It is no secret that greasy fast foods like burgers, french fries and fried chicken are bad for human health, but new research has found that they are also linked to depression.

The study, published March 15 in the Public Health Nutrition journal and conducted by researchers at the University of Las Palmas de Gran Canaria in Spain, found that regular fast–food consumers were 51 percent more likely to develop depression than those who ate little or none.

Researchers studied the behavior of 8,964 individuals with no history of depression for six years. When the study ended in 2011, 500 of them, many who were frequent consumers of fast food, had been diagnosed with depression.

According to the study, participants who consumed the most fast food were also more likely to be single and not get adequate exercise or consume enough healthy foods, such as fruits, vegetables and Omega-3 fatty acids.

Although the research was conducted in Spain, some experts find its conclusions relevant to the U.S., a nation where fast food is a dominant cuisine and, according to the Centers for Disease Control and Prevention website, an estimated one in 10 adults is depressed.

Dr. Will Lassek, professor of epidemiology at the University of Pittsburgh and co-author of “Why Women Need Fat,” said he was not surprised by the study’s conclusions. He added that high levels of Omega-6 fatty acids, which have been linked to depression in the past, and the lack of brain beneficial Omega-3s in processed food are major contributors.

“The more fast food you eat, the less Omega-3 you’re getting and the more Omega-6,” Lassek said. “It’s the perfect storm for having a higher risk for depression.”

He said he perceives the study as a warning sign of a future in which more people than ever are depressed and overweight.

Academy of Nutrition and Dietetics spokeswoman Amy Jamieson-Petonic said she believes the results are plausible but thinks more research on other factors that could contribute to depression should be evaluated before the research is accepted as fact.

“I think the study is one piece of the puzzle, but more research needs to be done to see what is truly happening at a cellular level,” Jamieson-Petonic said. “We also need to tease out other factors, such as stress levels, to find why folks [who] eat fast food are more prone to depression.”

According to study critics, the research was too vague to be significant because it did not examine external factors, like overall diet, workload, social behavior, stress and smoking, closely enough.

Dr. Joy Dubost, director of Nutrition and Healthy Living at the National Restaurant Association, also said she believes the study was too quick to blame fast food for consumers’ problems.

“Fast food is often targeted in this kind of research,” Dubost said. “People speculate and draw these conclusions when the scientific evidence is very limited, and that causes a fear tactic that isn’t beneficial

to consumers.”

In defense of fast food, she noted that nutritious options on fast-food menus was one of the top trends the National Restaurant Association’s “What’s Hot” survey found in 2012. Dubost said fast-food restaurants like Subway, McDonalds and others have continued to incorporate more healthy foods on their menus.

“Across the industry, you really do see a movement with our trend data where healthier options are more readily available,” she said. “People are demanding it, and restaurants are providing it.”