Background Cosmos caudatus is an annual plant known for its medicinal value in treating several health conditions, such as high blood pressure, arthritis, and diabetes mellitus. The α-glucosidase inhibitory activity and total phenolic content of the leaf aqueous ethanolic extracts of the plant at different growth stages (6, 8. 10, 12 and 14 weeks) were determined in an effort to ascertain the best time to harvest the plant for maximum medicinal quality with respect to its glucose-lowering effects. Methods The aqueous ethanolic leaf extracts of C. caudatus were characterized by NMR and LC-MS/MS. The total phenolic content and α-glucosidase inhibitory activity were evaluated by the Folin-Ciocalteu method and α-glucosidase inhibitory assay, respectively. The statistical significance of the results was evaluated using one-way ANOVA with Duncan’s post hoc test, and correlation among the different activities was performed by Pearson’s correlation test. NMR spectroscopy along with multivariate data analysis was used to identify the metabolites correlated with total phenolic content and α-glucosidase inhibitory activity of the C. caudatus leaf extracts. Results It was found that the α-glucosidase inhibitory activity and total phenolic content of the optimized ethanol:water (80:20) leaf extract of the plant increased significantly as the plant matured, reaching a maximum at the 10th week. The IC 50 value for α-glucosidase inhibitory activity (39.18 μg mL − 1 ) at the 10th week showed greater potency than the positive standard, quercetin (110.50 μg mL − 1 ). Through an 1 H NMR-based metabolomics approach, the 10-week-old samples were shown to be correlated with a high total phenolic content and α-glucosidase inhibitory activity. From the partial least squares biplot, rutin and flavonoid glycosides, consisting of quercetin 3- O -arabinofuranoside, quercetin 3- O -rhamnoside, quercetin 3- O -glucoside, and quercetin 3- O -xyloside, were identified as the major bioactive metabolites. The metabolites were identified by NMR spectroscopy ( J -resolve, HSQC and HMBC experiments) and further supported by dereplication via LC-MS/MS. Conclusion For high phytomedicinal quality, the 10th week is recommended as the best time to harvest C. caudatus leaves with respect to its glucose lowering potential. Electronic supplementary material The online version of this article (10.1186/s12906-019-2655-9) contains supplementary material, which is available to authorized users.

Aquilaria is a multipurpose plant with high commercial and medicinal values. The present study aimed to evaluate the effects of drying methods and extraction solvents on the phenolic and flavonoids content in A. malaccensis (AMA), A. subintegra (ASU), and A. sinensis (ASI). Leaf samples were either Air-Dried (AD) or Oven-Dried (OD) and extracted using 100% Ethanol (EE100), 70% ethanol (EE70), aqueous (AE), and hexane (HE). There was a significant difference (p<0.05) between the drying methods and the extracting solvents. The OD in AMA leaf samples gave the highest values of total phenolic compounds (TPC) (52.98 – 85.15 mg GAE/gm) and total flavonoid compounds (TFC) (2180.97 – 3733.45 QUE ppm). The highest DPPH radical scavenging was observed in OD-EE70-AMA with the IC50 value of 33.60 µg/mL. Meanwhile, OD-EE100-AMA gave the lowest IC50 value (0.13 µg/mL) in the a-glucosidase assay, followed by OD-EE70-AMA with IC50 value 0.69 µg/mL with no significant differences among them. OD-EE70-AMA was found to have a higher content of TPC and TFC with the most potent DPPH scavenging and a-glucosidase inhibition activities. A strong positive correlation was reported between bioassays and TPC or TFC, hence indicating that AMA leaf used in this study might have the potential as a natural antioxidant and an antidiabetic agent.