The green tea amino acid, L-theanine (L-THE) is associated with several health benefits, including improvements in mood, cognition and a reduction of stress and anxiety-like symptoms. This systematic review evaluated the effect of pure L-THE intake, in the form of orally administered nutritional supplements, on stress responses and anxiety levels in human randomised controlled trials. Following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) checklist, nine peer-reviewed journal articles were identified where L-THE as a supplement was compared to a control. Our findings suggest that supplementation of 200-400 mg/day of L-THE may assist in the reduction of stress and anxiety in people exposed to stressful conditions. Despite this finding, longerterm and larger cohort clinical studies, including those where L-THE is incorporated into the diet regularly, are needed to clinically justify the use of L-THE as a therapeutic agent to reduce stress and anxiety in people exposed to stressful conditions.
Tea has been consumed for thousands of years and is an integral part of people's daily routine, as an everyday drink and a therapeutic aid for health promotion. Consumption of tea has been linked to a sense of relaxation commonly associated with the content of the non-proteinogenic amino acid theanine, which is found within the tea leaves. The aim of this review article is to outline the current methods for synthesis, extraction and purification of theanine, as well as to examine its potential benefits related to human health. These include improvements in cognitive and immune function, cancer prevention, reduced cardiovascular risk and its potential usefulness as a functional food product.The relative concentration of L-THE in the Camellia sinensis plant varies between the plant's structures, over maturation and through the growth period. Recent studies have indicated that L-THE is distributed across the entirety of the plant with concentrations ranging between 1.2 and 6.2 mg/g fresh weight, with higher concentrations being expressed in the roots (6.2-13.7 mg/g). The biosynthesis of L-THE is proposed to occur in the roots of the plant and it is then transported towards the leaves [11]. The shading treatment and nitrogen fertilisation have been shown to influence the L-THE levels and the total free amino acid content in the Camellia sinensis plant [12]. Although most often related to GT, L-THE is present at similar levels in other types of teas made from the plant, including black, white and oolong teas [13]. Chemical, Physical and Flavour PropertiesL-THE is a water soluble non-proteinous amino acid [14] containing a glutamine backbone within the core of L-THE as well as existing as an ethylamide derivate of glutamate [15]. It is stable in acidic conditions but yields glutamic acid and ethylamine during base hydrolysis [8,16]. L-THE is insoluble in organic solvents such as chloroform and methanol, which facilitates the easy separation of L-THE from caffeine, catechins and other lipophilic tea constituents [8]. Aqueous L-THE solutions (1% (w/v)) stabilised at pH 5-6, were found to be stable (>1 year) under normal environmental conditions [8,16,17] and have a boiling point higher than that of water (range 214-216˝C) [8]. Furthermore, its systematic nomenclature is described as (2S)-2-amino-5-(ethylamino)-5-oxopentanoic acid (C 7 H 14 N 2 O 3 , M.W. = 174.2 g/mol) [18]. and L-glutamic acid-gamma-ethylamide have also been used to denote L-THE and it is also available under the proprietary name Suntheanine ® [5,19]. Similar to other amino acids in nature, theanine is a chiral species and occurs predominantly as the L-(S) enantiomer (Figure 1), whereas a synthetically derived theanine is typically a racemic mix of L-and D-enantiomers. As such, the use of synthetically derived theanine may not necessarily exhibit the same physiological effects as theanine found "naturally" occurring in foods [8].
l‐Theanine (l‐THE) is a nonproteinogenic amino acid derived from green tea (Camellia sinensis), which exhibits strong antioxidant‐like properties and contributes to the favourable umami taste sensation. Several studies have reported that the consumption of this amino acid has many therapeutic effects, including improvements in brain and gastrointestinal function, cancer drug therapeutic efficacies, antihypertensive effects, and improved immune function. Considering the recent Western commercialisation and popularity of green tea consumption as a nootropic agent in humans, the aims of this review were to consolidate the existing knowledge from ex vivo and in vitro animal models and attempt to highlight the applicability of l‐THE towards the human clinical trials. Considering the anti‐inflammatory and antioxidants effects of l‐THE presented in the current review, further research must translate the existing knowledge gained from animal and cell models to exploring the potential metabolic health benefits and moderating effects on the pathogenesis of conditions such as obesity, arthritis, depression, and type 2 diabetes in human trials. This will bridge the gap in literature and provide more insights into the mechanisms driving pathologies characterised by the inflammatory response and oxidative stress.
Consumption of L-Theanine (L-THE) has been associated with a sensation of relaxation, as well as a reduction of stress. However, these physiological responses have yet to be elucidated in humans where L-THE is compared alongside food or as a functional ingredient within the food matrix. The aim of this study was to determine the physiological responses of a single intake of a potential functional food product (mango sorbet) containing L-THE (ms-L-THE; 200 mgw/w) in comparison to a flavour and colour-matched placebo (ms). Eighteen healthy male participants were recruited in this randomised, double-blind, placebo-controlled trial. The participants were required to consume ms-L-THE or placebo and their blood pressure (BP) (systolic and diastolic), heart rate (HR), and heart rate variability (HRV) were monitored continuously over 90 minutes. Eleven males (age 27.7 ± 10.8 years) completed the study. Changes in area under the curve for systolic and diastolic blood pressure and HRV over the 90 minute observation period indicated no differences between the three conditions (all p > 0.05) or within individual groups (all p > 0.05). The values for heart rate were also not different in the placebo group (p = 0.996) and treatment group (p = 0.066), while there was a difference seen at the baseline (p = 0.003). Based on the findings of this study, L-THE incorporated in a food matrix (mango sorbet) demonstrated no reduction in BP or HR and showed no significant parasympathetic interaction as determined by HRV high-frequency band and low-frequency/high-frequency ratio. Further studies should be focussed towards the comparison of pure L-THE and incorporation within the food matrix to warrant recommendations of L-THE alongside food consumption.
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