Tryptophan

Tryptophan is an essential amino acid that dogs and cats must obtain from food, used to build proteins but best known as the precursor of serotonin, the neurotransmitter involved in regulating mood, behaviour and sleep. It is also the precursor of melatonin and, in many species, feeds into the synthesis pathway for niacin, or vitamin B3, though in cats this route is very limited, which is part of why cats have their own niacin requirement. Its link to serotonin has made tryptophan a focus of behavioural nutrition. Some so-called calming diets are enriched with tryptophan with the aim of supporting stress and anxiety management, for example during rehoming, travel or other stressful situations, although the scientific evidence for a clear effect remains moderate and variable across studies, so the benefit should not be overstated (peer-reviewed veterinary literature). Deficiency can impair growth and appetite. The richest sources are animal proteins such as turkey, meat, fish and egg. A practical subtlety is that how much tryptophan actually reaches the brain depends on its balance with other competing amino acids, not on its total intake alone, which complicates simple dose-response thinking. As one of the [essential amino acids](/glossary/essential-amino-acids), tryptophan adds a behavioural dimension to protein nutrition. See the [Petipedia glossary](/glossary).

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