To investigate, researchers from the Harvard School of Public Health in Boston followed nearly 55,000 nurses over 10 years. All the women, between 50 and 77 years old, were free of depression when the study began in 1996.
Over the next decade, five percent of the participants eventually developed clinical depression.
But researchers found the risk was the same regardless of how much DHA and EPA, two types of omega-3 fatty acids, women got from eating fish. Fish rich in omega-3s include salmon, trout, sardines and herring.
The researchers did find preliminary signs that a plant-based omega-3 called alpha-linolenic acid (ALA), found in walnuts, flaxseeds and canola oil, could play a role in mood.
For every increase of half a gram in daily intake of the substance, there was an 18-percent reduction in the risk of depression, although more studies are needed.
Previous studies have found mixed results as to whether taking omega-3 fatty acids can help depression symptoms.
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