This study evaluated the effects of a diet supplemented with α-casozepine and l-tryptophan on the stress response of anxious dogs. α-Casozepine has similarities to anxiolytic benzodiazepines, and tryptophan-induced serotonergic activity has been implicated in the regulation of mood, aggression, and susceptibility to stress. Forty-four privately owned dogs deemed anxious according to their Canine Behavioral Assessment and Research Questionnaire (C-BARQ) scores were fed a control diet for 8 weeks, then the study diet for 8 weeks after a 1-week transitional period. At the 7-week point of each period while the dogs were still on the respective diet, owners were asked to again complete the C-BARQ. In addition, 2 urine samples were obtained from each dog to measure the urine cortisol:creatinine ratio (UCCR), 1 under nonstressful home conditions (prestressor) and the second 2 hours after visiting a veterinary practice for nail clipping (poststressor).

After the dogs were on the study diet for 7 weeks, specific owner-reported anxiety-related behaviors, such as stranger-directed fear, nonsocial fear, and touch sensitivity, were reduced. This trend did not hold true for owner-directed aggression. Poststressor UCCR was increased from prestressor levels in dogs when fed either diet but was significantly lower when the study diet was fed as compared with the control diet. While the study diet appeared to be effective in reducing anxiety-related behavior and improved the ability to cope with stress, a placebo effect cannot be excluded. Further studies are needed to investigate whether the observed effects were attributable to the inclusion of one or both of these supplements. 

Commentary
A diet that lowers stress would be valuable to pet owner and veterinarian alike. Because there was only a small difference in tryptophan between the 2 diets fed, the milk-derived product α–casozepine was probably responsible for the difference in stress hormone release and owner perception of their dog’s fearfulness.Most of the dogs were toy breeds that are frequently poor patients and problem pets because their size leads them to be afraid of larger creatures in their world. The study diet is not currently available in the United States, but the active ingredient α-casozepine may be helpful.1—Katharine A. Houpt, DVM, PhD

Source
Effects of prescription diet on dealing with stressful situations and performance of anxiety-related behaviors in privately owned anxious dogs. Kato M,Miyaji K, Ohtani N, Ohta M. J VET BEHAV 7:21-26, 2012.

1. Effects of alpha-casozepine (Zylkene) versus selegiline hydrochloride (Selgian, Anipryl) on anxiety disorders in dogs. Beata C, Beaumont-Graff E, Diaz C, et al. J Vet Behav 2:175-183, 2007.