Garlic effect on diabetes mellitus

Diabetes mellitus is a group of diseases characterized by high blood glucose levels resulting from defects in insulin secretion, insulin action, or both. Abnormalities in the metabolism of carbohydrate, protein, and fat are also present [122]. Nutrition plays a primary role to control blood glucose level and further complication.

Animal studies

Garlic was effective in reducing blood glucose in streptozotocin-induced [123-126] as well as alloxan-induced [127-132] diabetes mellitus in rats and mice. Most of the studies showed that garlic can reduce blood glucose level in diabetic mice [127,133], rats [124,129,130], and rabbit [131,132]. Augusti & Sheela consistently showed that S-allyl cysteine sulfoxide (alliin), a sulfur containing amino acid in garlic (200 mg/kg b.wt.) has a potential to reduce diabetic condition in rat almost to the same extent as did glibenclamide and insulin [128-130]. Treatment of diabetic rats with garlic oil decreased serum acid and alkaline phosphatase, serum alanine and asparate transferases, as well as serum amylase in diabetic rats [123]. Aged garlic extract is also effective to prevent adrenal hypertrophy, hyperglycemia and elevation of corticosterone in hyperglycemic mice induced by immobilization stress [133]. Garlic intake (6.25% by weight in diet) for 12 days reduced hyperphagia and polydipsia but did not alter hyperglycemia and hypoinsulinaemia in streptozotocin-induced diabetic mice [125]. Ingestion of garlic juice resulted in better utilization of glucose in glucose tolerance test performed in rabbits [132,134]. The ethyl alcohol, petroleum ether and ethyl ether extracts of garlic produced a significant fall in blood sugar levels in rabbits [131]. Allicin at a dose of 250 mg/kg is 60% as effective as tolbutamide in alloxan-induced diabetic rabbit [132].

Human study

Hypoglycemic effect of garlic in human is not well studied. Chronic feeding of garlic oil and garlic powder [78,135] showed significant decrease in blood glucose level whereas some other studies [57,117,136] showed no change of blood glucose level. All human studies (Table- 5), except one or two, showing effect of garlic on blood glucose level on normal healthy individuals but not in diabetic patients. Thus the role of garlic in diabetic condition is yet to be confirmed.

Possible Mechanism

Though the exact mechanism/s of garlic as antidiabetic agent is still not clear but in-vivo [124,132] as well as in-vitro [128] studies showed that garlic acts as an insulin secretagogue in diabetic rats. Augusti & Sheela also proposed that antioxidant effect of S-allyl cysteine sulfoxide (isolated product from garlic) may also contribute for its beneficial effect in diabetes [128]. Another proposed mechanism is due to spare insulin from sulphydryl group. Inactivation of insulin by sulphydryl group is a common phenomenon. Garlic (allicin) can effectively combine with compounds like cysteine and enhance serum insulin [132]. Jain & Vyas proposed that garlic can act as an antidiabetic agent by increasing either the pancreatic secretion of insulin from the beta cells or its release from bound insulin [131].