The herbal supplement was also found to have significantly beneficial effects on glycosylated haemoglobin levels, total cholesterol and LDL cholesterol levels, and triglyceride levels on the diabetics who took part in the trial.
"The results are very encouraging, and we now need to do further large multi-centre studies," said lead author Fallah Huseini, from the Institute of Medicinal Plants, in Tehran, Iran.
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 U.S. mass market, with sales of over $3m (m) in 1997.
The researchers recruited 51 people with type-2 diabetes to take part in the four-month randomized, double-blind, placebo-controlled clinical trial. Subjects were assigned to receive either a daily milk thistle supplement (200 milligrams three times per day) or a placebo (three times per day).
The patients continued their conventional oral hypoglycaemic treatments and were examined at monthly.
The researchers, writing in the journal Phytotherapy Research on-line ahead of print (doi:10.1002/ptr.1988), reported that fasting glucose levels of the supplementation group decreased from 156 milligrams per decilitre (mg/dL) of blood to 133 mg/dL, while the placebo group's fasting glucose levels increased from 167 to 188 mg/dL.
Average glycosylated haemoglobin (HbA1c) levels also decreased in the milk thistle extract-supplemented group, by 1.04 per cent after four months, compared to the 1.16 per cent increase in the placebo group.
Blood levels of total cholesterol (12 per cent), LDL cholesterol (12 per cent) and triglyceride levels (25 per cent) were also significantly reduced as a result of the milk thistle extract supplements. No significant change in the placebo groups' total cholesterol or LDL cholesterol levels was measured, while triglyceride levels increased by 12 per cent.
"The results show that although there were no statistically significant differences between the two groups of patients at the beginning of the study, silymarin [milk thistle extract] significantly lowered HbA1c and fasting blood glucose levels in diabetic patients at the end of the study," wrote the researchers.
The researchers could not say however what the actual mechanism behind the effects was, and called for more large multi-centre studies to further investigate the potential role of milk thistle extracts in type-2 diabetes.
"We don't know the exact mechanism of action for this effect, but this work shows that silymarin could play an important role [against] type II diabetes," said Huseini.
An estimated 19 million people are affected by diabetes in the EU 25, equal to four per cent of the total population. This figure is projected to increase to 26 million by 2030.
In the US, there are over 20 million people with diabetes, equal to seven per cent of the population. The total costs are thought to be as much as $132 bn, with $92 bn being direct costs from medication, according to 2002 American Diabetes Association figures.