Phosphorus-restricted Diet For CKD Patients, 3 Major Issues To Be Clear

Hyperphosphatemia is a common complication in the decompensated phase of patients with chronic kidney disease (CKD)[1], which can appear in the early stage of CKD, and the incidence rate continues to increase with the progression of the disease until it reaches stage 5 of CKD Non-dialysis patients have been as high as 27.1% [2]. Hyperphosphatemia can also predict the progression of CKD disease, and is associated with the risk of all-cause death, cardiovascular death, and fracture, and has been considered as the key link in the control of mineral and bone disorder (MBD)[ 2]. In China, in terms of blood phosphorus detection rate, Chinese hemodialysis/peritoneal dialysis patients are 48.67% and 65.19% respectively[3]; in terms of blood phosphorus compliance rate, Chinese dialysis patients are 41.5%, which is lower than the standard level of 54.2% in developed countries [4,5]. The standard rate of blood phosphorus detection and blood phosphorus level in Chinese CKD patients is low, which urgently needs to be paid attention to.

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Improving calcium and phosphorus metabolism can start from four aspects: limiting phosphorus intake, adequate dialysis or increasing dialysis to remove phosphorus, rational use of phosphorus binders, and effective control of secondary hyperparathyroidism (SHPT). For non-dialysis CKD patients, dietary phosphorus restriction and the use of phosphate binders are the main approaches (learn more about phosphate-lowering therapy in non-dialysis CKD). Phosphorus-restricted diet is the cornerstone of blood phosphorus management in CKD patients, but there are many difficulties in clinical practice. Professor Wang Niansong was specially invited to interpret the three major problems of a phosphorus-restricted diet for CKD patients.

Patient adherence to the CKD diet is low

CKD patients have dietary restrictions such as sodium restriction, potassium restriction, low fat, phosphorus restriction, etc., and compliance is difficult to guarantee. But which patients are more likely to experience low adherence to the CKD diet? A large multicenter real-world prospective observational study, the G-CKD study, was conducted in Germany. Initiated in 2012, the study enrolled 5,217 nondialysis CKD patients and was followed up with questionnaires every 2 years thereafter. The observed endpoints included death, renal function progression, and cardiovascular events, and the CKD diet scoring system was used to evaluate patients' compliance with the CKD diet (Table 1) [6]. Part of the results of the first follow-up will be published in the European Journal of Clinical Nutrition in 2021. At the first follow-up, a total of 3,193 patients were included in the adherence correlation analysis, and the results showed that factors positively associated with a low adherence were younger age, less consumption of alcoholic beverages, insufficient physical activity, low education, low Effective glomerular filtration rate (eGFR), etc.; factors negatively correlated with low compliance include high BMI, male sex, smoking, etc. (Table 2) [6]. The G-CKD study revealed factors associated with low dietary adherence in CKD, providing evidence for early identification of patients with potential low commitment to clinical work. These patients should be paid attention to in clinical work, and education and management should be strengthened.

Phosphorus levels in foods are difficult to accurately assess

Phosphorus from food is mainly absorbed in the small intestine. Factors that affect intestinal net phosphorus absorption include phosphorus content in food, the bioavailability of phosphate, and the presence of phosphorus binders (natural or pharmaceutical). Phosphorus in food comes from 3 critical sources: natural phosphates found in raw or unprocessed foods, phosphates added to foods during industrial processing, and phosphates found in common and frequently consumed medications and dietary supplements of phosphate. Since the source of phosphorus in most foods is phosphorus-containing protein, CKD patients can choose foods to limit phosphorus intake by consulting the phosphorus/protein ratio (mg/g) of different protein sources[7] (different foods Please refer to Appendix 1[2] for the phosphorus content, and Appendix 2[2] for the phosphorus content of different beverages).

Phosphate in food additives and dietary supplements is often overlooked, but its phosphorus content should not be underestimated. Studies have shown that phosphorus in food additives can account for up to 40% of total phosphorus intake [7]. Unfortunately, there are still a large number of foods that do not label the phosphorus content in additives. This also brings great challenges to the management of blood phosphorus in CKD patients (for phosphorus-containing food additives, please refer to Appendix 3[2]).

People with CKD take multiple medications. The content of inorganic phosphorus in commonly prescribed drugs is usually low, and excipients are the main source of phosphorus in drugs. One of these, a single-center study published this year in the Journal of Renal Nutrition, tested 399 dosage forms of 204 drugs in 200 patients with CKD stages 3-5D. Among them, 58 (15%) dosage forms contained phosphorus (see Appendix 4 for details on the phosphorus content of drugs) [8]. And the phosphorus content may also vary between different brands of the same drug. In summary, the phosphorus content in drugs is usually low, but some dosage forms have high phosphorus content, the phosphorus content of the same drug of different brands may also be different, and the absorption of inorganic phosphate in CKD patients is unpredictable. still need to be cautious.

Risk of malnutrition

Malnutrition itself is a common complication of CKD. The prevalence of malnutrition in Chinese CKD patients is as high as 22.5% to 58.5%. Protein-energy wasting (PEW) can occur in the second stage of CKD. Non-dialysis patients 18% to 48% of patients can be combined with PEW, and the incidence of PEW in patients with end-stage renal disease (ESRD) can reach up to 75% [9].

Considering the actual situation in China, it is very difficult to promote professional nutrition guidance and a low-protein diet, which may increase the risk of PEW, decreased quality of life, and even death. Therefore, the Chinese CKD-MBD guidelines emphasize that one should not blindly pursue a low-protein diet to control blood phosphorus, but should seek a balance between protein intake and phosphorus intake [2]. (Causes, hazards and authoritative assessment models of malnutrition in CKD patients)

Phosphorus-restricted diets for CKD patients face many challenges, and it is necessary to make overall plans and take multiple measures in clinical work. For patients with low expected dietary compliance and existing PEW, early combined use of phosphorus-lowering drugs should be considered to improve the quality of life of patients and reduce mortality.

Summary

● Hyperphosphatemia is one of the common complications of CKD. The morbidity of CKD patients in China is high, but the management status is not good, and further attention should be paid.
● Phosphorus-restricted diet is the cornerstone of blood phosphorus management, but there are three major challenges: low compliance; the difficulty of accurately assessing the phosphorus content in food, and the risk of malnutrition.
● The G-CKD study reveals the related factors of low CKD dietary compliance, which can be used to identify patients with potential poor compliance and strengthen education and management.
● Do not blindly pursue a low-protein diet to control blood phosphorus, but seek a balance between protein intake and phosphorus intake.
● For patients with low expected dietary compliance and existing PEW, early combined use of phosphorus-lowering drugs should be considered to improve the quality of life of patients and reduce mortality.