Tryptophan

Description

Tryptophan is an essential amino acid. This means it must be obtained through the diet in adequate quantities to meet the body's needs.

Tryptophan is a precursor in the central nervous system of the neurotransmitter serotonin. Serotonin modulates mood and sleep patterns.

Tryptophan is also converted in the body to niacin (vitamin B-3) and picolinic acid. Giving high doses of vitamin B-6 along with tryptophan increases its conversion to niacin and decreases its uptake into the nervous system.

One standard test for vitamin B-6 deficiency is to supplement an individual with tryptophan and then look at the spill of xanthurenic acid in his or her urine. High levels of xanthurenic acid in the urine are indicative of the need for vitamin B-6. Low blood tryptophan levels have been reported in depressed patients and are corrected with tryptophan supplementation. Tryptophan has clinical impact on depression, particularly endogenous (unipolar) depression. Doses of 1,000-3,000mg divided throughout the day and taken on an empty stomach have been employed.

Tryptophan has been shown in double-blind trials to also be very helpful in treatment of insomnia when given in doses between 1,000 and 2,000mg per day one hour before bed. It has been found best to administer this amino acid along with a carbohydrate source such as fruit juice because insulin, which is secreted after the carbohydrate load, has been found to help tryptophan absorption into the nervous system. Giving tryptophan along with other neutral amino acids blocks its absorption and prevents an effective therapeutic result. Generally, tryptophan should be given away from a meal on an empty stomach along with a carbohydrate source for optimal therapeutic results.

See
L-tryptophan

Recommended Dietary Allowances

The RDA for tryptophan as been established as 16mg per day for women and 25mg per day for men. High doses of tryptophan have been shown, on occasion, to produce intestinal discomfort and nausea.

It is also known high doses of tryptophan which exceed the intestinal absorption efficiency can increase the risk of toxic reactions in the intestinal tract due to bacterial action, which produces substances such as indoxylsulfate.

Food Sources

Foods high in tryptophan include:

Cottage cheese - dry	450 mg/cup
Cottage cheese - creamed	336 mg/cup
Fish and other seafoods	800-1,300 mg/lb.
Meats	1,000-1,300 mg/lb.
Poultry	600-1,200 mg/lb.
Peanut, roasted with skin	800 mg/cup
Sesame seed	700 mg/cup
Dry, whole lentils	450 mg/cup

Method of Action

The metabolism of tryptophan is an interesting example of biological control of amino acids. Tryptophan in the gut can be converted by bacteria to indoxyl, which is then absorbed into the blood and sulfated by the liver to produce indoxyl sulfate in the urine, sometimes called indican. This is a substance related to amino acid malabsorption or intestinal toxicity reactions.

The primary breakdown pathway in humans is initiated by the enzyme tryptophan-2,3-dioxygenase. This enzyme has often been studied because of its regulation by hormone action, which is induced by glucocorticoids and the amount of tryptophan in the diet. This enzyme also produces oxidants such as singlet oxygen; therefore, activation of this enzyme is associated with increased risk to free radical pathology. This is one of the concerns about long-term, high-dose tryptophan therapy.

Tryptophan can also be converted to nicotinate mononucleotide, which produces NAD, a high energy-carrying substance used in energy metabolism. Also, this same pathway can manufacture nicotinic acid vitamin B-3 (niacin). Vitamin B-3 is also used as a source of NAD in the body and is about sixty times more efficient than tryptophan in this regard.

A high-tryptophan diet will partially overcome a deficiency in dietary niacin. Tryptophan is also the precursor of a variety of other metabolites, including serotonin, a neurotransmitter, and melatonin, a hormone secreted by the pineal gland which seems to be related to sleep.

Abstracts

References

Blackburn, G.L., J.P. Grant, V.R. Young. ed. Amino Acids Metabolism and Medical Applications.

Etienne, P., S.N. Young & T.C. Sourkes. Inhibition by Albumin of Tryptophan Uptake by Rat Brain. Nature. London, 262:14-5, 1976.

Mans, A.M., J.F. Biebuyck, S.J. Saunders, R.E. Kirsch & R.A. Hawkins. Tryptophan Transport Across the Blood-Brain Barrier During Acute Hepatic Failure. J. Neurochem., 33:409-18, 1979.

Munro, H.N. & M.C. Crim. The Proteins and Amino Acids. Modern Nutrition in Health and Disease, eds. R.S. Goodhart & M.E. Shils, Sixth Ed., Phila. Lea and Febiger, 1980.

Wurtman, R.J., Nutrients that Modify Brain Function. Scientific American. 246:50-9, 1982.

Young, V.R., M. Meguid, D.E. Meredith & D.M. Bier. Recent Developments in Knowledge of Human Amino Acid Requirements. Nitrogen Metabolism in Man. eds: J.C. Waterlow & J.M.L. Stephen. London: Applied Science Pub. 1981.