The in vitro study found that the milk thistle extract protected LDL cholesterol against oxidation in a dose-dependent manner, a result with potentially important implications since the oxidative modification of LDL has been reported to be a major part of the pathogenesis of atherosclerosis, and subsequently cardiovascular disease.
Cardiovascular disease (CVD), the cause of almost 50 per cent of deaths in Europe, and reported to cost the EU economy an estimated €169bn ($202bn) per year.
"Thus, it is possible that the extract prepared from the fruits of an easily accessible plant could be useful to prevent the progression of atherosclerotic events," wrote lead author Sunny Wallace from the University of Arkansas.
"However, before important conclusions that could lead to prevention strategies are drawn, it would be of interest to determine the atheroprotective effect of silymarin in vivo using an atherosclerosis-prone apolipoprotein E or LDL receptor knockout mouse model."
The results are reported in the Journal of Agricultural and Food Chemistry .
Study details Wallace and co-workers tested both the crude extract and purified forms of the major flavonolignans compounds that make up silymarin, the key flavanone in milk thistle, including silichristin (SC), silidianin (SD), silibinin (SBN), and isosilibinin (IS).
At doses of the milk thistle extract of 38, 75, 150, and 300 micromoles, LDL oxidation was inhibited by 18, 73, 82, and 86 per cent, respectively, said the researchers.
The individual flavonolignans were also associated with reductions in LDL oxidation with 300 micromole doses of SC, SD, SBN, and IS reducing LDL oxidation by 60, 28.1, 60, and 30.1 per cent, respectively, report Wallace and co-workers.
"These results showed that silymarin and SBN, likely through antioxidant and free radical scavenging mechanisms of action, inhibit the generation of oxidised LDL," concluded the researchers.
Significant further research is necessary if milk thistle and its extracts can be seen as providing a benefit to cardiovascular health.
Indeed, the researchers identified animal studies with mice as the next step.
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.
Previously, silibinin has linked to similar benefits against lung cancer ( Journal of the National Cancer Institute , Vol. 98, pp. 846-85) and liver cancer ( World Journal of Gastroenterology , Vol.13, pp. 5299-5305).
Source: Journal of Agricultural and Food Chemistry Volume 56, Issue 11, Pages 3966-3972 "Milk Thistle Extracts Inhibit the Oxidation of Low-Density Lipoprotein (LDL) and Subsequent Scavenger Receptor-Dependent Monocyte Adhesion" Authors: S. Wallace, K. Vaughn, B.W. Stewart, T. Viswanathan, E. Clausen, S. Nagarajan, D.J. Carrier