Dietary phosphorus in chronic kidney disease
Hyperphosphataemia is common in the later stages of chronic kidney disease and is a universal problem in end stage kidney disease, where it is associated with increased morbidity and mortality. Treatment includes a low phosphorus diet, that imposes numerous dietary restrictions, lacks a robust evidence base and in current practice fails to distinguish between phytate and non phytate bound phosphorus. The overall aim of this PhD was to provide the evidence basis for a revised low phosphorus diet, and to assess its effectiveness, safety and tolerability. Specific aims were 1) to define revised dietary recommendations for phosphorus in chronic kidney disease and to translate these recommendations into food based advice, 2) to conduct a proof of concept acute feeding study in prevalent haemodialysis patients using the modified diet and 3) to investigate the effectiveness of the modified diet in a multicentre randomised controlled trial in prevalent haemodialysis patients. To update the traditional low phosphorus diet, as chair of a national working group of Irish renal dietitians, I undertook a narrative review of the available renal nutrition literature. We combined the evidence with clinical experience and expertise to update the traditional low phosphorus diet. The modified diet, agreed by consensus, relaxed the restrictions of phytate bound phosphorus, and introduced previously disallowed foods such as pulses, peas, nuts and more whole grains. To ensure adequate but not excessive protein intake, we introduced guidance on bread and cereal intakes and included some advice based on phosphorus to protein ratios. Finally we increased the focus on avoiding phosphate additives. These changes were incorporated into a modified renal diet. The modified diet was initially evaluated against the standard low phosphorus diet in an in-centre, directly observed, one day, cross-over feeding study in order to examine the immediate postprandial effects of the modified diet. We demonstrated that the modified diet blunted post-prandial hyperphosphataemia, was not associated with increased hyperkalaemia and was well tolerated. Given the results of the acute feeding study, we conducted a national multicentre randomised controlled trial, comparing the modified low phosphorus diet with the standard low phosphorus diet in 74 prevalent patients treated with maintenance haemodialysis. The modified renal diet, with its broader food choice, achieved a significantly higher fibre intake, was well tolerated and provided comparable serum phosphate and potassium control as the standard renal diet. Based on the evidence from this project, a decision was taken by the Irish Renal Interest Group of the Irish and Dietetic Institute in January 2019, to endorse the modified low phosphorus diet and to implement it nationally.
Hyperphosphataemia, Phosphorus, Potassium, Diet, Phytate
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