postprandial glycemic response for glycemic control and optimal health.

The in vivo slow digestion property of octenyl succinic anhydride modified waxy corn starch (OSA-starch) in the presence of tea polyphenols (TPLs) was studied. Using a mouse model, the experimental results showed an extended and moderate postprandial glycemic response with a delayed and significantly decreased blood glucose peak of OSA-starch after cocooking with TPLs (5% starch weight base). Further studies revealed an increased hydrodynamic radius of OSA-starch molecules indicating an interaction between OSA-starch and TPLs. Additionally, decreased gelatinization temperature and enthalpy and reduced viscosity and emulsifiability of OSA-starch support their possible complexation to form a spherical OSA-starch-TPLs (OSAT) complex. The moderate and extended postprandial glycemic response is likely caused by decreased activity of mucosal α-glucosidase, which is noncompetitively inhibited by tea catechins released from the complex during digestion. Meanwhile, a significant decrease of malondialdehyde (MDA) and increased DPPH free radical scavenging activity in small intestine tissue demonstrated the antioxidative functional property of the OSAT complex. Thus, the complex of OSAT, acting as a functional carbohydrate material, not only leads to a flattened and prolonged glycemic response but also reduces the oxidative stress, which might be beneficial to health.

Slowly digestible starch (SDS) offers a range of health benefits due to its stabilizing and sustaining effect on blood glucose. Contributors to the slow digestion property of certain starches include their specific structure and their interaction with factors that inhibit effective digestion. In this study, the effects of tea polyphenols (TPLs) on the enzyme activities of pancreatic a-amylase and amyloglucosidase, which are used during starch digestion, were investigated by an in vitro Englyst test. Our results showed that TPLs noncompetitively inhibit the activity of pancreatic a-amylase (pH 6.9) but activate amyloglucosidase (pH 4.6) in a dose-dependent manner. However, when the two enzymes were employed together under the Englyst test condition at pH 5.2, a heterogeneous inhibition pattern was observed with a peak inhibition of $30% when the TPL content was $2.5 mg/mL, and a negative inhibition pattern when TPL content was greater than 4 mg/mL. In contrast, a similar degree of starch digestibility was observed in the presence of TPLs, which indicates that a modification of the Englyst assay warrants further studies to reflect the in vivo starch-digestion patterns, where a slower digestion of starch was observed in the presence of TPLs, which is corroborated by in vivo studies from the literature.