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

The amino acid L-tyrosine can be converted into norepinephrine, a neurotransmitter that affects mood. Women taking oral contraceptives have lower levels of tyrosine, and some researchers think this might be related to depression caused by birth control pills.⁸ L-tyrosine metabolism may also be abnormal in other depressed people⁹ and preliminary research suggests supplementation might help.^10,11 Several doctors recommend a 12-week trial of L-tyrosine supplementation for people who are depressed. Published research has used a very high amount—100 mg per 2.2 pounds of body weight (or about 7 grams per day for an average adult). It is not known whether such high amounts are necessary to produce an antidepressant effect.