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L-tyrosine is a non-essential amino acid precursor to the thyroid hormones, the neurotransmitters dopamine and norepinephrine, and the stress hormone epinephrine. As a non-essential amino acid, L-tyrosine can be created in the body from a separate amino acid, L-phenylalanine and typically doesn’t need to be directly consumed in food. However, some individuals have a rare genetic disorder, “phenylketonuria” (PKU), where they can’t process the amino acid L-phenylalanine. For these individuals, L-tyrosine becomes essential. Patients with PKU must replace L-phenylalanine with L-tyrosine, or they develop brain damage. The condition is typically tested for and identified at birth for affected individuals.
Since L-tyrosine is a neurotransmitter precursor, studies have explored its benefits for mental health. While some applications need more research, there are some interesting findings.
Some standard medications target dopamine and norepinephrine for the treatment of depression. As such, L-tyrosine would be a simple way to attempt to raise these neurotransmitter levels to produce a similar result. Interestingly, animal studies suggest the potential for antidepressant effects with L-tyrosine supplementation.
L-tyrosine depletion studies reduce levels of L-tyrosine in the brain by giving all of the amino acids except L-tyrosine and its precursor L-phenylalanine. Human studies found increasing indifference and discontent when L-phenylalanine and L-tyrosine were removed. Study subjects also displayed a bias towards sadness triggers during testing. The results suggest that L-tyrosine may affect mood when levels are deficient.
Old case reports describe patients who were unresponsive to many standard antidepressant medications that did well with L-tyrosine supplementation as a treatment for depression. Some of the case studies had evidence suggesting deficiencies in the neurotransmitter norepinephrine. Separate case reports out of France reported long-standing cures for depression in patients that were found to be deficient in dopamine. However, a single double-blind clinical trial didn’t find L-tyrosine helpful for depression, although no investigation was made into dopamine or norepinephrine levels in the study. Current research more strongly supports St. John’s wort, saffron, and potentially 5-HTP as natural treatments for depression. However, more research would help to understand better if L-tyrosine has a place in depression treatment or only helps in more isolated cases with specific neurotransmitter deficits.
ADHD is often thought of as a condition related to lowered dopamine levels in the brain. Stimulant medications, like ritalin and adderall, appear to work at least partially by increasing dopamine and norepinephrine. As such, it shouldn’t be too surprising that L-tyrosine is sometimes advocated as a treatment for ADHD. However, the research is somewhat nuanced, and studies are quite limited.
A small study that supplemented L-tyrosine, L-tryptophan, amphetamine, or placebo for only one week found no benefits on ADHD symptoms from L-tyrosine in children. Interestingly, L-tryptophan did improve parents’ reports on behavior. As the study was small and of such short duration, it’s hard to draw any firm conclusions. A separate small study in adults administered L-tyrosine for eight weeks. After two weeks, 75% of patients responded with improved ADHD symptoms. However, by six weeks, the benefits faded, and none of the patients appeared to maintain benefits long-term. The authors concluded that a tolerance to L-tyrosine developed, making it ineffective for continuous treatment of ADHD symptoms.
There is also one published ADHD case study, a child with PKU, the genetic disease that blocks L-tyrosine production from L-phenylalanine. At age three, the child started to display challenging behavioral symptoms. Due to the symptoms, the child’s parents sought help. Per a psychiatric evaluation, the child was diagnosed with ADHD. Since the child had PKU, the treating physician assumed the child might have a deficiency in L-tyrosine and provided the amino acid as a supplement. According to the authors, the child dramatically improved ADHD symptoms from L-tyrosine supplementation.
With the available evidence, it’s hard to draw any firm conclusions about L-tyrosine for ADHD. Larger, longer studies are needed to draw appropriate conclusions. There also needs to be an investigation as to whether a subset of patients might respond due to a greater biochemical need for the amino acid as potentially seen in cases of PKU. Research on zinc, iron, and dietary strategies suggest that there are likely additional options that provide benefits for ADHD, although they should be overseen by a knowledgeable healthcare provider.
By far, the most well-documented benefits of L-tyrosine appear to be its effects on brain function for individuals under stressful situations. It may have effects that are comparable to other herbal adaptogens, like ashwagandha and rhodiola. Many of the studies on L-tyrosine have been performed by different military groups with interest in the potential benefits of the amino acid.
Animal research shows that norepinephrine and dopamine levels can deplete during stressful situations. Providing L-tyrosine to these animals prevents the reduction in neurotransmitter levels and reduces stress-induced behavioral changes.
An initial human trial exposed army personnel to cold temperatures with reduced oxygen levels, similar to a high-elevation mountain environment. In the study, L-tyrosine, but not placebo, significantly reduced the negative effects. Supplementation of L-tyrosine had benefits on multiple symptoms, including headache, coldness, distress, fatigue, muscular discomfort, and sleepiness. On tasks requiring mental effort, including math exercises, map and compass skills, pattern recognition, and other measures, the subjects also performed more effectively with L-tyrosine.
In a study by the U.S. Army, subjects were stressed by reducing their core body temperature in cold water. From there, they were placed in a cold room and completed a number of tasks to assess both physical and mental performance. The most notable findings were improvements in marksmanship with L-tyrosine supplementation under this cold-stress situation. The authors noted that marksmanship is a task that requires both physical and mental performance.
A separate study from The Netherlands investigated the effects of L-tyrosine on cognitive function during noise stress. The subjects had to perform several stress-inducing mental tasks while wearing headphones that played noise consisting of a combination of sounds from a swimming pool, traffic, train noise, and a factory. With L-tyrosine, study subjects could perform the mental testing procedures better. In addition, L-tyrosine provided an initial drop in blood pressure, although the change normalized after an hour.
Sleep deprivation is also known to worsen both physical and mental performance. The U.S. Navy explored the effects of L-tyrosine on study subjects deprived of sleep for 24 hours. Both psychomotor skills and vigilance tasks were improved during sleep deprivation with L-tyrosine supplementation. A separate, similar study also explored L-tyrosine compared to caffeine, amphetamine, and another drug for performance under sleep-deprived conditions. While amphetamine was more effective, L-tyrosine still improved memory, logical reasoning, and visual vigilance-based tasks.
A small study on “interference control” also found some improvements with L-tyrosine. Interference control is measured through a number of methods. One is by presenting a circle or square on a computer screen, with each shape requiring a left (for circle) or right (for square) button press. However, each shape is presented randomly on the screen's left or right side. The object's location is irrelevant as to which response is correct, but it functions as a distraction to confuse the subject. L-tyrosine has been shown to improve performance on interference-control tasks.
A small trial explored the benefits of L-tyrosine during simple and multitasking performance. With tyrosine supplementation, memory was improved during more demanding multitasking situations. Another study by the U.S. Army applied cold stress and evaluated memory performance. In the study, subjects were immersed in cold, 50-degree Fahrenheit water that was utilized to drop core body temperature over 90 minutes. After the cold stress, short-term memory testing showed better performance with L-tyrosine than with placebo.
A similar study by the U.S. Navy also found benefits for short-term memory testing after cold exposure with L-tyrosine supplementation. Interestingly, when subjects were not exposed to cold beforehand, L-tyrosine had no effects on memory, likely indicating the benefits only occur under stressful situations.
L-tyrosine is an interesting amino acid that appears most likely to help improve mental function and memory during stressful activities and situations. While research is less clear, there may also be benefits for certain cases of depression with a metabolic need for higher amino acid levels. As for ADHD, the research is less certain.
Fortunately, the published studies don't suggest much in the way of side effects from L-tyrosine. The most commonly reported are headache, fatigue, heartburn, and nausea. When dosed in reasonable amounts, the amino acid is usually well tolerated.
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By Dr. Scott Buesing, N.D.
