Milk Thistle, known by its scientific name Silybum marianum, is a plant with medicinal properties that researchers suggest can support liver health. The first recorded use of milk thistle for liver health dates back over 2,000 years ago to Roman times. The Roman naturalist Pliny the Elder was the first known person to write about the use of milk thistle. His texts note milk thistle was useful for ‘carrying off the bile.’ (1) According to modern medicine, bile is a product of the gallbladder that carries toxins and other bodily waste that the liver filters out of the blood. (2)
Writings from throughout history show herbalists and physicians understood the connection between milk thistle and liver disorders. Today, research continues to confirm that milk thistle contains properties that support liver health. According to several studies, the benefits found in milk thistle come from silymarin, a combination of active ingredients found in the plant. Milk thistle does more than support liver health. As research continues so does the evidence that milk thistle offers support for the body in many other meaningful ways.
Milk Thistle vs. Blessed Thistle
Milk thistle should not be confused with its botanical relative, blessed thistle (Cnicus Benedictus). Blessed thistle is recommended to stimulate lactation in breastfeeding mothers, for antioxidant support, and healthy digestion. Milk thistle is most often recommended for liver support, but it is thought to share a few similar benefits with blessed thistle; however, its safety during breastfeeding is not well understood. (3)
What is Milk Thistle?
Milk thistle is a flowering plant in the Asteraceae family. Common names for milk thistle include blessed milk thistle, holy thistle, lady’s thistle, spotted thistle, Mediterranean milk thistle, or Scottish thistle. (4) Due to a close association with the Virgin Mary in Christianity, the plant is also known by the names St. Mary’s thistle, blessed virgin thistle, and Christ’s crown.
Milk thistle is native to southern Europe and Russia, North Africa, and some western regions of Asia. During the past several centuries, the cultivating of milk thistle has spread to the Americas, Australia, China, and more northern regions of Europe.
Milk thistle can grow over 60 inches tall. (5) Its most distinct feature is its flowering top that produces an array of red-purple tendril-like pistoles. The collar at the flower’s base has triangular spikes that wrap under the flower in a conical shape. Its oblong leaves taper into a lance-like point, have spiny edges, and white veins that when cut release a white fluid known as the ‘milk.’ The stem is hollow, grooved, and cottony to the touch. Long white pappus (hair-like structures) grow around the seed and provide lift so the seed can reach new growing locations. The leaves can be used for salads and the flowers roasted as a substitute for coffee.
Milk Thistle Benefits
Silymarin in Milk Thistle
The active compound found in milk thistle is known as silymarin, which contains flavonolignan isomers “silybins A and B, isosilybins A and B, silychristin, silydianin, and their flavonoid precursor taxifolin.” (6) Research shows these chemical compounds react within the body in a number of potentially beneficial ways. Specifically, silymarin supports detoxification pathways in the liver, kidneys, and the digestive system.
Researchers write that silymarin provides hepatoprotective activity in the liver. (7) Silymarin helps to inhibit free radicals that result from the organic compounds ethanol (alcohol), acetaminophen (ingredient in pain relievers), and carbon tetrachloride (gases found in the atmosphere) that can be present in the body as a result of consuming certain types of food or breathing. These are best known as free radicals. Free radicals ‘steal’ electrons from healthy cells, a process that can have a destabilizing action and cause irreparable cellular damage and results in more free radicals. (8)
The liver filters free radicals from the blood and places them into bile, a product of the gallbladder. The bile leaves the body through the kidneys or bowels. However, while in the liver these free radicals can negatively affect healthy liver cells. Over time, these free radicals can interfere with healthy liver function.
Silymarin helps to counteract this through the support of the antioxidant glutathione. Glutathione is an electron containing molecule that can donate an electron to free radicals, thus preventing them from stealing electrons from healthy cells. Unlike other cells, when glutathione loses an electron it does not destabilize. Instead, the body will produce a free electron and donate it to glutathione without the need to convert new antioxidants. (9) Silymarin also supports new cell growth in the liver through “protein synthesis in hepatocytes by stimulating RNA polymerase I activity.” (10) The growth and development of new cells have been identified in research studies as important in patients with cirrhotic alcoholism. (11)
Early research suggests that milk thistle may offer therapeutic potential to diabetes patients and related health complications. (12) Silymarin includes a compound that has properties similar to the peroxisome proliferator-activated receptor γ (PPAR γ ) agonist.
The research notes that “PPAR γ is the molecular target of thiazolidinediones, which are used clinically as insulin sensitizers to lower blood glucose levels in diabetes type 2 patients.” When administering PPAR γ to type 2 diabetes patients, doctors note a range of potential side effects, including “weight gain, fluid retention, and increased risk of heart failure.” (13) While the research has not showed effectiveness in large scale studies, researchers hope further studies of milk thistle will show it to be a potential alternative to diabetes patients.
Research shows that silybin has an immunostimulatory effect that results from a stimulating effect on neurons in the central nervous system. (14) When stimulated, signaling through nerves may help to promote immune system function. Research confirms that after ingesting milk thistle extract, immune system factors increased in the body.
Other research has found that milk thistle has immunomodulatory effects. (15) When compared with the herbs dong quai, ginseng, and green tea, both dong quai and milk thistle increased responsiveness of nonspecific immune factors, while ginger and green tea had immunosuppressive effects that are thought to be useful following organ transplants. Both effects on immune function may be useful in specific situations, but researchers did not further clarify for what purposes they could be useful.
Another study found that the antioxidants in milk thistle may help regulate inflammatory factors in the body. (16) Inflammation is an important immune system process that protects cells and tissue. Certain challenges can cause excessive inflammation that negatively affects the body. Research suggests that the antioxidants in milk thistle may support a healthy cardiovascular system by helping to regulate inflammation in arteries and vessels.
A study looking at the use of silymarin in patients with decreased cognitive function due to methamphetamine abuse showed positive results. (17) In an animal study, silibinin, an extract of silymarin, was administered before, during, and after methamphetamine use. Following the test, differences in brain composition were measured. Researchers found that silibinin decreased dopamine and serotonin levels in the prefrontal cortex and hippocampus. As a result, recognition memory improved. Researchers suggest silibinin may be worth further investigation, especially in those with cognitive disorders related to substance abuse.
The silymarin content of milk thistle is suggested to inhibit oxidative stress induced by ultraviolet (UV) radiation. When UV light hits the skin it creates oxidative stress that destabilizes lipids, proteins, and DNA. By applying a milk thistle cream to the skin, researchers hope to show that the herb has a protective effect. (18).
In studying the use of milk thistle cream on skin, silymarin was found to provide antioxidant-like effects against UV radiation. In an animal study a silymarin topical cream was applied to the skin prior to and after exposure to UV radiation. Measurements were taken at different periods to determine if silymarin would protect the skin from immediate and long-term UV damage. When the topical cream was applied before and just after UV exposure, skin inflammation was lower when compared to the group with no cream applied and cream applied longer after exposure. The research concludes that "the use of silymarin which can reduce UV-induced oxidative stress could be a promising chemopreventive candidate against solar UV radiation-induced oxidative stress-mediated skin disorders including photoaging and photocarcinogenesis in the human population.”
Dosage, Side Effects, and Warnings
One complication with taking silymarin is the low level of bioavailable flavonolignans that research shows is between 20% and 50%. The rest of the compound leaves the body as waste. Due to poor water solubility, bioavailability, and intestinal absorption, it is important to seek milk thistle supplements with specific levels of ‘standardized extracts.’ Recent developments of soluble silybin-derived compounds are believed to help address these concerns as they purportedly increase bioavailability.
Milk thistle is found in a variety of doses. Some studies have administered milk thistle in ranges as low as 10 mg and up to 800 mg at 3 or more times a day. (19) Studies have not identified safe dosages for short or long term use. Some supplements will list the standardize content of silymarin, which may appear up to 80% or more.
The most common reported side effect of milk thistle is abdominal discomfort and gastrointestinal disturbances. (20) Interactions may occur when taking milk thistle and enzyme-based drugs such as valium, diabetes medication, antibiotics, hepatitis C medication, and immunosuppressants.
Start Using Milk Thistle Supplements
Milk thistle is available as whole herb, in vegetable capsules, tinctures, and teas. Milk thistle is considered safe and well tolerated by most people. Individuals with existing health concerns and those who are undergoing therapies or treatments for a condition should consult with a primary care physician prior to use. Consider adding it to your daily regimen and see if it makes a difference in your life.