听力原文： Doctors in Chicago said on Monday they reached their conclusions on the fat question after examining 815 people aged 65 and older who did not have Alzheimer's at the start of a nearly four-year study. Those in the study were asked to recall their dietary habits during a more than two-year period before the study began. At the end of the study the researchers found that 131 people had developed Alzheimer's, the debilitating disease that leads to memory loss and eventual physical incapacity. People who consumed the most saturated fat -- the kind of fat that comes from meat, poultry, dairy products and palm or coconut oils -- had 2.3 times the risk of developing Alzheimer's compared with those who consumed the lowest amount of saturated fats, the researchers said. How many old people have been examined for the research?