Tyrosine is an amino acid used by the body to produce certain adrenal stress hormones and chemical messengers in the nervous system (neurotransmitters). Animal research shows that brain levels of these substances decline with stress, and that giving animals tyrosine supplements reverses this decline and improves various tests of performance in stressed animals.¹ In a controlled study, a protein drink containing 10 grams per day of tyrosine was more effective than a carbohydrate drink for improving mental performance scores in a group of cadets taking a stressful six-day combat training course.² A double-blind trial in humans found that one-time administration of 150 mg of tyrosine per 2.2 pounds of body weight helped prevent a decline in mental performance for about three hours during a night of sleep deprivation.³ Single administrations of tyrosine (100 to 150 mg per 2.2 pounds of body weight) have also helped preserve mental performance during physically stressful conditions such as noise or extreme cold in several controlled studies.^4,5,6,7