August 1, 2012
Normal Phosphorus in Early Kidney Disease: The FGF-23 Link
Phosphorus is a big deal for chronic kidney disease (CKD) patients, but often isn’t addressed until dialysis is needed. Thanks to a very efficient regulating hormone called fibroblastic growth factor-23 (FGF-23), blood phosphorus levels stay normal, usually until stages 4 & 5 CKD. What’s not obvious is that phosphorus can cause damage despite normal lab values, even in early kidney disease.
It’s well documented that excess phosphorus intake is bad, and we’re consuming more now than ever before. Here’s what we know:
- In the general population, risk for developing cardiovascular disease increases as phosphorus levels increase—even within the normal range.
- Lower phosphorus intake may help preserve kidney function in earlier stage CKD patients.
- In patients with early CKD a lower phosphorus diet is linked to better parathyroid hormone (PTH) control. Excessive levels of this hormone contributes to bone disease and calcification.
- Studies of phosphorus and dialysis patients show that high phosphorus levels above 5.5 mg/dL are linked to greater risk of hospitalization and death.
- The use of phosphate additives in foods has sky-rocketed over the past 10 years, contributing an additional 1000 mg of extra phosphate a day, and this type of phosphate is absorbed 90-100% compared to 40-60% absorption form phosphates naturally occurring in food.
- Almost all fast foods and 50% of the most popular grocery items have phosphate additives (start reading the ingredients on everything you buy).
The reason phosphorus stays normal until later stages of CKD is the hormone FGF-23 takes action to increase phosphorus excretion in the urine. This usually starts when glomerular filtration rate (GFR) is around 60-80. As FGF-23 increases so does risk of death for people with CKD.
FGF-23 can be reduced by low phosphorus intake and avoidance of phosphate additives. Vegetarian diets or, for meat eaters, eating less meat and replacing meat with plant proteins also helps lower FGF-23.
Checking PTH and FGF-23 levels may become more important than looking at phosphorus levels in the future for patients with early CKD. But more evidence is building up to support following a lower phosphorus diet even when phosphorus is normal on the lab report.
Stay tuned as we learn more about FGF-23 and phosphorus control in early kidney disease.
Kidney diet resources from DaVita.com