In adult dogs is supplementary tryptophan in the diet effective in reducing signs of anxiety?

Question In adult dogs, is dietary supplementation with tryptophan compared with no dietary tryptophan supplement effective in reducing signs of anxiety?   Clinical bottom line The category of research question was: Treatment. The number and type of study designs that were critically appraised were: Two studies were found to be considered appropriate in the level of hierarchy of evidence pyramid. One study was a randomised double-blinded, placebo-controlled study and the other was a single-blind, placebo-controlled study. Critical appraisal of the selected papers meeting the inclusion criteria collectively provide zero/weak/moderate/strong evidence in terms of their experimental design and implementation: Moderate. The outcomes reported are summarised as follows… One study found no overall significant influence of tryptophan in the diet as an aid in reducing anxiety and fear-related behaviour in anxious dogs in household conditions. In contrast, the second study reported lower anxiety-related behaviour in dogs by owners but did not find significant differences in cortisol levels based on urine cortisol-to-creatinine ratio. In view of the strength of evidence and the outcomes from the studies the following conclusion is made… Current evidence reveals two studies that have evaluated the effectiveness of tryptophan as a supplement to the diets of anxious canines for pet owners. One study conducted did not find a significant effect or interaction in reducing anxiety-related behaviour whilst the second study reported lower anxiety-related behaviour in dogs by owners but did not find significant differences in cortisol levels based on urine cortisol-to-creatinine ratios. The use of tryptophan as a supplementary component in the diet is thus not enough as a sole treatment to assist in reducing anxiety in anxious adult dogs.