The study by researchers at the National Cancer Institute is the largest to compare coffee drinkers with those who avoid it to determine whether the beverage can delay the risk of dying from ailments such as heart disease, diabetes or respiratory illness, said Neal Freedman, the lead study author. It’s unclear why coffee may be beneficial and more research is needed to study that question, he said.

The results “offer a little bit of reassurance to coffee drinkers who like drinking coffee that it won’t affect health,” said Freedman, an investigator at the NCI’s Division of Cancer Epidemiology and Genetics in Rockville, Maryland, in a May 14 telephone interview. “It doesn’t seem to increase one’s risk of dying.”

Still, “the association between coffee and mortality has been unclear,” he said. “This is an observational study so we don’t know for certain coffee is having a cause and effect.”

Americans drank 77.4 billion cups of coffee valued at $35.8 billion in the 12 months ended June 30, 2011, according to a Sept. 7 statement by the research firm StudyLogic. About 64 percent of U.S. adults drink coffee every day and 73 percent drink it weekly, according to the New York-based National Coffee Association. Americans consume about 3.2 cups of coffee a day, the group said.

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