Mechanism of Action
After crossing the blood-brain barrier via the large neutral amino acid transporter, L-Tryptophan is converted to 5-hydroxytryptophan (5-HTP) by tryptophan hydroxylase, then to serotonin by aromatic amino acid decarboxylase. Serotonin is the primary neurotransmitter governing mood, anxiety, aggression and sleep quality. L-Tryptophan is also the precursor to melatonin through sequential enzymatic steps in the pineal gland, making it relevant to sleep disruption associated with anxiety or ageing. Dietary protein competes with tryptophan for the blood-brain barrier transporter, which is why the ratio of tryptophan to other large neutral amino acids in the diet affects central serotonin synthesis.
Evidence Summary
L-Tryptophan has been studied directly in dogs. A study by DeNapoli et al (2000, Journal of the American Veterinary Medical Association) found that tryptophan supplementation combined with a low-protein diet reduced dominance aggression scores in dogs. A further study by Bosch et al (2009, Journal of Nutritional Science) found tryptophan supplementation reduced anxiety-related behaviours in dogs exposed to standardised stressors. These are among the only randomised dog-specific studies for any ingredient in the Stress and Anxiety formula.
In Petz Park Products
Stress and Anxiety for Dogs: L-Tryptophan 85mg per scoop. Note: L-Tryptophan is also present in the Stress and Anxiety for Cats formula. The cat formula is currently being updated and the confirmed dose will be added here once finalised.
Safety and Considerations
L-Tryptophan is safe at supplemental doses in dogs and cats. It should be used with caution in animals on serotonergic medications including SSRIs, MAOIs or tramadol due to the theoretical risk of serotonin syndrome at pharmacological doses. At supplemental doses the risk is low but veterinary oversight is recommended in any animal on these drug classes.