Tryptophan and L-tryptophan refer to the essential amino acid known for its role in the production of serotonin, an important neurotransmitter for regulating mood, sleep, and other brain functions. Tryptophan is also a precursor to melatonin, which regulates sleep-wake cycles.

• L-tryptophan specifies the biologically active form of the amino acid. Amino acids can exist in two enantiomeric forms, L (Levo) and D (Dextro), which are mirror images of each other. The L-form is the one utilized and recognized by human biological systems for the synthesis of proteins and neurotransmitters. Therefore, when discussing supplementation or biological functions, the L-form is referred to.
• Tryptophan. Without the "L-" prefix, this term refers to the amino acid in general, without specifying its spatial orientation. However, in practice, when tryptophan is mentioned in contexts related to nutrition, health, or biology, it almost always implies L-tryptophan, as it is the form that actively participates in biological processes in the human body.

In summary, although "tryptophan" may be used in a generic context, "L-tryptophan" is the more specific term that denotes the form of the amino acid active in the human body, involved in the synthesis of serotonin and melatonin, among other processes.

L-tryptophan is an essential amino acid used by the body for several crucial functions. Here's an overview of its applications and benefits.

• Serotonin Precursor. L-tryptophan is a precursor to serotonin, an important neurotransmitter for mood, sleep, and appetite regulation. Supplementing with L-tryptophan can help improve mood and combat depression (1).
• Improvement of Sleep Quality. Thanks to its ability to increase serotonin levels, which in turn can be converted into melatonin, the sleep hormone, L-tryptophan is often used to improve sleep quality and treat insomnia (2).
• Support for Immune Function. L-tryptophan plays a role in the immune system. Studies suggest it can help regulate immune response and keep the body protected from pathogens (3).
• Reduction of Anxiety and Stress. Supplementing the diet with L-tryptophan can help reduce levels of anxiety and stress, thanks to its influence on serotonin production (4).

Safety

 Although L-tryptophan is generally considered safe when taken in appropriate doses, it's important to consult a doctor before starting supplementation, especially for individuals with pre-existing medical conditions or those taking other medications.

References_________________________________________________________________________

(1) Turner EH, Loftis JM, Blackwell AD. Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan. Pharmacol Ther. 2006 Mar;109(3):325-38. doi: 10.1016/j.pharmthera.2005.06.004. Epub 2005 Jul 14. PMID: 16023217.

Abstract. This paper reviews the preclinical and clinical evidence regarding the use of the dietary supplement 5-hydroxytryptophan (5-HTP) for the treatment of depression. In the absence of supplementation with exogenous 5-HTP, the amount of endogenous 5-HTP available for serotonin synthesis depends on the availability of tryptophan and on the activity of various enzymes, especially tryptophan hydroxylase, indoleamine 2,3-dioxygenase, and tryptophan 2,3-dioxygenase (TDO). Factors affecting each of these are reviewed. The amount of 5-HTP reaching the central nervous system (CNS) is affected by the extent to which 5-HTP is converted to serotonin in the periphery. This conversion is controlled by the enzyme amino acid decarboxylase, which, in the periphery, can be blocked by peripheral decarboxylase inhibitors (PDIs) such as carbidopa. Preclinical and clinical evidence for the efficacy of 5-HTP for depression is reviewed, with emphasis on double-blind, placebo-controlled (DB-PC) trials. Safety issues with 5-HTP are also reviewed, with emphasis on eosinophilia myalgia syndrome (EMS) and serotonin syndrome.

(2) Doherty R, Madigan S, Warrington G, Ellis J. Sleep and Nutrition Interactions: Implications for Athletes. Nutrients. 2019 Apr 11;11(4):822. doi: 10.3390/nu11040822. 

Abstract. This narrative review explores the relationship between sleep and nutrition. Various nutritional interventions have been shown to improve sleep including high carbohydrate, high glycaemic index evening meals, melatonin, tryptophan rich protein, tart cherry juice, kiwifruit and micronutrients. Sleep disturbances and short sleep duration are behavioural risk factors for inflammation, associated with increased risk of illness and disease, which can be modified to promote sleep health. For sleep to have a restorative effect on the body, it must be of adequate duration and quality; particularly for athletes whose physical and mental recovery needs may be greater due to the high physiological and psychological demands placed on them during training and competition. Sleep has been shown to have a restorative effect on the immune system, the endocrine system, facilitate the recovery of the nervous system and metabolic cost of the waking state and has an integral role in learning, memory and synaptic plasticity, all of which can impact both athletic recovery and performance. Functional food-based interventions designed to enhance sleep quality and quantity or promote general health, sleep health, training adaptations and/or recovery warrant further investigation.

(3) Platten M, Nollen EAA, Röhrig UF, Fallarino F, Opitz CA. Tryptophan metabolism as a common therapeutic target in cancer, neurodegeneration and beyond. Nat Rev Drug Discov. 2019 May;18(5):379-401. doi: 10.1038/s41573-019-0016-5. 

Abstract. L-Tryptophan (Trp) metabolism through the kynurenine pathway (KP) is involved in the regulation of immunity, neuronal function and intestinal homeostasis. Imbalances in Trp metabolism in disorders ranging from cancer to neurodegenerative disease have stimulated interest in therapeutically targeting the KP, particularly the main rate-limiting enzymes indoleamine-2,3-dioxygenase 1 (IDO1), IDO2 and tryptophan-2,3-dioxygenase (TDO) as well as kynurenine monooxygenase (KMO). However, although small-molecule IDO1 inhibitors showed promise in early-stage cancer immunotherapy clinical trials, a phase III trial was negative. This Review summarizes the physiological and pathophysiological roles of Trp metabolism, highlighting the vast opportunities and challenges for drug development in multiple diseases.

(4) Kikuchi AM, Tanabe A, Iwahori Y. A systematic review of the effect of L-tryptophan supplementation on mood and emotional functioning. J Diet Suppl. 2021;18(3):316-333. doi: 10.1080/19390211.2020.1746725. 

Abstract. L-tryptophan (TRP), one of the essential amino acids in humans, is a precursor of serotonin, and hence its intake is closely related to the suppression of depressed and anxious moods. We did a systematic review of RCTs to examine the effects of tryptophan intake on the mood of healthy adults by searching PubMed, the Cochrane Library, and Ichu-shi according to PRISMA guidelines. As a result, 11 RCTs met the criteria and were accepted. Four RCTs showed the effects of tryptophan intake on negative feelings and happy feelings in healthy individuals, with significant differences between the treatment and the control groups. This suggests that TRP intake may be an effective approach to decrease anxiety and increase positive mood in healthy individuals. On the other hand, the effectiveness of TRP for aggressive feelings was not recognized. Reviewing these 11 RCTs, we concluded that taking 0.14-3 g of TRP per day in addition to the usual meal can be expected to improve the mood of healthy individuals. In order to estimate the optimum amount of TRP intake more accurately, further studies need to be conducted with more appropriate settings of intake period, intake frequency, and intake method.