The milk thistle, indigenous to western and Mediterranean Europe, North Africa, and the Middle East, is an annual or a biennial herbaceous plant that now grows throughout Eurasia and is naturalized in southern Africa, North and South America, and Australia. It grows a series of green, tapered oval leaves with spiky margins and whitish coloring along the veins, eventually sending up a number of flowering stalks to a height of 20 to 150 centimeters, each with a flowering head about 4 centimeters in diameter consisting of dozens of small, sharply pointed leaves (botanically bracts) surrounding a group of closely packed, pink-petaled flowers (figure 14.11). At maturity, the fruit case yields approximately 100 small, dry, seeded fruits, the medicinally important product.85
FIGURE 14.11 Milk thistle.
Milk thistle was well documented in the work of Greek and Roman scholars, and Theophrastus (ca. 371–287 B.C.E.), Pliny the Elder (23–79), and Dioscorides all mention its value for diverse medical concerns.86 An abundant plant throughout western Europe during the Middle Ages and early modern period, milk thistle was frequently referenced in medical texts for its health-related properties. For example, an English herbal of 1657 recommends milk thistle as an herbal tea or in powdered form for agues, prevention and cure of the plague, to “open obstructions of the Liver and Spleen,” to increase the production of urine, and to treat dropsy. The seeds were noted as particularly effective medicines and suggested “to cool the distemperature” of the liver and heart.87 In time, medical practice focused on milk thistle’s use for various ailments of the internal organs, and a medical text published in 1898 describes its efficacy to treat “chronic congestion of the kidneys, spleen and liver.”88
During the twentieth century, biochemists extracted a diverse set of chemicals from milk thistle seeds, known as flavonolignans, dihydroflavones, and other phenolic compounds.89 Collectively, the mixture of chemicals in milk thistle extract is called silymarin, and it has been standardized for clinical and commercial purposes.90 Silymarin exerts diverse effects on physiology, as demonstrated in laboratory tests. Silymarin components are antioxidants and have the property of stimulating cellular antioxidant proteins, thereby potentially reducing the effects of cellular damage from stress or age.91 These compounds also interfere with inflammation-related signaling, modulate the cellular immune response, and seem to be able to suppress cell proliferation in Petri dish tests, possibly leading to applications in cancer treatment.92Much of the current research on milk thistle extract has centered on understanding its effects on the liver, where it is thought to be especially useful.93
Milk thistle and its extract silymarin have a reputation as a detoxifier and liver protectant, although such assertions are largely based on laboratory rather than clinical studies. Milk thistle’s antioxidant, immunomodulatory, and anti-inflammatory properties were demonstrated to protect liver cells from hepatitis C virus infection, among other measures of liver health.94 While orally administered silymarin suffers from poor absorption in the human system, its constituents can be chemically modified in the laboratory to improve uptake and, in particular, activity in the liver.95 Despite the growing body of laboratory evidence for a number of liver-protective effects, challenges of dosing, formulation, and experimental design have rendered many human studies uninformative, and reliable clinical results for milk thistle have been slow to arrive.96 The uncertainty over possible therapeutic benefits notwithstanding, it seems from the numerous studies that milk thistle extract in current dosing strategies generates few side effects (comparable to placebo) and therefore can be considered safe.97 Ultimately, the results of large-scale, well-designed trials testing silymarin or other precisely measured forms of milk thistle extract to ameliorate the symptoms of cirrhotic, viral, and other types of liver disease will settle the open question whether the protective effects of the herb in the laboratory can be extended to human treatments.98
In addition to its potential role as a liver protectant and remedy for liver disease, milk thistle extract has been employed as an emergency measure in patients accidentally poisoned by eating Amanita mushrooms, whose toxins target the liver. Retrospective analyses and case studies generally support the idea that milk thistle extract was not harmful and, in the treating physicians’ judgment, contributed to better patient outcomes.99 Milk thistle has also been suggested as a galactagogue to increase milk production, but scientific research in this area is at an early stage, and possible mechanisms are unknown.100