Intragastric gelation of mixed soy protein isolate and alginate as well as its effect on postprandial glucose response and satiety
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The goal of the study is to investigate the effect of alginate on sucrose release and in vitro gastric digestion of soy protein isolate (SPI) in model beverages as well as to determine whether consumption of the model beverages would affect postprandial blood glucose response and appetite in healthy adults. Model beverages containing 5% w/v SPI, 0 to 0.20% w/v alginate and 10% w/v sucrose were prepared by heating the mixtures at 85 ᵒC for 30 min at pH 6.0 or 7.0. Characterizations of beverages included determination of zeta-potential, particle size analysis and rheological properties measurement. Digestion patterns and sucrose release were determined after 0 to 2 h in-vitro gastric digestion using SDS-PAGE and HPLC analysis, respectively. Results showed that increasing alginate concentration led to increased zeta-potential value, particle size as well as increased viscosity and pseudoplastic behavior; however, no phase separation was observed in any of the samples. In the absence of alginate, the SPI beverage could form a weak intragastric gel only at a pH of 6.0 after mixing with simulated gastric fluid (SGF), while at pH 7.0 a gel was formed only in the presence of alginate. Formation of the intragastric gel led to delayed protein digestion and slower release of sucrose. Higher resistance to digestion and a slower sucrose release rate were exhibited at increased alginate concentration, and to alesser extent, at pH 6.0. This suggests that electrostatic interaction between SPI and alginate that occurred when the beverages were under gastric condition could be responsible for the intragastric gelation. The hypothesis that beverages showing intragastric gel formation in the in vitro study could be translated into in vivo applications was tested in a clinical trial. In the clinical trial, after an overnight fast, twelve healthy subjects were asked to consume six standardized breakfast beverages in a randomized order: a 122 kcal sugar beverage (CONT), a 122 kcal sugar beverage with alginate (ALG), a 172 kcal sugar beverage with SPI at pH 7 (SPI-7) or pH 6 (SPI-6), a 172 kcal sugar beverage with mixed SPI and alginate at a pH 7 (SPI+ALG-7) or pH 6 (SPI+ALG-6). Subjects consumed one of the beverages at time 0. Blood samples were drawn at -15, 0, 15, 30, 45, 60, 90 and 120 min and questionnaires were completed immediately following the blood draw at each time point. Results showed that, compared to CONT, consumption of SPI-6, SPI+ALG-7 and SPI+ALG-6 significantly lowered peak blood glucose concentration and 1-h incremental area under the curve (AUC). SPI+ALG-6 also exhibited a significant reduction in 2-h AUC. No significant effect on appetite was found in any condition. Interactions between the protein and alginate during digestion and formation of an intragastric gel could play an important role in influencing postprandial blood glucose response. In conclusion, we demonstrated the possible formation of an intragastric gel resulted from the SPI and alginate mixture under certain conditions, which subsequently delayed protein digestion and sucrose release from the matrix. Compared with CONT, consumption of beverages that formed an intragastric gel (SPI+ALG-7 and SPI+ALG-6) attenuated the postprandial glycemic concentration in healthy adult subjects. These results could potentially lead to the formulation of SPI beverage with functionality to lower postprandial glycemic response.