Evaluation of Antioxidant Activity and Phenolic Compound Levels in Indigenous Vegetables of Thailand

Abstract

Introduction: Thailand is currently facing major public health challenges from non-communicable diseases (NCDs), largely driven by inappropriate dietary habits and lifestyle factors. These diseases progress gradually, leading to severe complications and high mortality rates. The consumption of local vegetables plays an essential role in reducing such risks, as they are rich sources of antioxidants that inhibit oxidative processes associated with chronic diseases. Complementary and Alternative Medicine (CAM) has emphasized the therapeutic potential of herbs and indigenous plants, recognizing their safety and efficacy. International research, such as studies on Moringa stenopetala extracts in Algeria, has revealed high levels of phenolics and flavonoids with strong antioxidant and anti-inflammatory activities. Similarly, several Thai studies report that local vegetables exhibit antioxidant activity exceeding 80% and contain bioactive compounds, particularly phenolics and flavonoids, which are associated with the prevention of diabetes, cancer, and cardiovascular diseases. Thai indigenous vegetables are valuable, easily accessible, and regionally diverse. Therefore, analyzing their antioxidant activity and phenolic content is crucial—not only to build fundamental knowledge and encourage sustainable consumption, but also to provide a foundation for developing raw materials for functional foods and herbal products that support preventive healthcare.

Objectives: To analyze the antioxidant activity and phenolic compound content of local vegetables in Thailand.

Methods: This experimental research analyzed 28 species of Thai indigenous vegetables. The study was divided into two parts: (1) Antioxidant activity was evaluated using the DPPH radical scavenging assay. DPPH solutions and Trolox standards at various concentrations were reacted with vegetable extracts, and absorbance was measured at 517 nm using a UV–Visible Spectrophotometer. The results were used to calculate the percentage inhibition (% inhibition) and the half-maximal inhibitory concentration (IC₅₀). (2) Total phenolic content was determined by the Folin–Ciocalteu colorimetric method, with gallic acid as the reference standard. Absorbance was measured at 765 nm, and results were expressed as milligrams of gallic acid equivalent per gram of sample (mg GAE/g).

Results: Antioxidant activity varied across vegetables from different regions. In the Northern region, Ocimum gratissimum exhibited the highest activity; in the Northeastern region, Spondias pinnata showed the greatest activity; in the Central region, Ocimum sanctum was the most potent; and in the Southern region, Curcuma longa demonstrated the highest antioxidant activity. Overall, Ocimum sanctum from the Central region exhibited the strongest activity (TEAC 198.95 mg/ml), followed by Curcuma longa from the Southern region (153.06 mg/ml). Regarding phenolic content, Ocimum gratissimum had the highest levels in the Northern region, Spondias pinnata in the Northeastern region, Ocimum sanctum in the Central region, and Parkia speciosa in the Southern region. Nationally, Parkia speciosa from the Southern region contained the highest phenolic content (213.34 mg/ml), followed by Ocimum sanctum from the Central region (122.47 mg/ml).

Conclusions: This study demonstrates that Thai indigenous vegetables are promising sources of antioxidants and phenolic compounds, particularly Ocimum sanctum, Curcuma longa, Ocimum gratissimum, Spondias pinnata, and Parkia speciosa Hassk. Their antioxidant capacity may be influenced by the quantity and type of bioactive compounds present, as well as environmental conditions affecting secondary metabolite synthesis. These findings provide a scientific basis for promoting the consumption of local vegetables for preventive nutrition and support the potential development of functional foods, herbal supplements, and precursors for novel therapeutic agents in the future.