Phosphorus is absorbed along the small intestine by both active and passive mechanisms. The main determinants of this absorption are the phosphorus content in the diet, its bioavailability and the presence of natural and/or pharmacological phosphorus binders.

The usual daily intake of phosphorus will vary significantly depending on the way people eat. For the typical American diet, young and middle aged men consume about 1600 mg /day, while comparably aged women consume about 1000 mg /day^{4, 5}. These estimates of dietary phosphorus intake reflect mostly the "natural" phosphorus content of foods. A true estimate of the dietary content of phosphorus, however, requires the consideration not only of the natural phosphorus contained in raw or unprocessed food, but also the amount of phosphorus added to foods during processing and the phosphorus content of dietary supplements.

In general, foods high in protein like meat, milk, dairy, eggs and cereals are also naturally high in phosphorus and contribute the largest amounts to the total dietary phosphorus intake on an average diet. Over the past few decades, however, increasing use of food processing practices, which include the addition of phosphorus, has determined a significant increase in the daily intake of this mineral⁶. In 1990, phosphorus containing food additives contributed an estimated 470 mg/day to the daily adult phosphorus intake in the USA, but this contribution will continue to increase as the people’s demand for convenience and fast foods escalates⁶. Consequently, depending on individual food choices, phosphorus intake could be increased by as much as 1.0 g/day simply increasing the percentage of processed foods in the diet⁷.

The bioavailability of phosphorus from various food sources is also an important consideration in the analysis of dietary phosphorus intake. In plants a large fraction of the phosphorus is in the form of phytate. Since the small intestine of humans does not secrete the enzyme phytase, which is necessary to degrade dietary phytate and release phosphorus, phytate phosphorus is not bioavailable in humans, unless the food is processed with phytase such as in the leavening of bread with phytase-containing yeast⁴. In contrast, phosphorus in meat is well absorbed since it is found mostly as intracellular organic compounds, which are easily hydrolyzed in the gastrointestinal tract releasing inorganic phosphorus for absorption. Phosphorus in milk is present in different fractions each one of them with different bioavailability. Phosphorus added during food processing is mostly in the form of inorganic salts and hence is almost completely absorbed and therefore, represents an even larger phosphorus burden.

The relative amount of calcium in the diet will also influence the intestinal absorption of phosphorus probably through binding and formation of insoluble calcium phosphate salts, which are not well absorbed.

As a result of the different factors mentioned above, usually only 60 % of phosphorus from a typical mixed diet is absorbed. Obviously, this percentage will change depending on the predominant food groups and the degree of processing of the foods present in selected diets. A diet with a predominance of fast food and processed meats as a protein source will provide higher fractional phosphorus absorption than a diet of similar "natural" phosphorus content but with a predominance of fresh, unprocessed foods.

The close association between phosphorus and protein content in food makes it very difficult to restrict dietary phosphorus content without simultaneously restricting dietary protein intake, especially protein of animal origin⁸. Thus, the minimal daily protein intake of 1.2 g/kg/day set for dialysis patients by the DOQI guidelines will almost invariably lead to hyperphosphatemia in chronic peritoneal² and hemodialysis patients³. These guidelines for minimal protein intake are inappropriate since they prevent reaching reasonable levels of serum phosphorus and considering the lack of evidence of any benefit of this high protein intake (Table 1)².

TABLE 1. Normophosphatemia in CAPD patients requires dietary P control

Changes in the food labeling regulations in the USA no longer require manufacturers to list the phosphorus content in the Nutrition Facts Panel of food as they do for sodium and potassium content⁴. Consequently, patients have to be specifically advised to read the ingredient list on food labels to help them select food products with lower levels of phosphate additives.

Phosphate binders could theoretically prevent hyperphosphatemia even in the presence of a high intake of phosphorus if their dose is progressively increased. In clinical practice, however, increasing the dose of phosphorus binders is limited by their own set of side effects. Calcium-containing phosphate binders, the most commonly used ones, will frequently lead to hypercalcemia, particularly when patients are simultaneously exposed to either vitamin D (calcitriol, doxercalciferol) or vitamin D analog (paracalcitol) preparations, which is almost the rule at present². Newer preparations such as sevelamer, which do not predispose to hypercalcemia, are very expensive alternatives.

Enhancing dialytic phosphorus removal should lead to normophosphatemia. In fact, experience with daily nocturnal hemodialysis has shown that ideal levels of serum phosphorus and, even hypophosphatemia, can be achieved with this form of dialysis⁹. However, daily hemodialysis is still not widely used and in the meantime, limiting dietary intake of phosphorus remains a cornerstone in the maintenance of a safe serum phosphorus concentration in dialysis patients.

In summary, current dialysis and phosphorus binder prescriptions are not sufficient to assure normophosphatemia in dialysis patients who are eating well. The important message is that limiting dietary intake of phosphorus requires prescription of a moderate protein intake (0.9 to 1.0 g/kg/day), increased consumption of a variety of fresh fruits and vegetables and restricted consumption of highly processed fast and convenience foods. Phosphorus added during food processing is an important source of this mineral because of its magnitude and high bioavailabilty. Because this information is often unknown to the consumer, it needs to be more extensively disseminated among health care professionals dealing with dialysis patients, a population of patients in whom a very precise determination of phosphorus intake is essential. The development of low-phosphorus containing food products would be a very useful addition for these patients

References

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