Comparative antidiabetic effects and mechanisms of actions of five Chinese and South African indigenous teas.
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Date
2020
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Abstract
The present thesis assessed the in vitro, ex vivo and in vivo anti-oxidative and antidiabetic
activities of five teas which are widely consumed in China or South Africa. Three of the
selected five teas are from South Africa, namely red rooibos (Aspalathus linearis), green
rooibos (Aspalathus linearis) and red honeybush (Cyclopia genistoides) tea. The remaining
two from China are jasmine green (Camellia sinensis) and zhengshanxiaozhong (ZSXZ) black
tea (Camellia sinensis). The different sequential solvent extracts following increasing polarity
index (dichloromethane, ethyl acetate, ethanol, and water) and hot water extract of different
teas were evaluated at in vitro and ex vivo conditions for their antioxidant properties, inhibitory
potentials on α-glucosidase, α-amylase and pancreatic lipase, effects on ameliorating Fe2+-
induced oxidative pancreatic or hepatic injury, as well as the glucose absorption inhibition in
small intestine and the glucose uptake stimulation in isolated psoas muscle of rats. Possible
bioactive components responsible for the activities of the extracts were identified by using Gas
Chromatography-Mass Spectrometry (GC-MS) analysis or liquid chromatograph-mass
spectrometry (LC-MS) analysis. In vitro and ex vivo tests presented promising antioxidant and
antidiabetic activities of these five teas. The red honeybush, jasmine green and green rooibos
teas, were further subjected to an in vivo intervention trial in a fructose-streptozotocin (STZ)
induced T2D model of Sprague-Dawley rats. Assays were carried out to reveal the effects of
these teas on lowering blood glucose level, improving oral glucose tolerance ability,
stimulating insulin secretion and hepatic glycogen synthesis and ameliorating some diabetes
related parameters such as serum lipid profile, hepatic and renal function tests and calculated
insulin resistance (HOMA-IR), β-cell function (HOMA-β) from the blood glucose and serum
insulin data. Furthermore, in vivo oxidative stress markers such as reduced glutathione,
superoxide dismutase and catalase activity and lipid peroxidation were analysed in harvested
organs (liver, kidney, heart and pancreas). The results of in vivo tests demonstrated that high
dose of jasmine green tea showing the best activity followed by the high dose of red honeybush
tea, low dose of jasmine green tea, high dose of green rooibos tea, low dose of red honeybush
tea, when lowest activity was observed for the low dose green rooibos tea. The results of this
study indicated promising anti-T2D properties of the above-mentioned teas. However, further
clinical trials are needed to ascertain the results of these in vitro, ex vivo and in vivo studies.
Description
Doctoral Degree. University of KwaZulu-Natal, Durban.