The low-fat, low-glycemic diet often promoted for general health and well-being may lower the risk of developing Alzheimer’s disease if adopted early in life, researchers say. But starting such an eating plan after symptoms surface doesn’t seem to help prevent deterioration of brain function, according to new research published online June 13 in Archives of Neurology. “This is not the first time this concept has emerged, that things you do in midlife or earlier on may have effects later on,” said Dr. Marc L. Gordon, chief of neurology at Zucker Hillside Hospital and an Alzheimer’s researcher at the Feinstein Institute for Medical Research in Manhasset, N.Y.