Phosphorus levels are elevated in patients with chronic kidney disease due to decreased urinary excretion. Higher levels of blood phosphorus are associated with increased mortality in patients on dialysis, patients with kidney disease not yet on dialysis, and in the general population. In animal studies, adding phosphorus to the diet causes calcification of arteries and progression of kidney disease.
In the petri dish in the lab, adding phosphorus to artery vascular smooth muscle cells results in a change of the cell to become a bone-like cell and to calcify. This and other data support the hypothesis that phosphorus is a true uremic toxin and a risk factor for adverse health in the more than 20 million individuals with kidney disease in the United States. Unfortunately, data from the National Health and Nutrition Examination Survey (NHANES) and other studies demonstrate that nearly all Americans eat food that contains far more phosphate than either the estimated average requirement or the recommended dietary allowance.
The approach to kidney patients with elevated phosphorus levels includes the use of phosphate-binding compounds, increased dialysis time, and diet adjustment. It is the latter that becomes tricky. It requires a savvy consumer to truly follow a low-phosphorus diet. Phosphorus is in all proteins, and thus any protein source will be high in phosphate (dairy, meat, or legumes/beans). However, in legumes/beans, the phosphate is bound to phytate. Humans lack the ability to digest phytate as they do not have the enzyme phytase (in contrast to most farm animals). Thus, there is decreased bioavailability, or intestinal absorption, of plant-based sources of phosphate. Short-term studies have demonstrated that vegetarian diets can reduce phosphorus levels and the hormonal elevations in parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF23) that result from increased phosphorus absorption. Whether such diets are efficacious and safe long term in kidney patients has not been studied.
A major source of phosphorus in the diet is not from the diet itself, but rather additives that contain inorganic phosphate salts. These additives will be nearly 100 percent bioavailable, meaning they are completely absorbed across the intestine. They are commonly used in canned and boxed food processing to improve taste, texture, color, and cooking time, and act as a preservative. They are also added to meat and poultry products to help retain moisture and protect flavor. Unfortunately, there is an increasing use of these additives by food manufacturers.
Foods that contain additives have nearly 70 percent higher phosphate content than similar foods without additives. These additives are listed on the U.S. Food and Drug Administration’s GRAS (Generally Recognized As Safe) list and specific quantitation on the food label is voluntary (and rarely listed). In contrast, these additives must be listed in the ingredients but these diverse chemical names can be confusing to patients, especially those with low health literacy. One study instructed patients to use a magnifying glass to look at foods and avoid the ones that included ingredients with the letters “Phosphor.” The result was a reduction in phosphorus levels in these patients. This should be a call to action to label food as “contains phosphate additives” so that patients and consumers alike know what they are eating. An alternative would be to ban the additives completely.