Tryptophan - Nutrition

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What is Tryptophan?

Tryptophan is an essential amino acid for dogs and cats. Tryptophan is classified as an aromatic amino acid and is both gluconeogenic and ketogenic. Unlike the other aromatic amino acids such as phenylalanine and tyrosine, tryptophan has a nitrogen-containing indole side ring. Dietary tryptophan is absorbed by a neutral amino acid transporter in the small intestine (particularly the jejunum) and plasma tryptophan is actively reabsorbed in the proximal tubule of the kidney.

Why is it Important?

Trypotophan is a component of structural proteins and the presence of the hydrophobic side-chain induces a fold in the protein structure. In addition to being required for normal growth, tryptophan is a precursor of niacin, key metabolic coenzymes nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP); and it is the precursor the neurotransmitters serotonin and melatonin[1].

Roles in the Body

NAD and NADP are formed during the degradation of tryptophan in both dogs and cats, though the major dietary precursor of NAD and NADP in both species is niacin (i.e. vitamin B3)[2]. Increased intake of tryptophan is able to lower the dietary requirement of niacin in dogs[3], but cats have a much lower production of NAD/NADP from tryptophan relative to dogs and tryptophan intake has very little impact on cat niacin requirements[2][4]. Neither dogs nor cats are able to produce sufficient amounts of NAD/NADP from tryptophan metabolism and a source of dietary niacin is still required by both species.

Tryptophan is a precursor to 5-hydroxytryptophan, which is further decarboxylated to form the neurotransmitter serotonin; in the pineal gland serotonin is methylated to form melatonin, which plays a role in regulating diurnal rhythms[1]. Supplementation of tryptophan has been evaluated as a behaviour modifying agent in dogs, although results have been inconsistent. One study feeding varied levels of dietary protein with and without tryptophan supplementation found that feeding a lower protein diet supplemented with higher levels of tryptophan may decrease territorial aggression in dogs, but these same dietary changes exerted no affect in dominance aggression or anxiety[5]. An additional study looking at anxiety in dogs showed no effect with tryptophan supplementation[6]. Tryptophan supplementation has also been shown to cause a transient increase in food intake in dogs[7].

Consequences of Tryptophan Deficiency

Dogs:

Puppies fed a tryptophan-deficient diet experience decreased food intake and weight loss[8].

Cats:

Feeding of a tryptophan-deficient diet to kittens resulted in decreased food intake and weight loss[9].

Toxicity

There are no reports of either acute or chronic toxicity related to feeding high doses of tryptophan in dogs. Although no adverse effects of feeding tryptophan at 10x the requirement was reported in kittens[10], feeding adult cats a diet containing 60g tryptophan/kg as fed (more than 50 times requirement) resulted in decreased food intake, and one cat died after prolonged feeding (42d)[11].

Dietary Sources

Sufficient tryptophan is found in plant and animal protein sources, such as muscle meat, eggs, dairy protein (e.g. casein), cereal grains, and pulses (i.e. legumes). Tryptophan also readily forms Maillard reaction products during heat processing, which can limit dietary tryptophan digestibility and absorption[12].

Diagnosing Tryptophan Deficiency

Diagnosis of tryptophan deficiency is based on fasted plasma amino acids.

References

  1. 1.0 1.1 Stipanuk MH and Watford M. Amino acid metabolism. In Biohemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 276.
  2. 2.0 2.1 Ikeda M, et al. Studies on the biosynthesis of nicotinamide adenine dinucleotide. II. A role of picolinic carboxylase in the biosynthesis of nicotinamide adenine dinucleotide from tryptophan in mammals. J Biol Chem 1965;240:1395-1401.
  3. Singal SA, et al. The role of tryptophan in the nutrition of dogs on a nicotinic acid deficient diet. J Biol Chem 1948;176:1051-1062.
  4. Da Silva, AC, et al. The domestic cat as a laboratory animal for experimental nutrition studies. III. Niacin requirement and tryptophan metabolism. J Nutr 1952;46:399-409.
  5. DeNapolo JS, et al. Effect of dietary protein content and tryptophan supplementation on dominance aggression, territorial aggression, and hyperactivity in dogs. JAVMA 2000; 217:504–508.
  6. Bosch G, et al. Dietary tryptophan supplementation in privately owned mildly anxious dogs. Appl Anim Behav Sci 2009;121:197-205.
  7. Fragua V, et al. Preliminary study: voluntary intake in dogs during tryptophan supplementation. B J Nutr 2011;106:S162-S165.
  8. Burns RA and Milner JA. Threonine, tryptophan and histidine requirements of immature beagle dogs. J Nutr 1982;112:447-452.
  9. Rogers QR and Morris JG. Essentiality of amino acids for the growing kitten. J Nutr 1979;109:718-723.
  10. Taylor TP, et al. Optimizing the pattern of essential amino acids as the sole source of dietary nitrogen supports near maximal growth in kittens. J Nutr 1996;126:2243-2252.
  11. National Research Council (NRC). Protein and Amino Acids. In Nutrient Requirements for Dogs and Cats. 2006 Washington, DC: National Academies Press p. 131-132.
  12. Dworschak E. Nonenzyme browning and its effect on protein nutrition. Crit Rev Food Sci 1980;13:1-40.



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