What is tryptophan
Tryptophan is an essential aromatic amino acid that has an α-amino group, an α-carboxylic acid group and an indole side chain that supports a cat’s overall health.
This neutral amino acid has the l-tryptophan and its d- tryptophan isomer and once absorbed in the small intestine (jejunum), it “competes with other amino acids for a carrier” in going through the blood-brain barrier notes Veterinary Practice.
Therefore, the presence of tyrosine, valine, leucine, phenylalanine, and isoleucine affects its relative uptake.
Some of its dietary sources include eggs, salmon, poultry, spinach, milk, soy products, nuts and seeds (pumpkin, squash, chia, sunflower, sesame, chew nuts, etc.), shellfish, beef, oats, among many others.
Cats need at least 0.25% for growth and reproduction and 0.16% for adult maintenance based on dry matter basis or 0.63g and 0.40g per 1000 kcal of cat food respectively 
Note that food processing may result in mallard reactions where a cross-linkage of amino acids and molecules of sugar occurs commonly noted in only tryptophan but also in lysine and proline. This will consequently minimize their bioavailability.
Deficiency and diagnosis
Weight loss and reduce food intake are noted in kittens in case of a deficiency and diagnosis is done on the basis of fasted plasma amino acids.
Tryptophan benefits or function
Tryptophan is used in protein synthesis and when the excess amount is available over other amino acids, it can work as a therapeutic supplement that increases serotonin secretion.
Therefore, it is a precursor of serotonin which is a neurotransmitter (works in the brain). According to PetMD, it helps in “behavior, awareness of pain, appetite, movement, body temperature, and function of the heart and lungs.”
Serotonin also helps help in mood balancing or control, reducing aggression, stress, and anxiety and thereby promoting calmness, a balanced mood, and other behavioral disorders. It may also help induce sleep in insomniac cats  and promote better sleep.
Signs of stress in cats include urine spraying, aggression, compulsive licking, not eat, hiding, excessive scratching, and vocalization, among others. They can be due to loud noises, thunderstorms, veterinary visits, boredom, grooming, new people, travel, small space, competition for resources, environmental change, separation and so on.
A research on the ‘Effect of dietary intake of L-Tryptophan supplementation on multi-housed cats presenting stress-related behaviors’ concluded that “supplementation had an effect in changing the frequency of the stress-related behaviors, decreasing anxiety signals.”
Secondly, it is a precursor for niacin (needed for serotonin production), nicotinamide adenine dinucleotide phosphate (NADP), nicotinamide adenine dinucleotide (NAD) as well as melatonin.
However, NADP and NAD are the principal sources are niacin or vitamin B3.
Tryptophan cat treats, supplements and chews
Most of these supplements are in the form of treats or chews except a few which are actual foods such as Royal Canin’s Feline Calm.
Besides this amino acid, most contain other vitamins, other amino acids, herbs such as meadow saffron, monkshood, silver nitrate, oat straw, jimson weed, l-theanine, ginger, catnip, valerian root, St. John’s wort, passionflower, hops, among others.
If you are looking for tryptophan for cats, try any of the following:
1. Royal Canin Feline Calm
This is not a supplement but a dry cat food that contains at least 0.34% tryptophan. It also has at least 9% crude fats and 34% crude protein. It is ideal for “cats facing stress-related digestive, skin and urinary sensitivities,” notes its manufacturer, Royal Canin
It also contains vitamin B3 or niacin, biotin, vitamin B6, vitamin B12, L-carnitine, rosemary extracts, marigold extracts, folic acid and trace minerals (zinc, manganese, copper, iron, iodide, selenium-containing compounds).
2. ThunderWunders Calming Chews
It also has thiamine and chamomile and will be helpful in relieving cat stress from various causes including separation, vet visits, thunderstorms, fireworks, travel and so on.
3. Nature’s Sunshine Nutr-calm
Found in the UK and also has PABA, passionflower and B vitamins (B6, B3, B1, B12, and inositol or B8 vitamin), bee pollen, lemon bioflavonoids, hops flower, inositol, choline, schizandra fruit, valerian root extract, among other ingredients.
According to VetRxDirect, it ensures a “healthy nervous system. It also works to help relieve anxiety, feelings of restlessness, and provide a sense of calm during stressful situations,” and maintains mental attitude.
ProQuiet is liver flavored for easy administration and it has essential amino acids, chamomile, B-vitamins, and L-tryptophan.
5. NutriCalm liquid
It has L-tryptophan, valerian root extract, catnip, and ashwagandha extracts, Kava Kava, calcium and magnesium for soothing and calming your cats.
6. Hemp Quiet Moments Cat Soft Chew
This is a calming aid which besides having l-tryptophan has melatonin, hemp seeds, ginger, omega-6 fatty acids, omega-3 fatty acids, thiamine mononitrate, and chamomile.
7. NaturVet –Quiet Moments Calming Aid for Cats Plus Melatonin
These soft chews help your feline friend to relax or calm down (reduce tension) in case of storms, travel, grooming, fireworks, and so on. Some of their ingredients include melatonin,
8. Sensible Pet Supplements ‘Calm’ for cats and dogs
It comes in capsule form and has L-tryptophan and L-theanine amino acids that will spur a sense of calmness in your feline friend in case of signs of stress due to vet visits, groomer visits, fireworks, loud noise, anxiety from separation, or any situation that is stressful.
9. Natural Relaxant – Calm and Relaxed – Pet Anxiety Relief
This Tryptophan cat treat will help in dealing with phobias and anxieties in cats and dogs due to things such as travel, separation, tension, motion sickness, vet visits, behavioral problems, environmental stressors and so on.
It raises brain serotonin levels that help reduce stress. Some of its ingredients include passionflower, chamomile, valerian, and l-tryptophan.
10. Feline Tranquil Formula with Tryptophan
These chewable chicken flavored tablets will stabilize your feline’s emotions. They are helpful during various stressing situations including fear of strangers, thunderstorms, or when your fluffy friend is aggressive.
Side effects of l-tryptophan are rare being an essential amino acid unless your pet is allergic to it or any other ingredients a specific brand may be having.
Possible side effects include vomiting, diarrhea, GI upsets and it may interact with other medications. Your vet will guide possible interactions.
Supplement dosages and toxicity
Dosages will be as advised by your vet or recommended by the manufacturer. Ensure you consult your vet before you begin on supplementation.
No toxicity was noted in kittens fed as much as 10 times the required amount but in adult cats, amounts 50 times normal diet can result in decreased food intake. There was an incidence of one cat who
died after prolonged feeding of more than 50 times the required amounts (NRC 2006).
Finally, PetMD lists this amino acid as a possible cause of serotonin syndrome in cats.
- National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 131-132.