Our study design was a parallel arm, two-week randomized controlled trial. Adult men and women who reported consuming meat and/or poultry at least once a day were recruited for this trial, which was announced as an investigation of the role of protein in brain function. Individuals who were pregnant or lactating, drank more than 12oz per day of alcoholic beverages, diagnosed with a mental disorder, or used substances that modulate mood were excluded. Participants (n = 39) provided written informed consent, and the study was approved by the Institutional Review Board at Arizona State University.
A general health history was completed at baseline; at baseline and at trial completion, dietary fatty acid intake and mood were assessed using a validated food frequency questionnaire [7
] and two validated self-report mood scales: the Depression Anxiety Stress Scales (DASS) [8
] and the Profile of Mood States (POMS) questionnaire [9
]. The DASS was designed to measure three distinct but related negative affective states in nonclinical and research populations: depression (DASS-D) assesses dysphoria and anhedonia; anxiety (DASS-A) assesses autonomic arousal and subjective anxiety; and stress (DASS-S) assesses nervous arousal and agitation. The POMS (Educational and Industrial Testing Service, San Diego, CA) is one of the most widely used and accepted mood scales in healthy populations and estimates the intensity of mood disturbance covering six mood domains: tension-anxiety (POMS-T); depression-dejection (POMS-D); anger-hostility (POMS-A); fatigue-inertia (POMS-F); confusion-bewilderment (POMS-C); and vigor-activity (POMS-V).
At baseline, participants were randomized in block sizes of three to ensure equal balance in the following diet groups: omnivore (OMN), fish (FISH), or vegetarian (VEG). OMN participants were directed to continue consuming meat and/or poultry at least once daily. FISH participants were directed to avoid meat and poultry and consume at least 3-4 servings of seafood weekly (eggs were permitted). VEG participants were directed to avoid all animal foods except dairy for the 2-wk trial period. Participants were given written diet instructions and directed to maintain their activity pattern and lifestyle habits. The research staff also administered a brief computer-based cognitive test at baseline and at trial completion to mask our focus on mood. Participants were contacted at least twice during the study period to promote diet compliance. A brief survey assessing diet compliance and physical side effects was administered at trial completion.
Descriptive statistics were reported for population characteristics (mean ± SE); all outcome measures were presented as medians with interquartile ranges (IQR) as data were not normally distributed. The DASS data were normalized by removal of one outlier (from VEG group; data point was > 3SD from mean) and transformed by square root function. The transformed DASS data were analyzed using univariate ANOVA to compare 2-wk change scores between groups. For all other data, non-parametric analyses were performed using the Kruskal Wallis test to compare 2-wk change scores between groups. The Chi-square test and one-way ANOVA test were used to examine comparisons between groups at baseline. Spearman's correlation was used to assess relationships between variables. The Statistical Package for the Social Sciences (SPSS 17 for Windows, 2009, Chicago IL) was used for all analyses and p ≤ 0.05 was considered significant.