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To explore the chemicals responsible for the health benefits of guava, water-soluble polysaccharides were extracted including GP90 and P90. They exhibited excellent α-glucosidase inhibition activity with an EC 50 of 2.27 μg/mL and 0.18 mg/mL. This suggests that their activities were 1379- and 17-fold higher than the positive control. The DPPH scavenging activities of GP90 was even higher than Vc at some concentrations. Upon further isolation, a novel polysaccharide termed GB90-1B was obtained. Monosaccharide analysis, methylation analysis, and NMR were used to analyze the structural characterization of GB90-1B. Structural analysis revealed that its backbone consisted of (1→5)-linked-α- l -arabinose, (1→2,3,5)-linked-α- l -arabinose and (1→3)-linked-α- l -arabinose. Branch linkages included (1→6)-linked-α- d -glucose, (1→)-linked-α- d -glucose and (1→)-linked-α- l -arabinose. The structure of the repeating unit of GP90-1B was predicted. [ABSTRACT FROM AUTHOR]