After being selected for the study, participants were asked to fill out questionnaires (in 1990 and 1994) about their eating habits to determine their usual amount and type of dietary protein. They were also questioned about other factors that could affect kidney function, such as age, weight, height, diabetes, high blood pressure, smoking habits, alcohol use, elevated cholesterol level, use of certain types of pain medication, and use of blood pressure medication. The researchers used the blood samples to evaluate how well the kidneys were working at each time point. This helped them to determine if kidney function was getting worse and, if so, how fast.