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Zinc oxide nanoparticles (ZnONPs) have shown antidiabetic activity in multiple studies and can be produced by different plant-mediated ("green") methods. This study aimed to compare ZnONPs prepared via different "green" approaches (heating at high temperatures (400 °C) vs. low temperature (70 °C)). The low temperature method involved addition of suspending agents (Tween 80 or gum arabic) and pH variations followed by lyophilization. The study evaluated the hypoglycemic potential of ZnONPs with the best properties (quantity of capped agents and stability) compared to the plant extract per se. The ZnONP synthesis involved a mixture of zinc nitrate hexahydrate as the zinc precursor and a plant extract with high antioxidant activity as the capping agent supplier. The results of the studies showed that the procedure using high-temperature heating resulted in almost uncapped nanoparticles with phytocompounds (0.01 % of phenolic compounds) and nanoparticle sizes larger than 300 nm. The low-temperature method produced ZnONPs with high retention of capping agents (92.90 % of phenolic compounds) and a size of approximately 200 nm. The use of Tween 80 with pH adjustment between 9 and 10 resulted in more stable nanoparticles than with gum arabic. These nanoparticles prepared with Tween 80, exhibited a pronounced in vivo antihyperglycemic activity at a much lower dose (10 mg ZnO/kg capped by 0.31 mg phenolic compounds per kg) than the extracts alone (400 mg extract/kg) following an oral glucose tolerance test. These results demonstrated that green-synthesized ZnONPs with a high retention rate of phytochemicals can induce antihyperglycemic effects at a low dose.