Oxidative stress is implicated in the pathogenesis of diabetes because it may cause insulin resistance and may also cause damage to pancreatic β-cells. Milk thistle is the common name for the herb Silybum marianum, a member of the Asteraceae family that also includes sunflowers, dandelions and marigolds. Milk thistle has been chemically analysed and found to contain a number of constituents, one of which includes a group of flavonolignans collectively called silymarin. Silymarin is biologically active in animals, including humans, and is known to possess antioxidant properties in mammalian tissues. The antioxidant properties of silymarin have been particularly well studied with regard protection from hepatotoxic compounds such as mushroom and drug poisoning. For this reason milk thistle is popular amongst bodybuilders who take oral anabolic steroids, which are 17-alpha alkylated and can be particularly liver toxic. The antioxidant effects of silymarin may give it useful properties in regulating insulin sensitivity.
Studies have assessed the effects of milk thistle extracts on the efficiency of glucose metabolism in humans. In particular the ability to improve the glucose metabolism in those with type 2 diabetes has been well reported. For example, in one randomised placebo controlled clinical trial1, subjects with type 2 diabetes were administered either 600 mg of silymarin or a placebo. Following the 4 month supplementation period, the subjects consuming the silymarin experienced reductions fasting blood glucose levels. The beneficial glycaemic effects were reflected in reductions in the levels of glycated haemoglobin in the plasma of the subjects. The silymarin treatment also caused reductions in total cholesterol, low density lipoprotein (LDL) cholesterol, fasting triglycerides and enzymes that are indicative of general tissue damage (serum glutamic oxaloacetic transaminase (SGOT), also called aspartate aminotransferase (AST); serum glutamic pyruvic transaminase (SGPT) also called alanine aminotransferase (ALT).
Therefore silymarin appears to produce beneficial biochemical changes in subjects with type 2 diabetes. These changes include improvements in blood sugar control, and improvements to the metabolic dysfunction that results from poor glycaemic control. The way that silymarin is able to lower plasma glucose levels is not clear. However, it is possible that silymarin can improve insulin sensitivity through its ability to scavenge free radicals. This is significant because oxidative stress induced by free radicals may be one of the mechanisms that initiates and propagates insulin resistance. Alternatively, the reduction in the circulating enzymes that signify tissue damage, may suggest that silymarin is able to protect tissues, particularly the liver, and this may have beneficial effects because the liver is particularly important for the regulation of the metabolism of glucose. The antioxidant effects of silymarin may also protect the β-cells of the pancreas, and thus improve the function of the insulin system.
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