Agriculture

Milk Thistle

Milk Thistle

Description

Milk thistle (Silybum marianum (L.) Gaertner, Asteraceae) is an annual or biennial that grows from 5 - 10 feet (1.5 - 3.0 m) tall, has large prickly-edged leaves covered with undulating white patches, and stems containing a milky juice. Typically, thistle-like, large, reddish-purple flower heads with sharp spines grow at the ends of flower stalks. The fruit is small, hard, shiny, and grey to black (an achene) with a silvery pappus or fluff.

History And Use

Milk thistle has long been used in Europe as a food. De-spined leaves were used in salads, while stalks, roots and flowers were cooked. Seeds were used as coffee substitute. It has been used as a medicine for over 2,000 years as milk stimulant, for liver, kidney and spleen problems, for jaundice, gall stones, and menstrual pain.

The active ingredients in milk thistle "seeds" (actually the achenes) are a group of related flavonolignan compounds collectively referred to as silymarin. Milk thistle is proven to be effective in the treatment of hepatitis, cirrhosis (e.g. due to excessive consumption of alcohol) and jaundice, and in protecting liver cells against toxins such as mushroom poisons from the death cup fungus (Amanita phalloides), chlorinated industrial solvents, and acetaminophen or certain other drugs’ overdose or prolonged treatment.

Economics And Marketing

In 1997, milk thistle ranked 12th among the top selling herb supplements in the U.S. mass market, with sales of over US$3 million. Prices in 1997 ranged from US$2.50 to $5.00/lb. Health Canada has registered two milk thistle products in the "Herbs and Natural Products" category, a liquid preparation and a capsule, and four milk thistle products in the "Homeopathic" category, including liquid, pellet, globule, granule, and tablet formulations. Milk thistle preparations are now under review by the U.S. Pharmacopeia. for possible inclusion in the USP and/or National Formulary.

Area Of Adaptation

Milk thistle is native to western and central Europe and northern India, but has become naturalized by escaping from cultivation in southern Europe, Africa, India, China, Australia, South America, and in many parts of North America. Much of the current commercial seed production for the European market comes from Argentina, while cultivation in Texas supplies some of the U.S. market. Milk thistle is very drought tolerant and prefers dry well drained soil in full sun. It is found along roadsides, in fields and waste places.

Site Selection And Seeding

There is evidence of genetic differences between populations of milk thistle with regard to the content of silymarin. Conditions such as rainfall and average temperature also affect silymarin production, with higher temperatures and drier conditions apparently increasing production. Milk thistle is very adaptable to many different growing conditions, but must have well drained soils. Due to its tendency to weediness, site selection should take into consideration containment so that its spread can be controlled if necessary. Seed is sown spring or fall, and takes two weeks to germinate. It self-seeds easily.

Fertilizer

Recent experiments show that fertilizing with nitrogen and potassium will increase seed yield, but appeared to have no effect on the amount of active ingredient in the seeds.

Weed Control

Milk thistle is often called a weed itself, and is a very good competitor. Hoeing and/or hand weeding in the early stages is the only requirement.

Insects And Diseases

Milk thistle is not bothered by many pests, and no diseases have been noted.

Irrigation

Milk thistle is very drought resistant and should not require irrigation unless severe conditions arise.

Harvesting

Seeds are ready for harvesting the first year. Mature seeds, which have the highest level of silymarin, are found in seed heads showing abundant silvery white fluff (pappus). Because plants may flower at different times and have different heights, once-over direct harvesting is difficult. Seed heads shatter easily, especially if over-mature in which case as many as one-half of the total seeds may be lost or have blown away from the centre of the seed head, making handling difficult. Seeds from the edge of the flower head tend to remain on the head late into winter. Since seeds are wind dispersed, they may become a weed problem in neighbouring fields. One solution is to cover each flower head with a mesh bag before the seeds mature. Heads should have finished flowering, and be cut with less than one inch of stem. A sunflower header can be used to combine the crop, or the heads can be hand-picked. Thick clothing and thick gloves are required for manual cultivation and harvesting due to the extremely sharp prickles.

Processing

The fluffy pappus must be removed from the "seed" (achenes) . The seed is usually dried, powdered and made into a tincture using ethyl alcohol. Since silymarin is nearly insoluble in water, aqueous extracts or teas are ineffective for liver treatment. It is best to use 95% alcohol to extract the seed. Most of the silymarin is concentrated in the protein layer of the seed husk (pericarp). The tincture should be bright yellow, indicating the presence of the resinous fraction which contains the silymarin.

Quality Control

As with any medicinal herb crop, quality control is essential to achieve a good selling price, particularly in the face of the anticipated intense competition in the next few years. Quality control procedures for production are described in Health Canada’s Good Manufacturing Practices: Supplementary Guidelines for the Manufacture of Herbal Medicinal Products, available from Health Canada’s Therapeutic Products Programme by mail or from their website at: www.hc-sc.gc.ca/hpfb-dgpsa/tpd-dpt/

The achene contains 1.5-3% silymarin, a mixture of flavonolignans including silybin (also known as silibinin, about 50%), and lesser amounts of dehydrosilybin, silychristin, silydianin, and isosilybin (a stereoisomer of silybin), plus some polymeric products of silybin known as silybinomers, primarily consisting of the trimer, tetramer and pentamer. While there are numerous other constituents with known pharmacological activity, such as quercetin, the extract of milk thistle achenes is standardized for quality control on the content of silymarin, e.g. 70% of the extract weight such that a 200 mg tablet contains 140 mg of silymarin, or 200 - 400 mg per daily dose. The content of silymarin is calculated as silybin. Testing methods for the content of silymarin components is being set up at the Food Development Centre in Portage la Prairie.

References

Barl, B., Loewen, D., and Svendsen, E. 1996.
Saskatchewan Herb Database. Department of Horticulture Science, University of Saskatchewan, Saskatoon, SK.

Foster, S. 1993.
Herbal Renaissance. Peregrine Smith Books, Layton, Utah.

Hagemann, R.C., Burnham, T.H., Golshahr, V., and Neubauer, D., eds. 1997.
Milk Thistle. The Review of Natural Products. Facts and Comparisons, St. Louis, MO.

Hobbs, C. 1995.
Milk Thistle: The Liver Herb. Botanica Press, Santa Cruz, CA.

IRI Scanner Data. 1998.
Top selling herb supplements in mass market - 1997. HerbalGram 42: 65.

Wahab, J., Slinkard, A., Barl, B. and Carrier, J. 1998.
Medicinal and Aromatic Plants: Research and Development in Saskatchewan.
Prairie Medicinal and Aromatic Plant Conference 1998, Saskatoon, SK, p.13.