The effects of stress on people's behavior are well documented and the main cognitive effects can be summarized as lack of attention, lack of precision, increased speed in information processing and reduction of short-term’s memory capacity. It is still unclear whether these behavioral effects of anxiety are related to depletion of norepinephrine levels. In a stressful state, the peripheral nervous system can affect physiological effects and lead to increased heart rate, blood pressure, muscle tone, skin conductance and breathing rate. In addition, biochemical changes occur, such as increases in the concentration of catecholamines.

The precursor of norepinephrine, dopamine and epinephrine is tyrosine, which is present in nutritional proteins. This study was designed to investigate whether tyrosine reduces the cognitive and physiological effects of stress on people exposed to a combination of mental and physical pressures and has been published in the Brain Research Bulletin Journal. Sixteen healthy subjects took part in the study and received placebo or tyrosine in a double-blind, randomized trial. The tests started 20 minutes after swallowing and lasted for 1 hour. Blood pressure levels were measured before and after the test.

Perceptual-motor tasks:
  1. Cognitrone: This program measures concentration and form perception. Five display fields appeared on the screen, four on the top and one on the bottom. The image displayed in the bottom field should be compared to the four numbers in the upper field. If the bottom shape is identical to one of the four numbers at the top, they had to press the «yes» button, and if there were no identical figures, no response is required.
  2. Vienna determination unit. The Determination apparatus is a complex multiple-stimuly reaction unit. The unit allows presentation of white, yellow, red, green and blue visual stimuli in ten different locations. Individuals had to respond by pressing one of the five reaction keys assigned to these colors. Two additional white lamps, separated from the colored lamps, required the left or right pedals to walk. The assay device has been found to be useful in simulating anxiety states that require decisions to be made under stress conditions.
  3. Vigilance / peripheral perception (double task). A circle consisting of small rings appears on the screen. A point travels along the circular path from one ring to the next. Sometimes a ring is bypassed and if this happens, people have to press it button. When performing this work, the peripherall perception of the individual is measured.
  4. Peripheral perception. Occasionally bright stimuli it is moving from the periphery of the control unit to its center. Once the persons understand a light motivation from the angle of his eyes, a foot pedal should be strengthened. This test is designed to test a person's ability to perceive and process peripheral visual information with particular emphasis on early understanding of the stimuli entering the field of view from both sides.
  5. Stroop task. Four color words (white, red, black and blue), randomly distributed on one sheet, were visualized in one of these colors in an incongruent state. The color of the word presented does not match the color name (eg, the word black is shown in blue). People had to call the color of 80 different words as quickly as possible, while during the test they heard random color names through headphones.
Short-term memory
Digit span. A 4-digit series is displayed on the computer screen, 1 every second. Individuals then have to press the corresponding digits on the computer keyboard. After a correct answer, a 5-digit sequence is displayed. If the answer is correct, the order is extended to 6 and, in case of correct answers, up to 9 digits.

Physical stress exposure
As an environmental stressor, a continuous broadband noise of sound pressure level of 90 dB that is induced by headphone. This noise consists of sounds like factory, traffic and trains. Industrial continuous noise of 20 minutes at sound levels of 75, 85, 95 dB has been found to increase diastolic blood pressure by 5-12%, mean blood pressure 3-7% and overall peripheral resistance by 11-13% in young healthy subjects. Thus, noise induction of 90 dB can be considered as stressful, as indicated by significant increases in hemodynamic markers.

The test was performed with receiving either 100 mg/kg L-tyrosine or placebo. Individuals received tablets containing 500 mg of L-tyrosine and 10 mg of vitamin B6 or non-active tablets with the same appearance as placebo. L-tyrosine was given in combination with vitamin B6, because this vitamin supports the conversion of tyrosine to norepinephrine.

The results of the study showed that tyrosine administration improved the performance of the Digit Span and Stroop trials while it reduced diastolic blood pressure 15 minutes after tyrosine administration compared to placebo. The performance of these two cognitive results took place at the end of the trial period and lasted about 1 hour.

  • Deijen, J. B., & Orlebeke, J. F. (1994). Effect of tyrosine on cognitive function and blood pressure under stress. Brain Research Bulletin, 33(3), 319-323.
Related product: Neuro Balance
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