Circadian Rhythm Modulation by Dietary Phosphorus in Chronic Kidney Disease (CKD)

In normal healthy individuals, there is a 12-hour and 24-hour circadian rhythm of serum phosphorus (serum P) with a nadir in early-mid morning, a rise to a minor peak in the afternoon (around 4:00PM), and a further rise to a major peak just after midnight. In a study of 6 healthy male patients, Portale et al found that the 12-hour component is modified by dietary P intake whereas the 24-hour rhythm in serum P is independent of the intake of phosphorus. Additionally, this trial demonstrated a large swing in amplitude between peak and nadir serum P in the normal healthy subjects (1.2 mg/dL) and also demonstrated a large difference in mean 24-hour serum P with normal vs. high dietary P intake, however no difference in fasting AM serum P results was observed. These findings may suggest the poor performance of serum P as a biomarker of intervention aimed at P load reductions via treatment options such as phosphate binders or salivary P-binder chewing gum.

While the circadian rhythm of serum phosphorus in healthy individuals has been described, no data exists on the circadian rhythm of salivary P, nor has the circadian rhythm of serum P been described in subjects with moderate kidney disease exposed to various levels of dietary P exposure. In recent years, saliva has been found to be a good indicator of the plasma level of various substances, one of which being phosphorus. Specifically, increased salivary P excretion has been reported in patients with CKD. Daily salivary secretion volume ranges between 500 - 700 mL/day, therefore salivary P has recently been established as a previously unaccounted for source of phosphorus, contributing a conservative estimate of 366 mg/day and 2500 mg/week of immediately bioavailable phosphorus.

There is a significant amount of data available regarding the utilization of serum P as a biomarker of intervention aimed at phosphorus load reductions; however, minimal information regarding the timing of phosphorus measurements (i.e. when serum P is assessed during a 24-hour period) is available. Wolf et al, have demonstrated that despite an 80% reduction in urinary P with dietary P restriction and treatment with P binders over a 2-week period, there was no change in serum P or in FGF-23. However, both biomarkers were sampled once every 3 days over a 2-week period without regard to the time of day. This suggests that single time point measurements of serum P in patients with CKD do not accurately reflect phosphate load. As such, it is plausible that mean 24-hour serum P, or mean 24-hour serum FGF-23 are better markers of phosphorus load in CKD than single measurements.

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.

This study is a single-center, exploratory trial in subjects with reduced kidney function. A total of 9 eligible subjects with an estimated GFR between 30 - 45 mL/min ± 10% and 3 normal healthy subjects as control will be sequentially exposed to three different Phosphorus diets (normal, low and high).