Study author Dr. Honglei Chen of the National Institutes of Health in Research Triangle Park in North Carolina said the study involved about 264,000 people ages 50-71 who drank soda, tea, fruit punch and coffee from 1995 to 1996.
About 10 years later, Chen and colleagues asked the study participants whether they had been diagnosed with depression since 2000. A total of 11,311 depression diagnoses were made, Chen said.
People who drank more than four cans or cups per day of soda were 30 percent more likely to develop depression than those who drank no soda, while those who drank four cans of fruit punch per day were about 38 percent more likely to develop depression than those who did not drink sweetened drinks, Chen said.
People who drank four cups of coffee per day were about 10 percent less likely to develop depression than those who drank no coffee. The risk appeared to be greater for people who drank diet than regular soda, diet than regular fruit punches and for diet than regular iced tea, Chen said.
"Our research suggests that cutting out or down on sweetened diet drinks or replacing them with unsweetened coffee may naturally help lower depression risk," Chen said in a statement. "More research is needed to confirm these findings, and people with depression should continue to take depression medications prescribed by their doctors."
The findings are scheduled to be presented at the American Academy of Neurology's 65th annual meeting in San Diego March 16-23.