Milk thistle (Silybum marianum) is a three foot high thistle that grows in rocky parts of Western Europe. The reddish purple flowers and shiny prickly leaves give it an easily recognisable appearance. Milk thistle is interesting nutritionally because it contains a number of bioactive phytochemicals that may have useful health properties. In particular, milk thistle contains a number of flavonolignans (phenolic compounds composed of part flavonoid and part lignan) that include silybin, silidianin and silychristin. These compounds are present in the seeds and leaves, but are found in their highest concentrations in the fruits. Collectively these compounds are referred to as silymarin, and they can be found in standardised extract that have been shown to have liver protecting effects. This protective ability of silymarin on the liver relates to the ability of flavonolignans to prevent the mechanisms by which liver damage can be induced. It is thought that this damage results from the generation of free radicals which increases the oxidative stress on hepatic tissue.
The antioxidant effects of silymarin have been well reported and as well as a direct antioxidant effect, silymarin also increases cellular levels of glutathione, presumably through its ability to spare reduced glutathione (GSH). In addition silymarin is an inhibitor of the enzyme lipoxygenase, an enzyme responsible for the transfer of oxygen molecules to long chain polyunsaturated fatty acids in the membranes of cells. This reaction yields leukotrienes, eicosanoid hormones that have pro-inflammatory effects. Milk thistle therefore has potent anti-inflammatory effects and can protect the liver from the deleterious effects of leukotrienes. The beneficial effects of milk thistle have been demonstrated in a number of animal models using chemically induced liver damage (using the death cap mushroom Amanita phalloides). Death Cap mushroom causes severe liver damage and death is induced in around 30 % of those who ingest it as a result of liver failure. Silymarin extract can counteract the poisonous effects of this mushroom and prevent the deaths of animals in 100 % of cases.
Another benefit of milk thistle is the its ability to increase protein synthesis rates in hepatic tissue. In this way milk thistle can rebuild damaged liver tissue and thus repair diseased tissue. Many studies have shown that milk thistle supplements containing silymarin are effective at treating patients with damaged livers such as might occur in cirrhosis, chronic hepatitis and both alcoholic and non-alcoholic fatty liver disease. Silymarin may also have beneficial effects at increasing the solubility of bile, and in this way may be a useful treatment and prevention of gallstones, which are largely caused by the insolubility of bile leading to obstructions in the gallbladder. Milk thistle is available as a supplement and the best supplements are standardised for around 70 to 80 % silymarin. Milk thistle can also be purchased as a tea and consumed in its whole herb form, but it makes sense to obtain a standardised extract because it is known that the presence of silymarin is required for its liver protecting effects.