ORLANDO, Fla.—The phosphorus content of a typical American diet significantly increases levels of fibroblast growth factor-23 (FGF-23), a pilot study suggests. The finding emphasizes the need to counsel CKD patients about limiting dietary phosphorus intake, according to researchers.

Dietary phosphorus increases secretion of FGF-23, a hormone that some researchers say has the potential to serve as a biomarker for monitoring phosphorus balance in CKD patients, however, has also been associated with negative clinical outcomes, such as increased mortality.

The study also showed that daily phosphorus restriction alone, or in combination with lanthanum carbonate, lowers 24-hour urine phosphate in pre-dialysis CKD patients without significantly changing serum phosphate or calcium levels.

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Researchers presented the study results here at the National Kidney Foundation’s 2010 Spring Clinical Meetings.

A team of academic investigators, that included lead author Tamara Isakova, MD, MMSc, of Massachusetts General Hospital in Boston, randomly assigned 16 patients with CKD (stages 3-4) and normal phosphate levels to receive one of four treatments for two weeks: 750 mg daily dietary phosphorus plus lanthanum carbonate 1000 mg three times daily (group 1); 1500 mg daily dietary phosphorus and lanthanum carbonate 1000 mg three times daily (group 2); 750 mg daily dietary phosphorus plus placebo (group 3); and 1500 mg daily dietary phosphorus plus placebo (group 4). Subjects had a mean estimated glomerular filtration rate of 40 mL/min/1.73 m2.

The most profound lowering in 24-hour urine phosphate was seen in group 1 (from 659 mg/day at baseline to 214 mg/day at day 3). FGF-23 levels increased early and significantly in group 4 (from 121.8 RU/mL at baseline to 178.5 RU/mL at day 3).

“Although we did not see a significant reduction in FGF-23 levels after the two weeks of treatment, there are several important findings of the study that will be of interest to mineral metabolism investigators and practicing nephrologists, especially given the tremendous recent attention to FGF-23 in CKD,” Dr. Isakova told Renal & Urology News.

One of these findings is the observation that FGF-23 levels increased in subjects who consumed 1500 mg of phosphorus daily for two weeks. “This intake of phosphorus is within the described range of phosphorus content in a typical American diet, and our findings underscore the importance of dietary counseling and education regarding phosphorus intake in CKD patients,” Dr. Isakova said.

Future studies with use of long-term phosphorus reduction strategies are needed to see if FGF-23 levels can be lowered in CKD patients with these interventions, she said.