Additionally, there are no published indicators of 24-hour mean serum P in patients with CKD. It is reasonable that either salivary P or FGF-23 might be optimal predictors. Therefore, this study will examine the 24-hour mean serum P and its relationship to both single and mean salivary P values as well as single and mean FGF-23 values in the controlled setting of specific low, normal and high phosphate diets. This examination and further understanding of mean serum P values and how they relate to the biomarkers of FGF-23 and salivary P with variation in intestinal phosphate load will inform the design and conduct of future interventional trials assessing reductions in phosphorus load.