Dietary tryptophan supplementation in privately owned mildly anxious dogs

Food composition has been reported to influence mood and behaviour in humans and animals and it could help to reduce unwanted behaviour in dogs. Anxiety-related behaviour is associated with the functioning of the central serotonergic system and here it was investigated if dietary supplementation with the serotonin precursor tryptophan (Trp) affects behaviour in privately owned dogs. For 8 weeks, privately owned dogs were fed a control diet (n = 66) or a diet containing 2.6-fold more Trp than the control diet (n = 72), using a randomised double-blinded, placebo-controlled approach. A third diet fortified with Trp, beet pulp, salmon oil, soy lecithin, and green tea extract was studied for its potential in 69 dogs. Owners reported on their dogs’ behaviour in the home-situation by filling out a web-based questionnaire before the onset of dietary treatment and after 4 and 8 weeks of feeding the diets. Thirty-four dogs fed the control diet and 39 dogs fed the Trp diet were subjected to behaviour tests before and after 8 weeks of dietary treatment. The tests included open-field situations and owner-separation procedures and were set up to measure anxiousness. Blood was collected after 8 weeks from dogs in the control (15 dogs) and Trp (15) groups for evaluation of plasma amino acid concentrations. Dietary effects on behaviour were investigated for significance by means of testing interactions between diet and time, using Residual Maximum Likelihood. Intake of the Trp supplemented diet significantly increased plasma Trp concentrations by 37.4% and its ratio with large neutral amino acids by 31.2% compared to the control diet but owners did not report on behavioural changes that could be attributed to a specific dietary treatment. Also, the dogs’ responses in the behavioural tests, including those in saliva cortisol, were unaffected after 8 weeks of consuming the Trp supplemented food. A number of significant changes in both owner-reported assessments and behavioural responses did occur over time, possibly mirroring a placebo-effect and/or influences of a new diet regardless of its specific composition. It is concluded that intake of diets supplemented solely with Trp or in combination with beet pulp, salmon oil, soy lecithin, and green tea extract does not change (anxiety-related) behaviour in privately owned dogs that do not show clear signs of abnormal behaviour. The influence of dietary Trp intake on behaviour of pathological anxious or chronically stressed dogs remains to be established