A single dose of 100 milligrams of the compound L-theanine—the amount found in four cups of green tea—improved reaction times and working memory of men and women in new research from Japan’s Central Research Institute, in Shizuoka. In the double-blind, placebo-controlled study, 69 adults that self-reported cognitive decline reacted more quickly to computer-generated promptings and answered more questions correctly after ingesting the compound compared to the placebo.