Study on antioxidant activity of certain plants in Thailand: Mechanism of antioxidant action of guava leaf extract.

Suganya Tachakittirungrod, Siriporn Okonogi, Sombat Chowwanapoonpohn, Department of Pharmaceutical Sciences, Faculty of Pharmacy, Chiang Mai University, Chiang Mai, Thailand; Food Chemistry, Volume 103, issue 2, 2007, Pages 381-388

The ethanol extracts from 24 samples plant species commonly found in Thailand were investigated and compared on their antioxidant activity by ABTS assay. The ethanol extract from the leaves of guava (Psidium guajava) showed the highest antioxidant capacity with the TEAC value of 4.908 ± 0.050 mM/mg. The results demonstrated that guava leaf extracts act as antioxidants by free radical scavenging and reducing oxidized intermediates.

The antimicrobial activities of Psidium guajava and Juglans regia leaf extracts to acne-developing organisms.

Qadan F, et al., Am J Chin Med. 2005;33(2):197-204.

The researched cultured the acne-causing bacteria Propionibacterium acnes (P. acnes), Staphyloccocus aureus and the skin cells from 38 acne patients. The extracts of Psidium guajava (guava leaf) and Juglans regia leaf were then tested for their ability to kill these acne-causing bacteria. The researchers found that the two extracts have significantly better antimicrobial effects on the Staphyloccocus bacteria in vitro (or in test-tube environments) when compared to tea tree oil, or the antibiotics doxycycline and clindamycin antibiotics. However, the extracts had lower antimicrobial effects on P. acnes compared to the chemical antibiotics.

In vitro effect against Giardia of 14 plant extracts

Ponce-Macotela M; Navarro-Alegría I; Martínez-Gordillo MN; Alvarez-Chacón R
Rev Invest Clin, 46: 5, 1994 Sep-Oct, 343-7

In this test-tube study researchers investigated several plants, including guava, which are used in Mexico to fight diarrhea and parasites. Researchers wanted to test the plants' ability to fight a specific parasite: Giardia, which is known to cause diarrhea. Guava was among the plants to have a clear anti-Giardia effect which was superior to tinidazol, the drug commonly used to treat a giardia infection.

Antibacterial activity of guava (Psidium guajava L.) and Neem (Azadirachta indica A. Juss.) extracts against foodborne pathogens and spoilage bacteria.

Department of Microbiology, University of Dhaka, Dhaka, Bangladesh. Foodborne Pathog. Dis., 2007 Winter;  4(4): 481-8.

Researchers determined the antibacterial activity of guava (Psidium guajava) and neem (Azadirachta indica) extracts against 21 strains of food-born pathogens: Listeria (5 strains), Staph aureus (four strains), E. Coli 0157:H7 (six strains), Salmonella (four strains), Vibrio parahaemolyticus and Bacillus cereus; as well as five bacteria that cause food spoilage: Pseudomonas aeroginosa, P. putida, Alcaligenes faecalis, and Aeromonas hydrophila (two strains). Guava showed the greatest anti-microbial activity against certain strains of Listeria, Staph aureous and Vibrio parahaemolyticus; however it had no anti-microbial effect against E. coli or Salmonella.  The results suggest that guava extract has compounds containing antibacterial properties that can potentially be useful to control food-born pathogens and spoilage organisms.

Guava fruit (Psidium guajava L.) as a new source of antioxidant dietary fiber.

Jiménez-Escrig A, Rincón M, Pulido R, Saura-Calixto F., Departamento de Metabolismo y Nutrición, Instituto del Frío, Consejo Superior de Investigaciones Científicas, Ciudad Universitaria, E 28040 Madrid, Spain. J. Agric Food Chem., 2001 Nov; 49(11):5489-93.

Guava pulp and peel fractions were tested, and both showed high content of dietary fiber (48.55-49.42%) and extractable polyphenols (2.62-7.79%). The antioxidant activity of polyphenol compounds was studied, and all fractions showed a remarkable antioxidant capacity correlating with their total phenolic content. These results indicate that guava could be a suitable source of natural antioxidants. Peel and pulp could also be used to obtain antioxidant dietary fiber, a new item which combines in a single natural product the properties of dietary fiber and antioxidant compounds.

Effect of guava (Psidium guajava Linn.) leaf soluble solids on glucose metabolism in type 2 diabetic rats.

Shen SC, Cheng FC, Wu NJ. Department of Medical Nutrition, I-Shou University, No.1, Sec. 1, Syuecheng Road, Dashu Township, Kaohsiung County 84001, Taiwan. Phytother Res. 2008 Nov;22(11)1458-64.

This study investigated the effect of aqueous and ethanol-soluble solid extracts of guava (Psidium guajava Linn.) leaves on hypoglycemia and glucose metabolism in type 2 diabetic rats. Acute and long-term feeding tests were carried out, and an oral glucose tolerance test (OGTT) to follow the changes in plasma glucose and insulin levels was performed to evaluate the antihyperglycemic effect of guava leaf extracts in diabetic rats. The results of acute and long-term feeding tests showed a significant reduction in the blood sugar level in diabetic rats fed with either the aqueous or ethanol extract of guava leaves (p < 0.05). Long-term administration of guava leaf extracts increased the plasma insulin level and glucose utilization in diabetic rats. The experiments provided evidence to support the antihyperglycemic effect of guava leaf extract and the health function of guava leaves against type 2 diabetes.

Fermented guava leaf extract inhibits LPS-induced COX-2 and iNOS expression in Mouse macrophage cells by inhibition of transcription factor NF-kappaB.

Choi SY, Hwang JH, Park SY, Jin YJ, Ko HC, Moon SW, Kim SJ. Technology Innovation Center for Life Science, Cheju National University, Jeju, South Korea; Phytotherapy Research, 2008 Aug; 22(8):1030-4

This study determined that fermented guava leaf extract has anti-inflammatory properties.