Scientists have studied the link between what you eat and your mood for some time and a number of things seem clear from the research. Dark chocolate? Full of something that increases your sense of well-being. Fruits high in antioxidants? Also a possible  natural antidepressant. Fried foods? Not so much.

The impact of fish, however, had been somewhat of a mystery, with studies showing mixed results about whether consumption might help.

Now a new study published in the Journal of Epidemiology & Community Health on Friday involving 150,000 people provides the strongest evidence yet that fish may belong in the list of "good mood" foods that may stave off depression.

The research involved what's called a meta-analysis -- pooling data from 26 previously published studies from 2001 and 2014 -- and found that people eating the most fish had a 17 percent reduction in depression risk compared with those eating the least amount of fish. The reduction was seen in both men and women, but the effect was slightly stronger in men.

One theory of how this works has to do with the omega-three fatty acids in fish. Scientists think that they may boost the activity of dopamine (a neurotransmitter that works with the brain's rewards center) and seratonin (which plays a role in people's sense of happiness and well-being).

The researchers also suggested two other possibilities. Perhaps the high quality of protein vitamins and minerals in fish may help protect against depression. Or maybe it isn't necessarily the fish itself but just that people who eat a lot of fish tend to have more nutritious diets.

There's an asterisk in the study, however. The reduction in depression risk was only seen in the studies that took place in Europe -- and not in those in North America, South America, Asia and Oceania.

How's that possible? The researchers wrote that "differences in fish type, fish preservation and cooking styles" may be at play. Because the researchers weren't conducting their own studies from scratch they had limited data about the kind of fish people ate. There were also differences in the methodology each study used to measure fish consumption, which included everything from measuring the amount of fish in grams to how many times it was eaten per week. Some studies even included other kinds of seafood in their measurement of "fish."

Study author Dongfeng Zhang, a researcher in the department of epidemiology and health statistics at Qingdao University in China, and his colleagues wrote that further research is needed to determine whether the association they found varies according to the type of fish but that their conclusion is that "fish consumption may be beneficial in the primary prevention of depression."

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