Previous studies using saw palmetto, including a meta-analysis of 18 clinical trials (JAMA Vol. 280, pp. 1604-1609) and 21 clinical trials (Cochrane Database of Systematic Reviews 2002 Iss. 3) had reported positive results for easing the mild-to-moderate symptoms.

The new double-blind randomised trial, published in the New England Journal of Medicine (Vol. 354, pp. 557-566), followed 225 men with an average age of 63 who had moderate-to-severe symptoms of BPH for one year. The men were divided into two groups, with one given 160 mg of Saw Palmetto twice a day (N=112), while the other group (N=113) received a placebo.

The effects of the saw palmetto were quantified using the American Urological Association Symptom Index (AUASI) and the urinary flow rate. Similar scores between the intervention and placebo groups were reported for AUASI, while the urinary flow of the test group improved by 0.43 ml per second.

“In this year-long randomised trial, we found that saw palmetto was not superior to placebo for improving urinary symptoms and objective measures of BPH,” wrote lead researcher Stephen Bent from the University of California, San Francisco.

The results of this study have been called “particularly puzzling” by Dr Andrew Shao, from the US Council for Responsible Nutrition (CRN), given the number of previous studies that reported positive effects.

“While this trial is an important piece of research, it should by no means close the book on saw palmetto research. Science is an evolutionary process, so it is inappropriate to simply discount the benefits previously found,” said Shao.

In an accompanying editorial, Robert DiPaola and Ronald Morton from the University of Medicine and Dentistry of New Jersey said: “Bent and colleagues tested a specific preparation of saw palmetto, leaving open the possibility that a different preparation or dose of saw palmetto might have been effective.”

Bent considered this limitation and responded: “We cannot completely address this possibility, because the active ingredient in saw palmetto, if one exists, is not known.”

Another concern with the study has come from the choice of sample population. Mark Blumenthal, from the American Botanical Council (ABC), echoed Shao when he stressed: “Most of the official international monographs that recognise the benefits of saw palmetto do so for stage 1 and 2 of BPH, ie the mild to moderate range of symptoms.”

“These include monographs from the German government and the World Health Organisation,” said Blumenthal.

A reported 2.5 million Americans use saw palmetto with yearly sales of $134 (€112) million, while in Germany over half of the nation's urologists prescribe plant-based extracts, of which saw palmetto is one of the main players.

The price of saw palmetto looks set to drop after Spanish company Euromed completed the construction of a saw palmetto processing plant in Central Florida. All of the world's supply of saw palmetto berries is harvested in one month in Florida.