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==What is Tryptophan?==

==What is Tryptophan?==

Tryptophan is an essential amino acid for dogs and cats. Tryptophan is classified as an [[Nutrition Glossary#Aromatic Amino Acids|aromatic amino acid]] and is both [[Amino Acids Overview - Nutrition|gluconeogenic and ketogenic]]. Unlike the other aromatic amino acids such as [[Phenylalanine and Tyrosine - Nutrition|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 Overview - Anatomy & Physiology|small intestine]] (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and plasma tryptophan is actively reabsorbed in the [[Nephron Microscopic Anatomy#Proximal Tubule|proximal tubule]] of the kidney.

Tryptophan is an '''[[Amino Acids Overview - Nutrition|essential amino acid]]''' for dogs and cats. Tryptophan is classified as an [[Nutrition Glossary#Aromatic Amino Acids|aromatic amino acid]] and is both [[Amino Acids Overview - Nutrition|gluconeogenic and ketogenic]]. Unlike the other aromatic amino acids such as [[Phenylalanine and Tyrosine - Nutrition|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 Overview - Anatomy & Physiology|small intestine]] (particularly the [[Jejunum - Anatomy & Physiology|jejunum]]) and plasma tryptophan is actively reabsorbed in the [[Nephron Microscopic Anatomy#Proximal Tubule|proximal tubule]] of the kidney.

==Why is it Important?==

==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 [[Vitamin B3 (Niacin) - Nutrition|niacin]], key metabolic coenzymes nicotinamide adenine dinucleotide  (NAD) and nicotinamide adenine dinucleotide phosphate (NADP); and it is the precursor the neurotransmitters serotonin and [[melatonin]]<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biohemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 276.</ref>.

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 [[Vitamin B3 (Niacin) - Nutrition|niacin]], key metabolic coenzymes nicotinamide adenine dinucleotide  (NAD) and nicotinamide adenine dinucleotide phosphate (NADP); and it is the precursor the [[Neurotransmitters - Anatomy & Physiology|neurotransmitters]] serotonin and [[melatonin]]<ref name="Stipanuk">Stipanuk MH and Watford M. Amino acid metabolism. In Biohemical and physiologic aspects of human nutrition. 2000 Philidelphia, PA: WB Saunders Company p. 276.</ref>.

==Roles in the Body==

==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)<ref name="Ikeda">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.</ref>. Increased intake of tryptophan is able to lower the dietary requirement of niacin in dogs<ref>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.</ref>, 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<ref name="Ikeda"/><ref>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.</ref>. '''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'''.  

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 (Niacin) - Nutrition|vitamin B3]])<ref name="Ikeda">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.</ref>. Increased intake of tryptophan is able to lower the dietary requirement of niacin in dogs<ref>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.</ref>, 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<ref name="Ikeda"/><ref>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.</ref>. '''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 - Anatomy & Physiology|pineal gland]] serotonin is methylated to form melatonin, which plays a role in regulating diurnal rhythms<ref name="Stipanuk"/>. 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<ref>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.</ref>. An additional study looking at anxiety in dogs showed no effect with tryptophan supplementation<ref>Bosch G, et al. Dietary tryptophan supplementation in privately owned mildly anxious dogs. Appl Anim Behav Sci 2009;121:197-205.</ref>.

Tryptophan is a precursor to 5-hydroxytryptophan, which is further decarboxylated to form the neurotransmitter serotonin; in the [[Pineal Gland - Anatomy & Physiology|pineal gland]] serotonin is methylated to form melatonin, which plays a role in regulating diurnal rhythms<ref name="Stipanuk"/>. 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<ref>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.</ref>. An additional study looking at anxiety in dogs showed no effect with tryptophan supplementation<ref>Bosch G, et al. Dietary tryptophan supplementation in privately owned mildly anxious dogs. Appl Anim Behav Sci 2009;121:197-205.</ref>.

==Dietary Sources==

==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<ref>Dworschak E. Nonenzyme browning and its effect on protein nutrition. Crit Rev Food Sci 1980;13:1-40.</ref>.

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 [[Nutrition Glossary#Maillard Reaction|Maillard reaction]] products during heat processing, which can limit dietary tryptophan [[Nutrition Glossary#Digestibility|digestibility]] and absorption<ref>Dworschak E. Nonenzyme browning and its effect on protein nutrition. Crit Rev Food Sci 1980;13:1-40.</ref>.

==Diagnosing Tryptophan Deficiency==

==Diagnosing Tryptophan Deficiency==

==References==

==References==

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[[Category:Amino Acids]]

[[Category:Amino Acids]]