The Impact of Peeling on Highland Barley’s Digestive Properties: In Vitro and In Vivo Insights

Highland barley is a low-glycemic-index grain, and its slow-digesting starch can help delay or prevent the onset and progression of type Ⅱ diabetes. Peeling processes can alter the composition of highland barley, potentially changing its digestive properties. This study explored how changes in nutritional components due to different peeling times (zero, one, two, and three times) affected the digestion and absorption of barley during the gastric and intestinal phases and the utilization of undigested substrates at the distal end of the digestive tract, as well as their impact on fasting blood glucose regulation. The findings indicated that highland barley with fewer peeling times, which retained its outer layer that is rich in dietary fiber, protein, and polyphenols, delayed starch digestion and exhibited better hypoglycemic effects. Compared to unpeeled highland barley, the starch digestion rates of highland barley with one, two, and three peeling times increased by 2.82%, 18.62%, and 26.43% (<i>p</i> < 0.05). Based on microstructure, at the same enzymatic digestion time, starch with fewer peeling times retained a more intact granule structure. In mice with dysregulated glucose and lipid metabolism induced by the HFD/STZ method, highland barley with fewer peeling times exhibited a stronger hypoglycemic effect (6.13 mmol/L and 6.07 mmol/L). Additionally, the highland barley dietary intervention improved the gut microbiota composition in these mice, restoring the <i>Firmicutes/Bacteroidetes</i> ratio balance and enriching various probiotics, such as <i>Akkermansia</i> and <i>Lactobacillus</i>. Furthermore, this effect was inversely proportional to the number of peeling times.