These results support the results of our cross-sectional study which found that vegetarians reported significantly better mood than their non-vegetarian counterparts . Although omnivores who consumed fish frequently and avoided meat and poultry for two weeks did not significantly improve their mood, those who adopted a lacto vegetarian diet did improve their mood. These data suggest that consuming a diet high in meat, fish, and poultry may negatively impact mental state. Beyond differences in the ratio of long-chain fatty acids, vegetarian diets are typically rich in antioxidants, potentially conveying mood protection for the VEG group via reduction of oxidative stress .
Early human evolution is theorized to have coincided with increasing reliance on animal source foods such as wild game and seafood , however, the average n-6 to n-3 fatty acid profile of modern grain-fed meat is 5X higher than grass-fed meat, a product similar to the wild game in our hunter-gatherer diet [15, 16]. The amount of meat and poultry consumed is important since very little AA is formed from LA ; in fact, diets high in short chain essential fatty acids down regulate conversion to longer chain metabolites, particularly AA, and experimental diets high in LA do not raise tissue AA [18, 19]. Preformed AA, however, is readily incorporated into tissues and competes for desaturases with EPA, increasing production of proinflammatory metabolites such as PGE2 and TNFa . This raises the risk for inflammation-based chronic diseases including depression since these metabolites are associated with altering mood-regulating mechanisms .
Participants in the FISH group consumed a relatively high fish intake (~270-364 mg/d of EPA/DHA vs the U.S. mean daily intake of ~88 mg/d) . Long-chain n-3 fatty acids incorporate steadily into brain phospholipids with a reciprocal decrease in n-6 fatty acid content . Numerous studies show mood protective effects of high fish intakes in nonvegetarian populations , however, mood scores in our FISH participants did not improve significantly within the study time frame.
Failure to monitor weight of the participants beyond the baseline BMI calculation was a study limitation, possibly missing weight fluctuations that could confound mood. Also, in light of the results, measures of blood fatty acid concentrations may have shown that long-chain n-3 intake and status in the vegetarians was not as disparate as expected, as observed in the recent EPIC-Norfolk study . Measures of blood inflammatory markers associated with mood would also have strengthened results.