Your alertness responds to the caffeine in a blend, not the theanine
Pooling 11 trials of caffeine-theanine blends, the gain in alertness over the first two hours tracked how much caffeine was in the mix more closely than how much theanine.
What the study found
The alertness tracked the caffeine dose, not the theanine dose
In the blends, one ingredient does most of the work. Pooling 11 trials, the gain in alertness and attention-switching over the first two hours tracked how much caffeine was in the mix more closely than how much theanine was.
The caveat · Doses varied, no single blend tested
The 11 trials used a spread of doses and designs rather than one fixed ratio, so this is a pattern across mixtures rather than a test of any single blend. The authors call theanine's contribution comparatively modest rather than absent, which leaves room for a larger pool to move it. And every trial pooled here tested combinations, so none of it measures theanine by itself.