Phosphorus is a mineral that plays an important role in the body, helping to build strong bones and teeth and serving as a component of cellular energy. Healthy kidneys maintain the correct balance of this mineral by excreting any excess through the urine. Chronic kidney disease (CKD) impairs this function, causing phosphorus to build up in the bloodstream, a condition known as hyperphosphatemia. When the kidneys fail, the body loses its primary way to eliminate this excess, which can lead to serious health complications, including bone disease and calcification of blood vessels. Dialysis is the primary treatment for kidney failure and is necessary to manage this mineral imbalance.
How Dialysis Removes Phosphorus
Dialysis removes phosphorus from the blood primarily through diffusion. This process relies on the movement of solutes, like phosphorus, from an area of higher concentration to an area of lower concentration across a semipermeable membrane. In both hemodialysis (HD) and peritoneal dialysis (PD), the patient’s blood or fluid, which contains high phosphorus, is separated from a specialized cleaning fluid, called dialysate, which has a very low or zero phosphorus concentration. The concentration gradient drives the phosphorus out of the patient’s blood and into the dialysate, clearing the mineral from circulation.
Standard hemodialysis, typically performed three times a week, removes 600 to 1,200 milligrams of phosphorus per session. Peritoneal dialysis, a continuous process, removes smaller amounts daily, averaging 300 to 360 milligrams over 24 hours. However, this removal is temporary and insufficient to manage the daily phosphorus load without additional therapies.
The Limitations of Dialysis in Phosphorus Management
Despite its removal capabilities, dialysis alone is typically inadequate for maintaining healthy phosphorus levels. The primary limiting factor for standard hemodialysis is its intermittent schedule, usually involving three, four-hour sessions per week. This leaves long inter-dialytic intervals—up to 68 hours between sessions—during which phosphorus levels rise unchecked as the patient continues to eat.
A significant challenge is the complex way phosphorus is stored and moves within the body, known as phosphorus kinetics. Most phosphorus is stored deep within the bone and soft tissues, not just in the bloodstream. Dialysis can only efficiently remove the phosphorus currently circulating in the blood.
Phosphorus moves slowly from tissue stores into the blood. As a dialysis session begins, blood phosphorus is rapidly cleared, but the rate at which it moves out of the tissue is too slow for complete removal in a typical short session. Consequently, the serum phosphorus concentration often reaches a plateau during the latter part of the treatment, meaning further dialysis time yields diminishing returns. This slow mobilization means that supplemental strategies are necessary.
Dietary Adjustments and Phosphate Binders
Because dialysis alone cannot fully resolve hyperphosphatemia, successful phosphorus management requires a combined approach: strict dietary control and the regular use of medications called phosphate binders. Dietary restrictions limit the amount of phosphorus entering the body, with a typical target intake for dialysis patients being between 800 and 1,000 milligrams per day.
A key focus is distinguishing between naturally occurring organic phosphorus and inorganic phosphate additives found in processed foods. Organic phosphorus, found in foods like meat, nuts, and beans, is absorbed at a rate of 40 to 60 percent. Inorganic phosphate additives, used to preserve or enhance processed items like fast food and packaged cheeses, are nearly 100 percent absorbed by the gut, making them a major concern. Patients are advised to check food labels for ingredients containing “phos,” such as phosphoric acid or sodium phosphate, and to limit these highly absorbed sources.
Phosphate binders are oral medications taken with meals and snacks to prevent consumed phosphorus from being absorbed into the bloodstream. These medications work in the gastrointestinal tract by chemically binding to dietary phosphorus, forming an insoluble compound that is excreted in the feces. Common types include calcium-based binders, such as calcium acetate, and non-calcium-based options, like sevelamer and lanthanum carbonate, which are used to avoid the risk of calcium overload. Effective phosphorus control depends on this three-part strategy: dialysis removal, phosphate binders, and conscious dietary choices.