*3.5. Protein*

Limited research has investigated the effects of different levels of dietary protein on anxiety symptom severity or disorder prevalence (Figure 8). Two thirds of the 12 animal studies assessing the impact of protein malnutrition reported a worsening of anxiety symptoms. Four human experimental studies delivered a high protein diet; three studies, involving participants without anxiety disorders, reported no effect on anxiety symptom severity. One experimental study involving three participants with elevated anxiety symptoms reported improvement following a high protein, low carbohydrate diet [32]. A very small amount of research has been completed comparing plant and animal sources of protein. One observational study reported a benefit with higher intake of animal protein [33], the other reported no difference [34]. The food additive monosodium glutamate (MSG), a modified amino acid, was associated with increased levels of anxiety in animal models.

**Figure 8.** Studies assessing protein and anxiety. ■ Higher intake or levels associated with decreased anxiety. ■ No association between intake or levels and anxiety. ■ Higher intake or levels associated with increased anxiety.

There was an association between higher anxiety symptoms and tryptophan depletion, among human experimental studies. Supplementation of tryptophan resulted in decreased anxiety symptoms in both animal and experimental studies. Of the studies assessing the effect of tryptophan depletion, 10 involved individuals with anxiety disorders; five reported no effect while five reported a worsening of anxiety symptoms. Of the studies administering a tryptophan supplement, two involved individuals with anxiety disorders [35,36]; both studies reported improvement.
