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Milk thistle (silybum marianum) is a plant that has been used for centuries. It is presently one of the most commonly used medicinal plants worldwide [1]. The extract of the active components of milk thistle, consisting of various flavonolignans and the isoflavanoid taxifolin, is known as silymarin. The flavanolignans in silymarin include silybinin (also spelled silibinin), isosilybinin, silydianin, silychristin, and dehydrosilibinin [2-3]. Silibinin is generally seen as the primary active component, although the other flavanolignans may also contribute to the therapeutic activity of silymarin [4-5].
The biological activity of silymarin can be attributed to many factors. One of the most important is its action as an antioxidant, and many of the other biological effects are downstream of this action. Silymarin scavenges free radicals and causes a significant reduction in lipid peroxidation, protecting and stabilizing cell membranes [2, 4]. Silymarin stimulates RNA and protein synthesis in the liver, leading to faster regeneration after injury [5-6]. [2]. Silymarin may also bind directly to receptor sites in liver cells, prevent toxins from binding to those sites [8]. The effects are many, and the protective effect of milk thistle can be seen as the sum of many different mechanisms [5].
Liver protection
There is a large body of literature discussing experimental studies on the hepatoprotective (liver protecting) properties of silymarin. In rodent models, silymarin (or silibinin) reduced or prevented liver toxicity caused by chroquine (an antimalarial and antirheumatoid agent), amiodarine (an anti-arrhytmic drug), D-galactosamine, carbon tetrachloride, acetaminophen, paracetamol, heavy metals, ethanol, pyrogallol, phenylhydrazine, radiation, and Amanita phalloides (a toxic mushroom species) [4, 6, 8-12]. These benefits have been measured by changes in liver enzymes (such as AST, ALT, and ALP), changes in serum bilirubin, decreased free radical generation, and other methods [4, 6, 9, 13-14].
Other effects
A number of other possible benefits of silymarin supplementation have been identified, related to its antioxidant effects. In humans, supplementation with silymarin significantly increases plasma antioxidant capacity [2]. In addition to protecting the liver from heavy metals, silymarin (or silibinin) may reduce other toxic effects of iron and mercury buildup [2, 31].
Dosage & toxicity
The majority of the literature indicates that silymarin is virtually non-toxic and associated with few side effects [2, 22]. No significant adverse effects with silymarin as monotherapy, and few negative drug interactions have been reported [2, 8]. In clinical trials, reports of side effects are similar to those reported with placebo treatment [8, 10]. There are some a few case reports of gastrointestinal disturbances and allergic skin rashes [17]. In vitro studies indicate that silymarin may actually be hepatotoxic in high enough concentrations, and high concentrations of silymarin inhibit some drug metabolizing enzymes, but these are not significant concerns with normal supplemental doses [5, 33].
Although there are many references to the low oral bioavailability of silymarin, supplementation in sufficient quantities results in increased silymarin concentrations in various tissues, especially the liver [4, 34]. The dose used in most clinical trials is 420 mg, with a range of 210-800 mg [8]. Assuming the milk thistle being used is 70-80% silymarin, 200-400 mg of milk thistle daily is recommended as a general supplement to maintain good liver health, and 500-1000 mg is recommended in situations of high liver stress.
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This article is for informational purposes only. Always consult a doctor before taking any supplement or starting a new diet or training program.
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