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5 Benefits and Side Effects of L-Theanine (3 Contraindications to Be Noted)

Due to its unique properties and beneficial functions, theanine has potential applications in the development of functional foods, such as as beverage ingredients or dietary supplements.

What are the benefits of theanine in empirical medicine? Are there any side effects or contraindications? For details, see the empirical discussion in the text

What is Theanine?

L-Theanine is a unique free amino acid, one of the most important secondary metabolites in the leaves of tea plants, giving tea a special caramel flavor, is the main source of tea aroma, and has a chemical structure similar to the nerve transmitter: glutamic acid

Similar to other natural amino acids, theanine is a chiral substance that exists mainly in the form of L-enantiomers (in this introduction, theanine refers to L-theanine).

Theanine is one of the important indicators to evaluate the quality of green tea, because its unique aromatic sweet taste helps to remove the bitterness of caffeine and the astringent nature of tea polyphenols, which greatly affects the flavor of tea

In tea plants, theanine is biosynthesized by glutamic acid and ethylamine by theanine synthetase, accounting for about 50% of the total amino acids of tea, about 1%-2% of the total dry weight of green tea, and a cup of green tea contains about 8-30 mg of theanine.

What are the recommended benefits of theanine?

1. Beneficial for generalized anxiety disorder

Generalized anxiety disorder is one of a family of anxiety disorders characterized by pervasive, diffuse anxiety associated with multiple areas, with symptoms including chronic, pervasive anxiety and apprehension, accompanied by nonspecific physical and psychological symptoms (irritability, fatigue, difficulty concentrating, irritability, muscle tension, or sleep disturbance).

Effective treatments include psychotherapy (usually cognitive-behavioral therapy) and pharmacotherapies such as selective serotonin reuptake inhibitors and serotonin-norepinephrine reuptake inhibitors.

It affects up to 5% of children and adolescents and 3% to 6% of adults, is twice as common in women as men, and is more common in people aged 45 to 59 years, and in addition to significant personal, social, and economic costs, untreated conditions increase the likelihood of secondary morbidity, including major depressive disorder, other anxiety disorders, and increased risk of suicide

A double-blind, randomized, placebo-controlled trial (10 weeks, 46 patients diagnosed with DSM-5 generalized anxiety disorder) noted that adjuvant L-theanine (450 to 900 mg per day) may be helpful in self-reported sleep satisfaction and milder symptoms of insomnia, although it failed to reduce the Hamilton Anxiety Scale (HAMA) and insomnia severity index. Note 1

*Conclusion: For generalized anxiety disorder, adjuvant L-theanine therapy is not significantly helpful in improving anxiety symptoms, and is limited by the small sample size, which needs to be further verified

2. Beneficial for depression

Major depressive disorder is one of the most common mental illnesses, manifested not only as severe disorders of affect and mood, but also with cognitive dysfunction, sleep and appetite disorders, fatigue, and other metabolic, endocrine, or inflammatory changes.

In addition to genetics (50% genetics), depression may also be caused by traumatic emotional experiences or other disease factors such as stroke, multiple sclerosis, Parkinson’s disease, Cushing’s disease, and hypothyroidism.

The prevalence of depression is higher in women due to hormonal factors (between 1.5 and 2.5 times that of men), especially in the premenstrual changes, pregnancy, miscarriage, postpartum period, and menopause stage.

An open-label study (8-week study of 20 patients with depression) showed that oral theanine preparations had beneficial effects on depression, anxiety, sleep disturbances, and cognitive impairment symptoms. Note 1

The mechanism behind theanine may be related to increasing levels of central nervous system inhibitory neurotransmitters such as glycine, glutamate, and gammaminebutyric acid (GABA).

*Conclusion: Theanine has a positive help in patients with depression, but it is limited by the small sample size and needs to be further verified by more large placebo-controlled clinical trials

3. Enhance pressure resistance

Stress is defined as a response to intrinsic or extrinsic stimuli that cause homeostasis to drift, and prolonged exposure to stress increases the risk of various health problems and concerns, such as mental health problems, substance abuse, and cardiovascular problems, and can even weaken the immune system, increasing susceptibility to colds and common infections.

In everyday life, there are many stressful situations such as: financial, family responsibilities, work stress, exams, psychological stress, trauma, surgery and physical stress caused by various diseases.

Different stressors affect hormones and the nervous system, leading to a variety of abnormal behaviors and physiological reactions such as: irritability, anxiety, low hope, loss of cognitive function, changes in appetite or sleep patterns, irritability… wait

One randomized, double-blind, placebo-controlled, crossover trial (34 healthy adults) noted that theanine-based beverage intake helped reduce stress and salivary cortisol responses to multitasking stressor tests. Note 2

In addition, a significant increase in alpha waves in the brain was observed in people with high anxiety traits (but not related to the stress-reducing response described above)

*Conclusion: Theanine intake can help enhance stress resistance, but more large studies are still needed to further validate

4. Beneficial for schizophrenia

Schizophrenia is a chronic, disabling disorder often associated with dysfunction in the social, cognitive, and emotional spheres.

It usually occurs in early adulthood or late adolescence and therefore has a low incidence (15.2 per 100,000) but a relatively high prevalence (7.2 per 1000).

The main features of schizophrenia are: positive symptoms (hallucinations, delusions), negative symptoms (emotional oblusence, apathy), and cognitive impairment.

Anxiety disorders are also common in schizophrenia and are a strong predictor of subjective quality of life.

A double-blind, randomized, placebo-controlled study (8 weeks in 60 patients with schizophrenia and schizoaffective disorder treated with antipsychotics) noted that theanine improved anxiety, positive, and primal symptoms (measured by the Positive and Negative Syndrome Scale) compared with placebo. Note 3

*Conclusion: The combination of theanine in addition to formal drug therapy can improve the treatment outcome, but more large trials are needed to confirm the results

5. Prevent colds

The common cold, defined as an upper respiratory tract infection, affects any part of the mucous membranes of the nasal respiratory tract, with symptoms including sneezing, runny nose, headache and general malaise, and nearly half of patients experience a sore throat and 40% cough.

Cold symptoms usually last 1 to 2 weeks, and most patients feel better after the first week. Only a small percentage of cold cases do not resolve themselves (e.g., infants, the elderly, immunodeficients, and chronic patients).

Cold viruses are transmitted mainly through hand-to-hand contact followed by entry into the nostrils or eyes, not commonly known as airborne foam.

A randomized, double-blind, and placebo-controlled trial (35-day trial of 176 healthy male volunteers) showed that L-theanine and cystine supplements significantly reduced the incidence of cold and fever symptoms (but had no significant effect on the duration of colds). Note 5

*Conclusion: Supplementation with theanine and cystine may help prevent the common cold, but more large trials are needed to confirm this

Does theanine have side effects?

Short-term use of theanine supplements at appropriate doses is generally considered safe. In animal experiments, oral theanine has not been toxic even at high doses. Note 6

Safety precautions

1. Do not use by pregnant and nursing women (as the relevant safety is still unknown)

2. Do not use with blood pressure lowering drugs (because theanine seems to lower blood pressure, may lead to low blood pressure, related drugs are: captopril, enalapril, losartan, valsartan, diltiazem, amlodipine, hydrochlorothiazide, furosemide and others

3. Do not use it with central nervous system stimulants drugs, because theanine can ease the nervous system. Combining the two may reduce the effectiveness of stimulant drugs, such as diethylpropion, epinephrine, phentermine, pseudoephedrine, etc

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