One of my biggest jobs in this specialty is clarifying what vegetarian eating IS...and what it is NOT. Unfortunately, the vast majority of people I know who are vegetarian define that by describing what they DON'T eat, rather than what they DO eat. That is why the word "vegetarians" is in quotes in my title, because it refers to what many vegetarians consider the definition, and that is absolutely not what I define it as.
My definition of vegetarian is a person who replaces the essential nutrients found in animal based foods with non-animal sources.
People who don't eat fish have an incredibly difficult time getting omega-3 fatty acids, because they are primarily found in seafood. In addition, if they're eating more salads, thinking they are "healthy", they may be getting excessive amounts of the proinflammatory omega-6 fatty acids, which are often the base for commercial salad dressings.
"Vegetarians" with a more disordered bent to their habits, who are filling up on baked goods and processed foods, are also prone to excessive omega-6 fatty acids.
Which may explain the findings of this most recent study. Women experiencing psychological distress and symptoms of depression were divided into two groups. (It is my experience that of the two genders, women are the guiltiest when it comes to not eating meat and subsisting on salads and carbs.) The first group received 1.05 grams of EPA (a pretty hefty dose, given that most fish oil capsules have only 20-30% of that amount) plus .15 grams DHA. The second group received a placebo. They received this dose for 8 weeks.
The women on the fish oil supplement showed a degree of decrease in symptoms that the women on placebo did not.
Of the two omega-3 fatty acids found in fish oil, EPA is the one that is primarily found in fish. So if you're not eating fish, and you are having trouble with depression, chances are your food choices have something to do with that!
If you can't, off the top of your head, list five significant sources of DHA, your only other source of EPA (it can be converted when DHA stores are sufficient and there is excess in your diet), you're not getting enough. For a list of food products containing marine-algae based DHA...click here.
And, if your diet is heavy on processed foods and salads, and you are using salad dressings based on soybean or corn oil...you're likely breaking down whatever omega-3's are in your system before you can even benefit from them.
Maybe now you can see why I'm so fussy about where the line should officially be drawn between vegetarian and omnivore. It's not at all about what you don't eat...it's about what you DO eat.
Lucas M, Asselin G, Mérette C, Poulin MJ, Dodin S. Ethyl-eicosapentaenoic acid for the treatment of psychological distress and depressive symptoms in middle-aged women: a double-blind, placebo-controlled, randomized clinical trial. Am J Clin Nutr. 2009 Feb;89(2):641-51. Epub 2008 Dec 30.