A flavanone compound in milk thistle, silibinin, stopped lung cancer growth and spread in mice, says research from the University of Colorado.
"Our research findings suggest that milk thistle (with active compound silibinin) could inhibit lung cancer growth and progression… We expect that after supplement consumption a sufficient level of silibinin must be achieved in blood circulation/lung to have an anticancer effect," lead researcher Rana Singh told NutraIngredients.com.
Lung cancer is the most common form of cancer worldwide with over 1.2m new cases diagnosed annually, according to the European School of Oncology. It has one of the lowest survival rates with only 25 per cent of patients surviving more than one year after diagnosis (England and Wales).
It should be stressed that the new research, published in the Journal of the National Cancer Institute (Vol. 98, pp. 846-85), did not use milk thistle dietary supplements, but pure silibinin, the active component in milk thistle.
"Milk thistle extract dietary supplements generally contain 80 per cent silymarin, a flavonolignan mixture; and silymarin contains approximately 40 per cent silibinin," explained Singh.
"Therefore, we expect about 32 per cent (w/w) silibinin in milk thistle dietary supplements," he said.
The researchers divided 90 mice into six groups and injected 75 mice with cancer-causing urethane (cancer development in two to three weeks) and 15 mice with a saline solution (control).
After two weeks of eating a normal diet, the urethane-treated mice (five groups of 15) were fed a diet supplemented with different doses of silibinin (zero, 0.033, 0.10, 0.33, 1.0 per cent).
After 18 weeks, the silibinin supplemented group had between 32 and 38 per cent less tumours than the urethane-only group, across all the dose range. After 29-weeks, the silibinin supplemented group had between 64 and 70 per cent less tumours than the urethane-only group, across all the dose range.
Mice that received the 1.0 per cent silibinin supplement had 93 per cent fewer large tumours than the urethane-only group.
In terms of cancer spread, the tumours in the silibinin supplemented mice had between 41 and 74 per cent fewer cells that tested positive for proliferation (spread) markers.
The formation of blood vessels (angiogenesis) was also found to be reduced in the silibinin groups, with reductions up to 89 per cent recorded.
"Our most clinically relevant observation was that silibinin prevented tumours from growing beyond a small size in a dose-depended fashion," said the researchers.
"We believe that this inhibition is due, at least in part, to the inhibitory effect of silibinin on angiogenesis."
The mechanism behind the effects is not clear, but the researchers suggest that the flavanone may act on the expression of two enzymes (iNOS and COX-2), as well as the vascular endothelial growth factor (VEGF), which promote tumour angiogenesis.
The mechanism continues to be studied, said the researchers.
The authors caution that the animal model of lung cancer, while similar, does not mirror human lung cancer.
A comparable dose in a 70 kg human would be in the range of about 2 to 20 grams per day for the three lower doses, said Singh.
While this may sound like a lot, Singh told NutraIngredients.com that research has indicated such doses of silibinin are not toxic.
"A recent study in human prostate cancer patients suggest that silibinin consumption up to about 6.7 g/day (equivalent to 20 g/day silibinin-phytosome in three equal doses) does not show any significant adverse health effect.
"This study is still continuing with escalated doses of silibinin to find out the maximal tolerated dose in the patients," said Singh.
Milk thistle (Silybum marianum) has been used for a long time as a food in Europe. Young leaves are used in salads, the stalks eaten like asparagus, and the heads boiled like artichoke.
According to the Manitoba Agriculture, Food and Rural Initiatives (Canada) milk thistle ranked 12th among the top selling herb supplements in the US mass market, with sales of over $3m in 1997.