There is more evidence to back up a long-standing theory that smokers are less likely to develop Parkinson's disease than people who do not use tobacco products, researchers reported on Monday.

The apparent protective effect of tobacco against the degenerative nerve disease has been observed for years but a University of California Los Angeles School of Public Health report said a new review of existing studies seems to confirm it, with long-term and current smokers at the lowest risk.

The review also found that the effect seems to extend beyond cigarettes to pipes and cigars, and possibly to chewing tobacco, and that it persisted among those who had stopped smoking years earlier.

What would cause such a preventive effect is not well understood, said the report in the Archives of Neurology, but studies on test animals suggested two possibilities.

One is that carbon monoxide or other agents in tobacco smoke exert a protective effect and promote survival of brain neurons that produce dopamine, which allows muscles to move properly and is lacking in Parkinson's cases.

Cigarettes may also somehow prevent the development of toxic substances that interfere with proper neurological functioning.

While there have been a number of previous studies, most were too small to be conclusive, the report said. So the UCLA researchers looked at 11 studies done between 1960 and 2004 covering more than 11,800 people, of whom 2,816 had Parkinson's disease.

"Our analyses confirmed prior reports of an inverse association between cigarette smoking and Parkinson's disease," the study said.

"Although we found that current smokers and those who had continued to smoke to within five years of Parkinson's disease diagnosis exhibited the lowest risk, a decrease in risk (13 percent to 32 percent) was also observed in those who had quit smoking up to 25 years prior to Parkinson's disease diagnosis," it said.

"Other tobacco products also appeared to be protective. Men who smoked pipes or cigars had a 54 percent lower risk. The number of chewing tobacco users was small, but there was a suggestion of reduced risk associated with this product as well.