Dietary Phosphorus Limits in CKD: When Enough Is Enough

Chronic kidney disease (CKD) disrupts normal electrolyte homeostasis by decreasing the kidneys’ ability to excrete excess phosphorus in the urine. As phosphorus builds up in the blood, it contributes to reduced conversion of vitamin D to its active form, calcitriol, by the kidneys, leading to disruptions in bone and mineral metabolism. 
Because excess phosphorus in the blood causes calcium to be leached from the bone into the bloodstream, it can lead to significant reductions in bone density. Consequently, patients with CKD can experience increased falls and vascular calcifications (calciphylaxis), which can lead to the development of blood clots and ulcers. 
This disruption must be corrected to prevent further vascular damage, including heart attack and stroke. This is often done using medications called phosphorus binders, in which an active ingredient binds to phosphorus in the gastrointestinal tract to prevent its excess absorption. However, the binding capacity of these medications is limited and typically requires additional dietary restriction of phosphorus. In my experience, such dietary restrictions are usually the more influential factor in promoting phosphorus control.
In the past, this has meant limiting all phosphorus-rich foods, including:
Meat
Poultry
Dairy products
Chocolate
Colas
Nuts
Seeds
Legumes
Whole grains
Beer
This list encompasses a vast array of foods naturally occurring in the Western diet. As such, patients limiting consumption of these foods can struggle to achieve adequate protein, fiber, vitamin, and mineral intake. This is especially concerning in the CKD population, which is prone to malnutrition. 
Adherence with dietary guidance can be difficult for kidney patients owing to the extensive restrictions of the renal diet, which may require limiting protein, sodium, phosphorus, and potassium. Additionally, patients with concomitant diabetes, gout, or a history of kidney stones may have to restrict carbohydrates, purine-rich foods, and high-oxalate foods, respectively. 
Although these various restrictions are necessary for optimal health, it may simply be unrealistic for all patients to adhere to them. This is attested to by results showing that approximately 70% of dialysis patients are nonadherent with dietary and fluid restrictions. 
Kidney Disease–Improving Global Outcomes does not recommend decreasing a specific source of phosphorus while treating hyperphosphatemia. However, their 2017 guidelines on CKD-mineral and bone disorder do recommend considering the source and its additional nutrition implications in the course of medical nutrition therapy. 
Dietary phosphorus can be found in many forms, most notably as an inorganic additive to thicken, stabilize, and to add flavor and acidity; an organic protein-bound compound in animal products; and as phytate or phytic acid in plant products. Some meat and dairy products, such as red meat or milk, may not be ideal for kidney patients, not only because of their relatively high levels of digestible phosphorus but also their protein and potassium content. Meat products may also be infused with phosphorus additives to maintain stability, further increasing their available phosphorus content.
Phosphorus-rich foods such as legumes are also rich sources of potassium and calcium. Although potassium and calcium may be protective against CKD progression, it can become necessary to restrict these electrolytes in later stages of the disease. Substituting animal-based proteins for diets high in plant-based protein sources has been linked to improvements in serum phosphorus, metabolic acidosis, and hypertension. 
In a study published in the Journal of Renal Nutrition, researchers compared the total phosphorus and digestible phosphorus content of 21 food and drink items. The tested items contained phosphorus from both inorganic and organic sources, including grains and grain products, legumes, colas, meats, and beer.
Researchers found that nearly 100% of total phosphorus found in highly processed foods containing inorganic phosphorus additives, such as cola, were digested and absorbed. Legumes — even those with a very high content of total phosphorus, such as sesame seeds — had much lower quantities of digestible phosphorus owing to high levels of phytic acid. 
Phytic acid must be hydrolyzed for significant absorption in the human intestinal tract. Processing by cooking, fermenting, or soaking causes hydrolysis of such products. 
Cooked beans yielded anywhere from 6% to 42% of total phosphorus as digestible, whereas studies suggest that phosphorus in beer is digested and absorbed at nearly 100%. This indicates that multiple methods of processing, such as the malting, soaking, cooking, and fermenting used in beer production, are probably needed to maximize phosphorus availability. 
Baked goods, including breads with phosphorus additives such as baking powder, yielded much higher rates of digestible phosphorus — nearly 100% of total available phosphorus. This is probably due to the highly available nature of inorganic phosphorus additives. 
The complex restrictions required with the renal diet may be one of the most frustrating aspects of care for the kidney patient. On a traditional renal diet, limits are placed on animal protein owing to its protein and potassium content; fruits and vegetables owing to their potassium content; legumes owing to their phosphorus and potassium content; and processed foods owing to their sodium, potassium, and phosphorus content.
However, diet encompasses more than just nutrition. It includes culture, lifestyle, connection, and socioeconomic capabilities. For these reasons, dietary recommendations should be both practical and efficient. 
It may not be realistic for a patient to cut out all phosphorus-rich foods. But, if we can work with patients to make a few highly effective changes, such as cutting out colas or processed meat products, we can help them get the most bang for their buck and encourage compliance by reducing the complexity of the renal diet while still addressing phosphorus consumption.