Dietary phosphorus restriction in advanced chronic kidney disease: merits, challenges, and emerging strategies

Breast Cancer and Bone Mineral Density in a U.S. Cohort of Middle-Aged Women: Associations with Phosphate Toxicity

Breast cancer is associated with phosphate toxicity, the toxic effect from dysregulated phosphate metabolism that can stimulate tumorigenesis. Phosphate toxicity and dysregulated phosphate metabolism are also associated with bone mineral abnormalities, including excessive bone mineral loss and deposition. Based on shared associations with dysregulated phosphate metabolism and phosphate toxicity, a hypothesis proposed in the present mixed methods-grounded theory study posits that middle-aged women with incidence of breast cancer had a greater magnitude of changes in bone mineral density over time compared with women who remained cancer-free. To test this hypothesis, a mixed-effects model was used to analyze the associations of breast cancer incidence with spinal bone mineral density changes in the U.S. Study of Women's Health Across the Nation. Compared with women in the cohort who remained cancer-free, women who self-reported breast cancer had higher bone mineral density at baseline, but had more rapid losses in bone mineral density during follow-up visits. These findings agree with the hypothesis that a greater magnitude of changes in bone mineral density over time is associated with breast cancer in a cohort of middle-aged women. The findings also have implications for studies investigating dysregulated phosphate metabolism and phosphate toxicity as causative factors of bone metastasis in metastatic breast cancer. Additionally, the authors previously found increased breast cancer risk associated with high dietary phosphate intake in the same cohort of middle-aged women, and more studies should investigate a low-phosphorus diet to reduce bone mineral abnormalities and tumorigenesis in breast cancer patients.

The efficacy and safety of cuttlebone for lowering serum phosphate in patients with end-stage renal disease: a meta-analysis of randomized controlled trials

Background: The efficacy of cuttlebone for treating hyperphosphatemia in patients with end-stage renal disease and its safety remained unclear. Methods: Randomized controlled trials comparing the efficacy of cuttlebone with conventional interventions were retrieved from MEDLINE, EMBASE, Cochrane Library, Airiti Library, and other major Chinese databases until 1 February 2023. The primary outcome was circulating phosphate concentration, while secondary outcomes included circulating calcium and intact parathyroid hormone levels, calcium-phosphorus product, and treatment-related side-effects. Results: Analysis of nine studies published between 2000 and 2019 including 726 participants showed a lower circulating phosphate concentration in the cuttlebone group than in controls [mean difference (MD) = -0.23, 95% CI: -0.39 to -0.06, p = 0.006, I2 = 94%, 726 patients] and a dose-dependent effect of cuttlebone against hyperphosphatemia. Therapeutic benefits were noted after both short-term (1-2 months) and long-term (3-6 months) treatments. Besides, patients receiving hemodialysis showed a better response to cuttlebone than those receiving peritoneal dialysis. There was no difference in circulating calcium level (mean difference = 0.03, 95% CI: -0.01 to 0.07, p = 0.17, I2 = 34%, 654 patients), while patients receiving cuttlebone showed lower circulating iPTH level and calcium-phosphorus product (MD = -43.63, 95% CI: -74.1 to -13.16, p = 0.005, I2 = 76%, 654 patients), (MD = -0.38, 95% CI: -0.38 to -0.01, p = 0.04, I2 = 83%, 520 patients). No difference in the risks of constipation, gastrointestinal discomfort, and elevated blood calcium was noted between the two groups. Conclusion: Compared with conventional phosphate-binding agents, cuttlebone more efficiently suppressed hyperphosphatemia with a dose-dependent effect. The limited number of included studies warrants further clinical investigations to verify our findings. Systematic Review Registration: https://www.crd.york.ac.uk/prospero/, identifier CRD42023396300.

Hyperphosphatemia increases inflammation to exacerbate anemia and skeletal muscle wasting independently of FGF23-FGFR4 signaling

Elevations in plasma phosphate concentrations (hyperphosphatemia) occur in chronic kidney disease (CKD), in certain genetic disorders, and following the intake of a phosphate-rich diet. Whether hyperphosphatemia and/or associated changes in metabolic regulators, including elevations of fibroblast growth factor 23 (FGF23) directly contribute to specific complications of CKD is uncertain. Here, we report that similar to patients with CKD, mice with adenine-induced CKD develop inflammation, anemia, and skeletal muscle wasting. These complications are also observed in mice fed high phosphate diet even without CKD. Ablation of pathologic FGF23-FGFR4 signaling did not protect mice on an increased phosphate diet or mice with adenine-induced CKD from these sequelae. However, low phosphate diet ameliorated anemia and skeletal muscle wasting in a genetic mouse model of CKD. Our mechanistic in vitro studies indicate that phosphate elevations induce inflammatory signaling and increase hepcidin expression in hepatocytes, a potential causative link between hyperphosphatemia, anemia, and skeletal muscle dysfunction. Our study suggests that high phosphate intake, as caused by the consumption of processed food, may have harmful effects irrespective of pre-existing kidney injury, supporting not only the clinical utility of treating hyperphosphatemia in CKD patients but also arguing for limiting phosphate intake in healthy individuals.

Dietary strategies for gut-derived protein-bound uremic toxins and cardio-metabolic risk factors in chronic kidney disease: A focus on dietary fibers

Chronic kidney disease (CKD) is associated with altered composition and function of gut microbiota. The cause of gut dysbiosis in CKD is multifactorial and encompasses the following: uremic state, metabolic acidosis, slow colonic transit, dietary restrictions of plant-based fiber-rich foods, and pharmacological therapies. Dietary restriction of potassium-rich fruits and vegetables, which are common sources of fermentable dietary fibers, inhibits the conversion of dietary fibers to short-chain fatty acids (SCFA), which are the primary nutrient source for the symbiotic gut microbiota. Reduced consumption of fermentable dietary fibers limits the population of SCFA-forming bacteria and causes dysbiosis of gut microbiota. Gut dysbiosis induces colonic fermentation of protein and formation of gut-derived uremic toxins. In this review, we discuss the roles and benefits of dietary fiber on gut-derived protein-bound uremic toxins and plant-based dietary patterns that could be recommended to decrease uremic toxin formation in CKD patients. Recent studies have indicated that dietary fiber supplementation may be useful to decrease gut-derived uremic toxin formation and slow CKD progression. However, research on associations between adherence of healthy dietary patterns and gut-derived uremic toxins formation in patients with CKD is lacking.

Pilot Randomized Controlled Trial of a Standard Versus a Modified Low-Phosphorus Diet in Hemodialysis Patients

INTRODUCTION

The standard low-phosphorus diet restricts pulses, nuts, and whole grains and other high phosphorus foods to control hyperphosphatemia. We conducted a randomized controlled trial to evaluate the effectiveness, safety, and tolerability of the modified diet, which introduced some pulses and nuts, increased the use of whole grains, increased focus on the avoidance of phosphate additives, and introduced the prescription of low-biological-value protein such as bread.

METHODS

We conducted a multicenter, pragmatic, parallel-arm, open-label, randomized controlled trial of modified versus standard diet in 74 adults on hemodialysis with hyperphosphatemia over 1 month. Biochemistry was assessed using monthly laboratory tests. Dietary intake was assessed using a 2-day record of weighed intake of food, and tolerability was assessed using a patient questionnaire.

RESULTS

There was no significant difference in the change in serum phosphate between the standard and modified diets. Although total dietary phosphorus intake was similar, phytate-bound phosphorus, found in pulses, nuts, and whole grains, was significantly higher in the modified diet (P < 0.001). Dietary fiber intake was also significantly higher (P < 0.003), as was the percentage of patients reporting an increase in bowel movements while following the modified diet (P = 0.008). There was no significant difference in the change in serum potassium or in reported protein intake between the 2 diets. Both diets were similarly well tolerated.

CONCLUSION

The modified low phosphorus diet was well tolerated and was associated with similar phosphate and potassium control but with a wider food choice and greater fiber intake than the standard diet.

Revising Dietary Phosphorus Advice in Chronic Kidney Disease G3-5D

We summarize how practicing dietitians combined available evidence with clinical experience, to define revised dietary recommendations for phosphorus in chronic kidney disease G3-5D. As well as a review of the evidence base, 4 priority topics were reviewed. These were translated into 3 nutrient level recommendations: the introduction of some plant protein where phosphorus is largely bound by phytate; consideration of protein intake in terms of phosphorus load and the phosphorus to protein ratio; and an increased focus on avoiding phosphate additives. This review summarizes and interprets the available evidence in order to support the development of practical food-based advice for patients with chronic kidney disease.

Considerations and controversies in managing chronic kidney disease: An update

PURPOSE

Current considerations and controversies surrounding the management of chronic kidney disease (CKD) are reviewed.

SUMMARY

Patients diagnosed with CKD require a unique clinical approach to prevent medication toxicities and ensure appropriate management of disease-progressing comorbidities, and they require attention to commonly occurring complications that may affect disease control and impact quality of life, including anemia and CKD-bone-mineral disorder (CKD-BMD). Many CKD-related comorbidities put patients at increased cardiovascular risk, including diabetes, hypertension, and hyperlipidemia. Although there are clinical guidelines to help clinicians manage CKD and its related complications and comorbidities, there are many clinical controversies surrounding optimal treatment. Recent literature and clinical studies bring into question multiple controversies regarding the optimal management approach to the patient living with CKD, including the appropriateness of iron and erythropoiesis-stimulating agents (ESAs) for the treatment of anemia and vitamin D supplementation for the prevention of CKD-BMD. While available guidelines can provide clinicians with guidance regarding the appropriate management of the patient with CKD, they often differ dramatically in the optimal approach to the management of comorbidities and complications. For a patient with CKD, the pharmacist has an important role to ensure optimal outcomes, by appropriately managing comorbid conditions and optimizing drug dosing.

CONCLUSION

Multiple controversies regarding the optimal management of patients with CKD, including the appropriateness of iron and ESAs for treatment of anemia and vitamin D supplementation for the prevention of CKD-BMD. Available guidelines differ dramatically in the optimal approach to the management of comorbidities and complications.

Use of phosphate-binders and risk of infection-related and all-cause mortality in patients undergoing hemodialysis: The Q-Cohort Study

The use of phosphate (P)-binders allows hemodialysis patients to take in more protein and thus may maintain a good nutritional status. Protein-energy-malnutrition increases the risk of infection-related death. The association between use of P-binders and the relative risks of infection-related death remains unknown in hemodialysis patients. A total of 2926 hemodialysis patients registered to the Q-Cohort Study was followed up for 4-years. The association between use of P-binders and the risks for infection-related and all-cause mortality were estimated by Cox proportional hazards risk model with multiple adjustments by conventional and propensity-score based approaches. During the follow-up period, 106 patients and 492 patients died of infection and any cause, respectively. Cox proportional hazards models with multivariable adjustments including nutritional confounders showed that the incidence of infection-related death was significantly lower in patients with P-binders use compared with those without (hazard ratio [95% confidence interval] for infection-related mortality 0.63 [0.40-0.99]). The results remained significant even after applying four different propensity score-based analyses. Notably, use of P-binders was associated with a lower risk of all-cause mortality. Further studies including randomized controlled clinical trials and observational studies analyzed by an instrumental variable model will provide more robust evidences for the associations observed in our study.

MICS, an easily ignored contributor to arterial calcification in CKD patients

In chronic kidney disease (CKD), simultaneous mineral and skeleton changes are prevalent, known as CKD-mineral bone disorder (CKD-MBD). Arterial calcification (AC) is a clinically important complication of CKD-MBD. It can increase arterial stiffness, which leads to severe cardiovascular events. However, current treatments have little effect on regression of AC, as its mechanisms are still unclear. There are multiple risk factors of AC, among which Malnutrition-Inflammation Complex Syndrome (MICS) is a new and crucial one. MICS, a combined syndrome of malnutrition and inflammation, generally begins at the early stage of CKD and becomes obvious in end-stage renal disease (ESRD). It was linked to reverse epidemiology and associated with increased cardiovascular mortality in ESRD patients. Recent data suggest that MICS can trigger CKD-MBD and accelerate the course of AC. In this present review, we summarize the recent understanding about the aggravating effects of MICS on AC and discuss the possible underlying mechanisms. A series of findings indicate that targeting MICS will provide a potential strategy for treating AC in CKD.

Fetuin-A decrease induced by a low-protein diet enhances vascular calcification in uremic rats with hyperphosphatemia

Although dietary phosphate restriction is important for treating hyperphosphatemia in patients with chronic kidney disease, it remains unclear whether a low-protein diet (LPD), which contains low phosphate, has beneficial effects on malnutrition, inflammation, and vascular calcification. The effects of LPD on inflammation, malnutrition, and vascular calcification were therefore assessed in rats. Rats were fed a normal diet or diets containing 0.3% adenine and low/normal protein and low/high phosphate. After 6 wk, serum and urinary biochemical parameters, systemic inflammation, and vascular calcification were examined. The protective effect of fetuin-A and albumin were assessed in cultured vascular smooth muscle cells. Rats fed the diet containing 0.3% adenine developed severe azotemia. LPD in rats fed high phosphate induced malnutrition (decreases in body weight, food intake, serum albumin and fetuin-A levels, and urinary creatinine excretion) and systemic inflammation (increases in serum tumor necrosis factor-α and urinary oxidative stress marker). LPD decreased the serum fetuin-A level and fetuin-A synthesis in the liver and increased serum calcium-phosphate precipitates. A high-phosphate diet increased aortic calcium content, which was enhanced by LPD. Reduced fetal calf serum in the medium of cultured vascular smooth muscle cells enhanced phosphate-induced formation of calcium-phosphate precipitates in the media and calcification of vascular smooth muscle cells, both of which were prevented by fetuin-A administration. Our results suggest that phosphate restriction by restricting dietary protein promotes vascular calcification by lowering the systemic fetuin-A level and increasing serum calcium-phosphate precipitates and induces inflammation and malnutrition in uremic rats fed a high-phosphate diet.

Government Quality Measures: First Do No Harm

A technical expert panel (TEP) was convened to evaluate all available literature in the area of bone and mineral metabolism and advise the Centers for Medicare and Medicaid services on the creation and collection of quality measures. The proposed quality measures were evaluated for importance, usability, feasibility, and scientific acceptability. All are important parameters to avoid unintended harmful consequences of a quality measure causing harm to patients or to the delivery of their health care.

Association of dietary phosphate and serum phosphorus concentration by levels of kidney function

BACKGROUND

The health implications of dietary phosphorus intake and the role of kidney function in managing serum phosphorus homeostasis are well studied. However, examining the source of dietary phosphorus intake and its impact on serum phosphorus has not been characterized in population studies.

OBJECTIVE

This study aimed to distinguish the association of food sources of organic phosphorus and inorganic phosphate additives with serum phosphorus concentration.

DESIGN

A cross-sectional analysis of 24-h food recall data from 7895 adult participants in the National Health and Nutrition Examination Survey 2003-2006 was performed. Phosphorus content of foods was categorized as organic or inorganic. Correlations of serum phosphorus to clinical and dietary intake variables were achieved by using multiple regression analysis.

RESULTS

After controlling for estimated glomerular filtration rate (eGFR), body mass index (BMI; in kg/m²), and albumin-to-creatinine ratio, a significant increase in serum phosphorus occurred with dairy foods with inorganic phosphates [parameter estimate (PE) ± SE: 0.07 ± 0.02 mg/dL, P < 0.01] or without inorganic phosphates (PE: 0.02 ± 0.01, P < 0.001) and cereals/grains with inorganic phosphates (PE: 0.005 ± 0.002, P < 0.01). Significantly higher serum phosphorus occurred when eGRF was <30 (PE: 0.24 ± 0.08, P < 0.0001), but eGFR 30-44 (PE: -0.11 ± 0.04, P < 0.01) and 45-60 (PE: -0.10 ± 0.04, P < 0.01) were associated with lower serum phosphorus; higher serum phosphorus was associated with BMI <18.5 (PE: 0.18 ± 0.05, P = 0.0009) but lower with BMI ≥35-39 (PE: -0.09 ± 0.03, P = 0.0013) or ≥40 (PE: -0.10 ± 0.03, P = 0.014).

CONCLUSIONS

This analysis shows that dairy products and cereals/grains having inorganic phosphate additives significantly increase serum phosphorus concentration, despite being consumed less frequently than foods without phosphate additives. It seems prudent for the Nutrient Facts Label to include phosphorus but also for food manufacturers to consider alternatives to phosphate additives.

The Envy of Scholars: Applying the Lessons of the Framingham Heart Study to the Prevention of Chronic Kidney Disease

During the past 50 years, a dramatic reduction in the mortality rate associated with cardiovascular disease has occurred in the US and other countries. Statistical modeling has revealed that approximately half of this reduction is the result of risk factor mitigation. The successful identification of such risk factors was pioneered and has continued with the Framingham Heart Study, which began in 1949 as a project of the US National Heart Institute (now part of the National Heart, Lung, and Blood Institute). Decreases in total cholesterol, blood pressure, smoking, and physical inactivity account for 24%, 20%, 12%, and 5% reductions in the mortality rate, respectively. Nephrology was designated as a recognized medical professional specialty a few years later. Hemodialysis was first performed in 1943. The US Medicare End-Stage Renal Disease (ESRD) Program was established in 1972. The number of patients in the program increased from 5,000 in the first year to more than 500,000 in recent years. Only recently have efforts for risk factor identification, early diagnosis, and prevention of chronic kidney disease (CKD) been undertaken. By applying the approach of the Framingham Heart Study to address CKD risk factors, we hope to mirror the success of cardiology; we aim to prevent progression to ESRD and to avoid the cardiovascular complications associated with CKD. In this paper, we present conceptual examples of risk factor modification for CKD, in the setting of this historical framework.

A Pilot Randomized Trial of Financial Incentives or Coaching to Lower Serum Phosphorus in Dialysis Patients

OBJECTIVE

Among chronic hemodialysis patients, hyperphosphatemia is common and associated with mortality. Behavioral economics and complementary behavior-change theories may offer valuable approaches to achieving phosphorus (PO4) control. The aim was to determine feasibility of implementing financial incentives and structured coaching to improve PO4 in the hemodialysis setting.

DESIGN AND METHODS

This pilot randomized controlled trial was conducted in 3 urban dialysis units for 10 weeks among 36 adults with elevated serum PO4 (median >5.5 mg/dL over 3 months).

INTERVENTIONS

Twelve participants each were randomized to: (1) financial incentives for lowering PO4, (2) coaching about dietary and medication adherence, or (3) usual care. PO4 was measured during routine clinic operations. Each incentives arm participant received the equivalent of $1.50/day if the PO4 was ≤5.5 mg/dL or >5.5 mg/dL but decreased ≥0.5 mg/dL since the prior measurement. The coach was instructed to contact coaching arm participants at least 3 times per week.

MAIN OUTCOME MEASURES

The outcome measures included: (1) enrollment rate, (2) dropout rate, and (3) change in PO4 from beginning to end of 10-week intervention period.

RESULTS

Of 66 eligible patients, 36 (55%) enrolled. Median age was 53 years, 83% were black race, and 78% were male. Median baseline PO4 was 6.0 (interquartile range 5.6, 7.5). Using stratified generalized estimation equation analyses, the monthly decline in PO4 was -0.32 mg/dL (95% CI -0.60, -0.04) in the incentives arm, -0.40 mg/dL (-0.60, -0.20) in the coaching arm, and -0.24 mg/dL (-0.60, 0.08) in the usual care arm. No patients dropped out. All intervention arm participants expressed interest in receiving similar support in the future.

CONCLUSIONS

This pilot trial demonstrated good feasibility in enrollment and implementation of novel behavioral health strategies to reduce PO4 in dialysis patients.

Rationale and Approaches to Phosphate and Fibroblast Growth Factor 23 Reduction in CKD

Patients with CKD often progress to ESRD and develop cardiovascular disease (CVD), yet available therapies only modestly improve clinical outcomes. Observational studies report independent associations between elevated serum phosphate and fibroblast growth factor 23 (FGF23) levels and risks of ESRD, CVD, and death. Phosphate excess induces arterial calcification, and although elevated FGF23 helps maintain serum phosphate levels in the normal range in CKD, it may contribute mechanistically to left ventricular hypertrophy (LVH). Consistent epidemiologic and experimental findings suggest the need to test therapeutic approaches that lower phosphate and FGF23 in CKD. Dietary phosphate absorption is one modifiable determinant of serum phosphate and FGF23 levels. Limited data from pilot studies in patients with CKD stages 3-4 suggest that phosphate binders, low phosphate diets, or vitamin B3 derivatives, such as niacin or nicotinamide, may reduce dietary phosphate absorption and serum phosphate and FGF23 levels. This review summarizes current knowledge regarding the deleterious systemic effects of phosphate and FGF23 excess, identifies questions that must be addressed before advancing to a full-scale clinical outcomes trial, and presents a novel therapeutic approach to lower serum phosphate and FGF23 levels that will be tested in the COMBINE Study: The CKD Optimal Management With BInders and NicotinamidE study.

Wakame (Undaria pinnatifida ) modulates hyperphosphatemia in a rat model of chronic renal failure

In chronic renal failure, inorganic phosphate (Pi) retention speeds up the progression to end-stage renal disease. The current therapy for hyperphosphatemia in patients with chronic renal failure consists of dietary Pi restriction combined with administration of Pi binders, but each therapy has practical problems. Thus, the discovery of foods or nutrients that inhibit Pi absorption may be useful for the treatment of hyperphosphatemia. In the present study, we investigated whether wakame (Undaria pinnatifida) is a useful food for the prevention of hyperphosphatemia in a rat model of renal failure. Feeding a diet containing 5% wakame significantly decreased plasma and urinary Pi levels and increased the amount of fecal Pi. In addition, wakame significantly reduced plasma blood urea nitrogen and plasma Pi levels in 5/6 nephrectomized rats fed a high-Pi diet. Biochemical analyses showed that the reduction of intestinal Pi absorption is the main reason for the decrease in plasma Pi levels in rats fed a diet containing wakame. In addition, feeding alginic acid and fucoidan, major components of wakame fiber, was effective in reducing plasma Pi levels in normal rats. Finally, we concluded that wakame may be a useful food for the prevention of hyperphosphatemia in rodents.

Effect of oral ferric citrate on serum phosphorus in hemodialysis patients: multicenter, randomized, double-blind, placebo-controlled study

BACKGROUND

Hyperphosphatemia is a common complication in dialysis patients that can be treated by oral phosphate binders. We investigated the efficacy and safety of oral ferric citrate as a phosphate binder for Taiwanese patients with end stage renal disease and with hyperphosphatemia who were undergoing hemodialysis.

METHODS

This was a prospective, double-blind, placebo-controlled, randomized trial carried out in 5 hospitals in Taiwan. Ferric citrate (4 or 6 g/day) or placebo was administered for 56 days. Serum calcium, phosphorous levels, calcium × phosphorus product, serum ferritin level, transferrin saturation, and adverse events were recorded.

RESULTS

A total of 166 patients completed the trial. The placebo group had relatively constant serum data. Serum phosphorus declined significantly in the 6 g/day group (p < 0.05 for 4 and 8 weeks) and the 4 g/day group (p < 0.05 for 4 and 8 weeks). There were similar changes in the calcium × phosphorus product. The serum ferritin level increased significantly in the 6 g/day group (p < 0.05) and the 4 g/day group (p < 0.05). There were similar changes in transferrin saturation. Most adverse events were mild to moderate and were comparable among the three groups.

CONCLUSIONS

A 56-day treatment with ferric citrate effectively controlled hyperphosphatemia and was well tolerated in maintenance hemodialysis patients. There were also moderate increases in serum ferritin and transferrin saturation.

Effects of water soaking and/or sodium polystyrene sulfonate addition on potassium content of foods

In this study, we determined, by atomic absorption spectrophotometry, the potassium amount leached by soaking or boiling foods identified by children suffering from chronic renal failure as "pleasure food" and that they cannot eat because of their low-potassium diet, and evaluated whether addition of sodium polystyrene sulfonate resin (i.e. Kayexalate®) during soaking or boiling modulated potassium loss. A significant amount of potassium content was removed by soaking (16% for chocolate and potato, 26% for apple, 37% for tomato and 41% for banana) or boiling in a large amount of water (73% for potato). Although Kayexalate® efficiently dose-dependently removed potassium from drinks (by 48% to 73%), resin addition during soaking or boiling did not eliminate more potassium from solid foods. Our results therefore provide useful information for dietitians who elaborate menus for people on potassium-restricted diets and would give an interesting alternative to the systematic elimination of all potassium-rich foods from their diet.

A phosphate-centric paradigm for pathophysiology and therapy of chronic kidney disease

Extracellular phosphate is toxic to the cell at high concentrations. When the phosphate level is increased in the blood by impaired urinary phosphate excretion, premature aging ensues. When the phosphate level is increased in the urine by dietary phosphate overload, this may lead to kidney damage (tubular injury and interstitial fibrosis). Extracellular phosphate exerts its cytotoxicity when it forms insoluble nanoparticles with calcium and fetuin-A, referred to as calciprotein particles (CPPs). CPPs are highly bioactive ligands that can induce various cellular responses, including osteogenic transformation of vascular smooth muscle cells and cell death in vascular endothelium and renal tubular epithelium. CPPs are detected in the blood of animal models and patients with chronic kidney disease (CKD) and associated with adaptation of the endocrine axes mediated by fibroblast growth factor-23 (FGF23) and Klotho that regulate mineral metabolism and aging. These observations have raised the possibility that CPPs may contribute to the pathophysiology of CKD. This notion, if validated, is expected to provide new diagnostic and therapeutic targets for CKD.

Hyperphosphatemia in patients with ESRD: assessing the current evidence linking outcomes with treatment adherence

In recent years, the imbalance in phosphate homeostasis in patients with end-stage renal disease (ESRD) has been the subject of much research. It appears that, while hyperphosphatemia may be a tangible indicator of deteriorating kidney function, lack of phosphate homeostasis may also be associated with the increased risk of cardiovascular events and mortality that has become a hallmark of ESRD. The need to maintain phosphorus concentrations within a recommended range is reflected in evidence-based guidelines. However, these do not reflect serum phosphorus concentrations achieved by most patients in clinical practice. Given this discrepancy, it is important to consider ways in which dietary restriction of phosphorus intake and, in particular, use of phosphate binders in patients with ESRD can be made more effective. Poor adherence is common in patients with ESRD and has been associated with inadequate control of serum phosphorus concentrations. Studies indicate that, among other factors, major reasons for poor adherence to phosphate binder therapy include high pill burden and patients’ lack of understanding of their condition and its treatment. This review examines available evidence, seeking to understand fully the reasons underlying poor adherence in patients with ESRD and consider possible strategies for improving adherence in clinical practice.

Gastrointestinal phosphate handling in CKD and its association with cardiovascular disease

Increases in serum concentrations of parathyroid hormone (PTH) and fibroblast growth factor 23 (FGF-23) and ultimately phosphate and decreases in 1,25-dihydroxyvitamin D level are thought to play a central role in the progressive nature of kidney disease and the development of cardiovascular disease in patients with chronic kidney disease. The initial changes in PTH and FGF-23 levels are adaptive to maintain serum phosphate concentration and phosphate load within defined levels by increasing urinary excretion of phosphate. Less well appreciated is the unanticipated finding that absorption of phosphate from the gastrointestinal tract is not downregulated in chronic kidney disease. This maladaptive response maintains higher levels of phosphate absorption, thereby contributing to the phosphate burden. Moreover, in response to a low-phosphate diet, as often is prescribed to such patients, gut phosphate absorption may be enhanced, undermining the potential beneficial effects of this intervention. Given the poor response to limiting phosphate intake and the use of phosphate binders, we suggest that research efforts be oriented toward better understanding of the factors that affect phosphate absorption in the gastrointestinal tract and the development of agents that directly inhibit phosphate transporters in the small intestine and/or their associated binding proteins.

A physiologic-based approach to the evaluation of a patient with hyperphosphatemia

Phosphate is required for skeletal mineralization, cellular energy regulation, synthesis of cell membranes and nucleic acids, and a variety of cell signaling pathways. Extracellular serum phosphate concentration is determined by the balance of gastrointestinal phosphate absorption, skeletal turnover, distribution in intracellular compartments, and renal phosphate excretion. An integrated system of hormones, receptors, and phosphate transporters regulates phosphate homeostasis, and a variety of hereditary and acquired perturbations in these regulators can result in hyperphosphatemia. Although kidney failure is the most common cause of hyperphosphatemia encountered by nephrologists, hyperphosphatemia that presents in patients with early stages of chronic kidney disease or normal kidney function should prompt a detailed evaluation that can be diagnostically challenging. In this teaching case, we describe a case of hyperphosphatemia out of proportion to the degree of decrease in glomerular filtration rate. We present a practical parathyroid hormone-based diagnostic approach that illustrates the current understanding of phosphate regulation in clinically meaningful terms for the practicing nephrologist. Finally, we illustrate how measurement of fibroblast growth factor 23 could be integrated in the future when the test becomes more widely available.

Outcomes associated with intradialytic oral nutritional supplements in patients undergoing maintenance hemodialysis: a quality improvement report

BACKGROUND

Insufficient clinical data exist to determine whether provision of oral nutritional supplements during dialysis can improve survival in hypoalbuminemic maintenance hemodialysis patients.

STUDY DESIGN

Retrospective matched-cohort study.

SETTING & PARTICIPANTS

All oral nutritional supplement program-eligible in-center maintenance hemodialysis patients with albumin level ≤3.5 g/dL in quarter 4 of 2009 without oral nutritional supplements in the prior 90 days at Fresenius Medical Care, North America facilities.

QUALITY IMPROVEMENT PLAN

Monitored intradialytic oral nutritional supplements were provided to eligible maintenance hemodialysis patients upon physician order, to continue for a year or until serum albumin level was ≥4.0 g/dL.

OUTCOME

Mortality (including deaths and withdrawals), followed up until December 31, 2010.

MEASUREMENTS

Both an intention-to-treat (ITT) and an as-treated analysis was performed using a 1:1 geographic region and propensity score-matched study population (using case-mix, laboratory test, access type, 30-day prior hospitalization, and incident patient status) comparing patients treated with intradialytic oral nutritional supplements with usual-care patients. Cox models were constructed, unadjusted and adjusted for facility standardized mortality ratio and case-mix and laboratory variables.

RESULTS

The ITT and as-treated analyses both showed lower mortality in the oral nutritional supplement group. The conservative ITT models with 5,227 matched pairs had 40% of controls subsequently receiving oral nutritional supplements after January 1, 2010 (because many physicians delayed participation), with comparative death rates of 30.1% versus 30.4%. The corresponding as-treated (excluding crossovers) death rates for 4,289 matched pairs were 30.9% versus 37.3%. The unadjusted ITT mortality HR for oral nutritional supplement use was 0.95 (95% CI, 0.88-1.01), and the adjusted HR was 0.91 (95% CI, 0.85-0.98); the corresponding as-treated HRs were 0.71 (95% CI, 0.66-0.76) and 0.66 (95% CI, 0.61-0.71) before and after adjustment, respectively.

LIMITATIONS

Limited capture of oral nutritional supplement intake outside the facility and potential residual confounding from unmeasured variables, such as dietary intake.

CONCLUSIONS

Maintenance hemodialysis patients with albumin levels ≤3.5 g/dL who received monitored intradialytic oral nutritional supplements showed survival significantly better than similar matched patient controls, with the as-treated analysis highlighting the potentially large effect of this strategy in clinical practice.

Update on fibroblast growth factor 23 in chronic kidney disease

Chronic kidney disease (CKD) is a public health epidemic that affects millions of people worldwide. Presence of CKD predisposes individuals to high risks of end-stage renal disease, cardiovascular disease and premature death. Disordered phosphate homeostasis with elevated circulating levels of fibroblast growth factor 23 (FGF23) is an early and pervasive complication of CKD. CKD is likely the most common cause of chronically elevated FGF23 levels, and the clinical condition in which levels are most markedly elevated. Although increases in FGF23 levels help maintain serum phosphate in the normal range in CKD, prospective studies in populations of pre-dialysis CKD, incident and prevalent end-stage renal disease, and kidney transplant recipients demonstrate that elevated FGF23 levels are independently associated with progression of CKD and development of cardiovascular events and mortality. It was originally thought that these observations were driven by elevated FGF23 acting as a highly sensitive biomarker of toxicity due to phosphate. However, FGF23 itself has now been shown to mediate “off-target,” direct, end-organ toxicity in the heart, which suggests that elevated FGF23 may be a novel mechanism of adverse outcomes in CKD. This report reviews recent advances in FGF23 biology relevant to CKD, the classical effects of FGF23 on mineral homeostasis, and the studies that established FGF23 excess as a biomarker and novel mechanism of cardiovascular disease. The report concludes with a critical review of the effects of different therapeutic strategies targeting FGF23 reduction and how these might be leveraged in a future randomized trial aimed at improving outcomes in CKD.

Dietary phosphorus intake and mortality in moderate chronic kidney disease: NHANES III

BACKGROUND

Dietary phosphorus intake is usually restricted in dialysis patients but the associations of dietary phosphorus intake with mortality in moderate chronic kidney disease (CKD) are unknown. Therefore, we examined these associations in National Health and Nutrition Examination Survey III.

METHODS

Dietary phosphorus intake was estimated from 24-h dietary recalls administered by trained personnel. CKD was defined as estimated glomerular filtration rate (eGFR) <60 mL/min/1.73 m(2). Time to mortality was examined by Cox regression models taking into account the complex survey design.

RESULTS

1105 adults with CKD were studied. Phosphorus intake was 1033 ± 482 mg/day (mean ± SD), eGFR was 49.3 ± 9.5 mL/min/1.73 m(2) and serum phosphorus was 3.5 ± 0.5 mg/dL. Compared to those in the lowest tertile of phosphorus intake (mean 532 ± 161 mg/day), those in the highest third (1478 ± 378 mg/day) had similar serum phosphorus levels (3.6 ± 0.5 versus 3.5 ± 0.6 mg/dL, P = 0.113) and modestly higher eGFR (50.0 ± 8.1 versus 47.5 ± 12.0 mL/min/1.73 m(2), P = 0.014). After adjustment for demographics, comorbidity, eGFR, physical activity, energy intake and nutritional variables, phosphorus intake was not associated with mortality [hazard ratio (HR) 0.98 per 100 mg/dL increase, 0.93-1.03].

CONCLUSIONS

High dietary phosphorus intake is not associated with increased mortality in moderate CKD, presumably because serum phosphorus levels are maintained in the normal range at this level of GFR. Interventional trials are needed to define optimal phosphorus intake in moderate CKD.