The debate over coffee's health risks continues to brew. A new study, out Thursday, finds that heavy coffee consumption is associated with a higher death risk in men and women younger than 55.

In the  study published online in the journal  Mayo Clinic Proceedings , men younger than 55 who drank more than 28 cups of coffee a week (four cups a day) were 56% more likely to have died from any cause. Women in that age range had a twofold greater risk of dying than other women. The study looked at 43,727 men and women ages 20-87 from 1971 to 2002.

"From our study, it seems safe to drink one to three cups of coffee a day," says the study's second co-author Xuemei Sui. "Drinking more than four cups of coffee a day may endanger health," adds Sui, assistant professor of exercise science with the Arnold School of Public Health at the University of South Carolina in Columbia. She defines a cup of coffee as 6 to 8 ounces.

The study did not find a higher death risk for adults 55 and older. Sui says there may be a bias — the research may not include unhealthy older people because they might have already died.

The reasons for the higher death risk among younger adults are not clear since experts through the years have found both health benefits and problems associated with coffee.

Sui says the caffeine in coffee can elevate heart rate as well as raise blood pressure and blood sugar levels. However, coffee is a major source of antioxidants, she adds.

Sui says the study didn't find a significant association between coffee consumption and heart disease death. Further research is needed to look at any connection between coffee and cancer, she adds.

Gregg Fonarow, co-chief of clinical cardiology,  David Geffen School of Medicine at UCLA , says, "Differences in other dietary factors, marital status, and other socioeconomic factors that were not adjusted for in this study may account for some or all of these observations."

Fonarow, who was not involved in this research, says observational studies that survey people about their coffee intake and tie that to how many died from any cause have yielded mixed results.

Consider a 2012  study that found that coffee drinkers ages 50-71 had a lower risk of death than their peers who did not consume coffee. In that study, researchers from the National Cancer Institute, part of the National Institutes of Health, and AARP found that the more coffee consumed, the more a person's death risk declined.

Joseph DeRupo, a spokesman for the  National Coffee Association of USA , says the new study "presents findings that are out of step with prevailing science as well as with widely accepted research methods."

Because coffee still stirs debate, Sui says more research is needed. In the meantime, people should watch their coffee intake, she adds. "Avoid excessive coffee drinking."