Feeling less stressed on theanine did not mean doing the task better
In one trial of 26 adults, a drink with 200mg of theanine lowered rated stress at one hour and saliva cortisol two hours later. Scores on the stressful task itself did not change.
What the study found
Stress ratings and cortisol fell, and task performance did not move
In one trial of 26 healthy adults, one drink carrying 200mg of L-theanine lowered how stressed they said they felt an hour later, and their saliva cortisol two hours after that, against placebo. Task performance was unchanged: what eased was the feeling and the hormone, not the work.
The caveat · Only the stress reactors analysed
Twenty-six of the 34 who finished were analysed, and they were the ones who had reacted to the task at baseline. The drink was not theanine alone: phosphatidylserine, alpha-glycerylphosphorylcholine and chamomile came with it, in small doses. An industry grant funded the trial, and the paper says the funder had no hand in its design or analysis. A brain-wave finding in the same trial turned up only in the more anxious participants and did not track the stress or cortisol results, so it stands unexplained.