Drinking more than four sweetened beverages a day is definitely not great for your waistline, but it also might not be good for your mood.

A new study by the U.S. National Institutes of Health finds that fruit and soft drink drinkers are more likely to be diagnosed with depression than those who didn't regularly drink sweetened beverages.

And interestingly, those who drink diet drinks made with artificial sweeteners were linked to an even higher likelihood of depression.

The study was not able to show that soft drinks actually cause depression -- only that the two appeared to be linked. But the study was a large one and the researchers say the link they spotted was significant.

Beginning in 1995, the researchers from the National Institute of Environmental Health Sciences had more 250,000 older adults who were between the ages of 50 to 71 to fill out questionnaires about the kinds of drinks they drank.

They then asked the participants again 10 years later about whether they had been diagnosed with depression since the year 2000. The participants reported a total of 11,311 depression diagnoses.

The researchers then looked back at their questionnaire responses and found that those who drank more than four cans of soda a day had a 30 per cent greater risk of depression than those who consumed none.

The same amount of fruit punch was tied to a 38 per cent higher risk. The risk was even greater for people who consumed diet drinks, whether soda, punch or iced tea.

Coffee, though, had the opposite effect. People who drank four cups of coffee each day had about a 10 per cent lower risk of developing depression than those who didn’t drink the stuff, the research found.

The findings were announced this week by the American Academy of Neurology and will be presented at the American Academy of Neurology’s annual meeting in San Diego in March.

Dr. Sean Wharton, an internal medicine physician at Toronto East General Hospital and the director of the Wharton Medical Clinic, notes that soft drinks – even diet drinks -- have been linked to many health problems, such as stroke and obesity. But he says it’s not clear in any of the studies if the drinks themselves were responsible.

“These are large association-based studies. So there no actual biological or scientific reason we have at this stage to think that this actually true,” he told CTV News Channel. “It may just be that those who drink higher amount of diet sodas have other things that are connected to having a higher risk of depression.”

He says such epidemiological studies try to eliminate as many confounding factors as they can, so they can control for other factors that might affect the results. But these approaches often miss important factors.

“There might have also been other factors that they don’t know about that may have been connected to these patients that make them drink diet sodas. So it may not be the diet soda but the other conditions that predispose them,” he said.

It’s also possible that the findings applied only to seniors -- the age group studied -- and younger populations might not have the same results.

Still, lead researcher Dr Honglei Chen, of the National Institutes of Health in North Carolina, said it might be a good idea to drink fewer sweetened and artificially sweetened drinks.

"Our research suggests that cutting out or down on sweetened diet drinks or replacing them with unsweetened coffee may naturally help lower your depression risk," he said in a statement.