In her May 4 column titled “Tea can help prevent dementia,” Dr. Sadja Greenwood wrote about a research study from the National University of Singapore, concluding: “tea drinking reduces the risk of cognitive impairment in older people by 50 percent and as much as 86 percent for those genetically at risk of Alzheimer’s.” I was puzzled by that 50 percent claim, since it ran counter to what I knew as a neuroscientist. If the 50…