|ISSN No. 1606-7754 Vol.15 No.3 December 2007|
Anti-diabetic and hypolipidaemic properties of garlic (Allium sativum) in streptozotocin-induced diabetic rats
Martha Thomson, Zainab M. Al-Amin, Khaled K. Al-Qattan, Lemia H. Shaban and Muslim Ali
Department of Biological Sciences, Faculty of Science, Kuwait University, Kuwait.
In this study the hypoglycaemic, hypocholesterolaemic and hypotriglyceridaemic effects of garlic were studied in streptozotocin (STZ)-induced diabetic rats. Compared to normal (non-diabetic) rats, STZ-induced diabetic rats had approximately 200% higher serum glucose, 50% higher serum cholesterol and 30% higher serum triglyceride levels as well as 86% higher urinary protein levels. Daily treatment of STZ-induced diabetic rats with an extract of raw garlic (500mg/kg intraperitoneally) for seven weeks significantly lowered serum glucose, cholesterol and triglyceride levels. Compared to control diabetic rats, garlic-treated rats had 57% less serum glucose, 40% lower serum cholesterol levels and 35% lower triglyceride. In addition, urinary protein levels in garlic-treated diabetic animals were 50% lower compared to the diabetic controls. In contrast, the increased urine output and water intake of diabetic rats were not affected by garlic treatment. These results indicate that raw garlic possesses a beneficial potential in reversing proteinuria in addition to reducing blood sugar, cholesterol and triglycerides in diabetic rats. Therefore, garlic could be of great value in managing the effects and complications of diabetes in affected individuals.
Key words: Diabetes, Garlic, hypoglycaemic activity, hypolipidaemic activity, proteinuria
Garlic (Allium sativum) is one of the most popular herbs used worldwide to reduce various risk factors associated with cardiovascular diseases. Garlic, a member of the Liliaceae family, is a common food for flavour and spice and it is one of the herbs most commonly used in modern folkloric medicine. Garlic was an important medicine to the ancient Egyptians as listed in the medical text Codex Ebers (ca. 1550 BC) especially for the working class involved in heavy labour because it was an effective remedy for many aliments such as heart problems, headache, bites, worms and tumours.
Garlic is stated to possess many therapeutic benefits. Garlic’s strong odour is largely due to sulphur-containing compounds (e.g. S-allylcysteine sulphoxide), which are believed to account for most of its medicinal properties.1 Actually, garlic contains a variety of effective compounds that exhibit anticoagulant (anti-thrombotic),2,3,4,5,6 antioxidant,7,8 antibiotic,9,10,11 hypocholesterolaemic,12 hypoglycaemic,1 as well as hypotensive activities.12-13
As mentioned above, although a large number of sulphur- thiosulphinates are present in sufficient quantities at normal consumption levels (3-5 g per day). Allicin has been shown to be important in many health effects of garlic.14 However, the anti-cancer effect of garlic might be shared between allicin and other unidentified compounds.15 Garlic contains about 1% alliin, which is converted enzymatically by allicinase to allicin, and other sulphur-containing compounds.16
Garlic has been found to be effective in lowering serum glucose levels in STZ-induced as well as alloxan-induced diabetic rats and mice. Most of the studies showed that garlic can reduce blood glucose levels in diabetic mice, rats and rabbits.14 Augusti and Sheela consistently showed that S-allyl cysteine sulphoxide, (allicin), a sulphur-containing amino acid in garlic (200 mg/kg body weight), had a potential to reduce the diabetic condition in rats almost to the same extent as did glibenclamide and insulin.17-18 Aged garlic extract was also effective in preventing adrenal hypertrophy, hyperglycaemia and elevation of corticosterone in mice made hyperglycaemic by immobilization stress.19 In addition, Liu and co-workers reported that both garlic oil and diallyl trisulphide improved glycaemic control in STZ-induced diabetic rats.20 Ingestion of garlic juice resulted in better utilization of glucose in glucose tolerance tests performed in rabbits, while allicin at a dose of 250 mg/kg was 60% as effective as tolbutamide in alloxan-induced diabetic rabbits.21
In contrast, garlic powder intake (6.25% by weight in diet) for 12 days reduced hyperphagia and polydipsia, but did not alter either hyperglycaemia or hypoinsulinaemia in STZ-induced diabetic mice.22 Similarly, Baluchnejadmojarad and Rohgani found no hypoglycaemic effect of an aqueous extract of garlic in rats with STZ-induced diabetes although they did observe a significant effect of garlic on vascular reactivity.23-24 Liu and co-workers have speculated that these inconsistent results are at least partly due to the use of different preparations or derivatives of garlic in the different studies.20 Staba and coworkers have established that the chemicals present in a garlic product are largely dependent on the processing conditions, such as temperature, duration of preparation, and extraction solvents used.25
In humans, the hypoglycaemic effect of garlic is not well documented. Most reports have shown a significant effect of garlic on blood glucose of normal healthy individuals but not in diabetic patients. Thus the role of garlic in diabetes treatment/prevention in humans is yet to be confirmed.13
The aim of the present study was to investigate the efficacy of an aqueous extract of raw garlic in controlling serum glucose, cholesterol, triglyceride and urine protein levels in STZ-induced diabetic rats treated daily intraperitoneally (IP) for a period of 7 weeks. Since there have been variable reports about the use of different preparations of garlic, as discussed above, an aqueous extract of raw garlic was used in the present study. Use of this preparation is also consistent with our previous work with garlic.4-5, 26, 27,28.