Can A Vegetarian Diet Improve Mood?

Looks like it can.

Vegetarian Diets Are Associated With Healthy Mood States: A Cross-Sectional Study In Seventh Day Adventist Adults, Nutrition Journal, 2010

This was a cross-sectional study of 138 men and women who completed a food frequency questionnaire, Depression Anxiety Stress Scale (DASS), and Profile of Mood States (POMS) questionnaire. The objective was to compare the mood states of vegetarians (who don't eat omega-3 rich fish) with the mood states of omnivores.

Results:

Vegetarians (VEG) reported significantly less negative emotion than omnivores (OMN).

VEG reported significantly lower mean intakes of EPA, DHA, as well as the omega-6 fatty acid, arachidonic acid (AA), and reported higher mean intakes of shorter-chain α-linolenic acid (ALA, omega-3) and linoleic acid (LA, omega-6) than OMN.

Participants with low intakes of EPA, DHA, and AA and high intakes of ALA and LA had better mood.

Conclusion:

"While dietary intake of EPA and DHA has an important role in brain function, we found no evidence that the absence of direct intake of these fatty acids in vegetarians adversely affects mood state."

Where do vegetarians get EPA and DHA? The body can manufacture longer chain EPA and DHA (good source: fish) from shorter chain ALA (good source: flax). The rate of manufacture varies by a host of factors, e.g. gender, age, health state, and presence of other nutrients.

So, where was vegetarians' better mood coming from? If not from more EPA and DHA? They didn't say, not directly. Although the omnivores did eat significantly more arachidonic acid (AA), and:

"AA is a key substrate for the synthesis of proinflammatory eicosanoids and downstream cytokines, which can adversely impact mental health via a cascade of neuroinflammation."

AA is a relatively longer chain (20 carbons), very unsaturated (4 double bonds), omega-6 fatty acid. Very little is found in plants. Most comes from animal sources - meat, eggs, dairy. The body can also make it from linoleic acid (LA), an omega-6 found in high amounts in the ubiquitous corn and soy oils, but may not if there is a lot of ALA present (greens, flax).

Just because AA can be proinflammatory doesn't mean it always is. It depends on how the body treats it. However, people with arthritis tend to improve when they eat less meat, eggs, and dairy, foods high in AA.
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Here's a follow-up study, a more telling randomized control trial:

Preliminary Evidence That Vegetarian Diet Improves Mood, American Public Health Association Annual Conference, 2009.

In a pilot randomized controlled trial, we investigated the effect on mood of removing meat and poultry from the diet of healthy adult omnivores for two weeks. Thirty-nine participants were randomly assigned to one of three diet groups: the control group maintained regular intake of flesh foods (CON); the fish group consumed 3-4 servings/wk of seafood but restricted all meat and poultry (FISH), or the vegetarian group restricted all animal protein sources except for dairy (VEG). At baseline and at end of trial, participants completed a validated food frequency questionnaire (FFQ) and two validated psychometric scales, the Profile of Mood States (POMS) and the Depression Anxiety and Stress Scales (DASS).

Results: After 2 weeks, total mean scores for both psychometric tests declined significantly in VEG participants. Significant reductions in VEG were observed in total fats, dietary eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA), arachidonic acid, and protein intake. FISH participants significantly increased their EPA/DHA intakes and decreased their saturated fat intake but mood scores were not significantly reduced.

Conclusion: The complete restriction of flesh foods significantly reduced mood variability in omnivores. To our knowledge, this is the first trial investigating the mood effects of a vegetarian diet.

Here's Dr. Greger explaining both of these studies, as well as the contribution of arachidonic acid to mood state:


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