Gymnema's antidiabetic activity appears to be due to a combination of mechanisms, including reduction of intestinal absorption of glucose (Shimizu et al 2001), inhibition of active glucose transport in the small intestine (Yoshioka 1986), suppression of glucose-mediated release of gastric inhibitory peptide (Fushiki et al 1992), increased activity of the enzymes responsible for glucose uptake and use (Shanmugasundaram et al 1983), stimulation of insulin secretion (Persaud et al 1999, Sugihara et al 2000) and increasing the number of islets of Langerhans and number of pancreatic beta cells (Prakash et al 1986, Shanmugasundaram et al 1990). Gymnema montanum (a related species) has also been shown to have antidiabetic, antiperoxidative and antioxidant effects in diabetic rats (Ananthan et al 2003) and antioxidant activity evident in the liver, kidney (Ananthan et al 2004) and brain tissue (Ramkumar et al 2004).

Animal studies suggest that gymnema will reduce blood sugar levels in response to a glucose load in streptozotocin-induced mildly diabetic rats (Okabayashi et al 1990) and alloxan-induced diabetic rats (Shanmugasundaram et al 1983, Srivastava et al 1985), but will not affect blood sugar levels in normal or spontaneously hypertensive rats (Preuss et al 1998). Gymnema extract has been found to return blood sugar and insulin levels to normal in streptozotocin-induced diabetic rats after 20-60 days and to double the number of pancreatic islet and beta cells (Shanmugasundaram et al 1990), as well as maintaining stable blood glucose levels in rats given beryllium nitrate (Prakash et al 1986).

There are currently two negative studies. One study using a dose of 120 mg/kg/day oral gymnema did not find improvements in insulin resistance in insulin-resistant, streptozotocin-induced diabetic rats (Tominaga et al 1995) and the other study, from Brazil using dried powdered leaves of gymnema, found no effect on blood glucose, body weight or food or water consumption in non-diabetic and alloxan-diabetic rats (Galletto et al 2004).

Gymnemic acids have demonstrated hypoglycaemic activity in dexamethasone-induced hyperglycaemic mice (Gholap & Kar 2005) and gymnema, together with other ayurvedic herbs, has been shown to have hypoglycaemic activity in streptozotocin-induced diabetic mice (Mutalik et al 2005) and rats (Babu & Prince 2004). Gymnema has also been shown to protect the lens against sugar-induced cataract by multiple mechanisms (Moghaddam et al 2005) and protect against the adverse effects of lipid peroxidation on brain and retinal cholinesterases, suggesting a use in preventing the cholinergic neural and retinal complications of hyperglycaemia in diabetes (Ramkumar et al 2005).

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