By Toni Clarke

(Reuters) - The U.S. Food and Drug Administration is poised to release initial data from a landmark study into how Americans use tobacco products, but researchers expect many key questions about e-cigarettes to remain unanswered.

The five-year Population Assessment of Tobacco and Health (PATH) Study of about 46,000 people, begun in 2011, is expected to provide a wealth of data about smoking behavior that could shape regulations ranging from warning labels and advertising restrictions to new product approvals.

"It is going to provide the most fine-grain, comprehensive, highest quality data on tobacco use that has ever been collected in the United States," said Stanton Glantz, a tobacco control expert at the University of California, San Francisco.

The FDA gained authority under a 2009 law to regulate cigarettes, smokeless tobacco and roll-your-own tobacco. It must issue new rules before it has authority to regulate e-cigarettes, cigars, hookahs and water pipes, among others. It has issued a draft proposal and is currently reviewing public comment before releasing final rules.

Some experts say the PATH study may not be able to capture crucial details about the use of electronic cigarettes, vapor tanks and other devices as tastes and technology are evolving so rapidly.

"While all this data that is being accumulated through the PATH study is great, it is unclear to me how much is going to be useful for setting the policy and regulations of the future." said Scott Ballin, a health policy consultant based in Washington, D.C.

The study is being funded by the FDA and administered by the National Institutes of Health. Responses from about 20,000 participants will be released on Thursday at the annual meeting of the Society for Research on Nicotine and Tobacco in Philadelphia.

Each participant is being followed over time, allowing researchers to assess trends, such as which products are gaining in popularity and which are fading. Researchers are also collecting biological samples that will allow scientists to compare subjective responses about, for example, nicotine exposure, with physical data.

Kurt Ribisl, a professor at the University of North Carolina Gillings School of Global Public Health, said the size of the study means researchers will gain unprecedented detail about specific groups of people.

Such detail could prove valuable to the agency as it weighs whether or not to ban menthol cigarettes, which are particularly popular in African American communities.

It is unclear whether the study will provide enough detail about e-cigarette use to answer public health questions about whether flavors in e-cigarettes attract youngsters to the products. Neither is there likely to be much data about average doses of nicotine consumed, since many vaping devices can be customized to release different levels of nicotine.

"When you get to e-cigarettes it's really complicated," Ribisl, who also sits on the FDA's tobacco products scientific advisory committee. "It’s a vexing and complex issue."

(Reporting by Toni Clarke in Washington; Editing by Diane Craft)