The role of dietary phosphorus restriction in the conservative management of chronic renal disease

Plasma metabolite biomarkers related to secondary hyperparathyroidism and parathyroid hormone

BACKGROUND

Hyperphosphatemia, hypocalcemia, and elevation of parathyroid hormone (PTH) are typical abnormalities of uremic patients with Secondary hyperparathyroidism (SHPT). However, metabolic imbalance associated with SHPT is not well understood.

METHODS

A total of 15 SHPT patients with an intact parathyroid hormone (iPTH) level > 600 pg/mL were set as preoperative (PR) group, 15 age- and gender-matched controls who had undergone parathyroidectomy plus forearm transplantation because of hyperparathyroidism and achieved an iPTH level <150 pg/mL were set as postoperative (PO) group. Metabolite profiling of these 30 uremic patients and five healthy controls (HC) was performed using ultra performance liquid chromatography-mass spectrometry.

RESULTS

Five differential metabolites, including allyl isothiocyanate, L-phenylalanine, D-Aspartic acid, indoleacetaldehyde, and D-galactose correlated with PTH were identified in this study. Taking them as a biomarker signature, PR group can be distinguished from HC group with an area under the curve (AUC) of 0.947 (95% CI, 0.76-1) and PO group with an AUC of 0.6 (95% CI, 0.38-0.807).

CONCLUSIONS

The serum metabolome correlated with PTH is successfully demonstrated for a better understanding of the pathogenesis of SHPT.

Phosphate balance in ESRD: diet, dialysis and binders against the low evident masked pool

Phosphate metabolism is crucial in the pathophysiology of secondary hyperparathyroidism and vascular calcification. High phosphate levels have been consistently associated with unfavorable outcomes in dialysis patients, but several limitations are still hampering a resolutive definition of the optimal targets of phosphate serum levels to be achieved in this cohort. Nonetheless, hyperphosphatemia is a late marker of phosphate overload in humans. Clinical nephrologists routinely counteract the positive phosphate balance in dialysis patients through nutritional counseling, stronger phosphate removal by dialysis and prescription of phosphate binders. However, the superiority against placebo of phosphate control by diet, dialysis or binders in terms of survival has never been tested in dedicated randomized controlled trials. The present review discusses this conundrum with particular emphasis on the rationale supporting the value of a simultaneous intervention against phosphate overload in dialysis patients via the improvement of dietary intakes, dialysis efficiency and an individualized choice of phosphate binders.

Management of natural and added dietary phosphorus burden in kidney disease

Phosphorus retention occurs from higher dietary phosphorus intake relative to its renal excretion or dialysis removal. In the gastrointestinal tract the naturally existing organic phosphorus is only partially (∼60%) absorbable; however, this absorption varies widely and is lower for plant-based phosphorus including phytate (<40%) and higher for foods enhanced with inorganic phosphorus-containing preservatives (>80%). The latter phosphorus often remains unrecognized by patients and health care professionals, even though it is widely used in contemporary diets, in particular, low-cost foods. In a nonenhanced mixed diet, digestible phosphorus correlates closely with total protein content, making protein-rich foods a main source of natural phosphorus. Phosphorus burden is limited more appropriately in predialysis patients who are on a low-protein diet (∼0.6 g/kg/d), whereas dialysis patients who require higher protein intake (∼1.2 g/kg/d) are subject to a higher dietary phosphorus load. An effective and patient-friendly approach to reduce phosphorus intake without depriving patients of adequate proteins is to educate patients to avoid foods with high phosphorus relative to protein such as egg yolk and those with high amounts of phosphorus-based preservatives such as certain soft drinks and enhanced cheese and meat. Phosphorus rich foods should be prepared by boiling, which reduces phosphorus as well as sodium and potassium content, or by other types of cooking-induced demineralization. The dose of phosphorus-binding therapy should be adjusted separately for the amount and absorbability of phosphorus in each meal. Dietician counseling to address the emerging aspects of dietary phosphorus management is instrumental for achieving a reduction of phosphorus load.

Nutritional knowledge in hemodialysis patients and nurses: focus on phosphorus

OBJECTIVE

To assess the knowledge of adult hemodialysis patients and nurses working in dialysis units, specifically with regard to knowledge of phosphorus and other nutrients related to dietary management of end-stage renal disease.

DESIGN

Cross-sectional cohort study.

SETTING

Hemodialysis unit.

SUBJECTS

One hundred ninety-one hemodialysis patients and 105 dialysis nurses, as well as 86 control hospital employees who are not health professionals.

INTERVENTION

Nutritional knowledge was assessed by a 25-item chronic kidney disease knowledge assessment tool for nutrition, which includes 15 questions on phosphorus and 10 questions on protein, sodium, and potassium knowledge.

RESULTS

The scores obtained by patients were much lower than those of nurses (11.6 ± 3.9 vs. 16.0 ± 2.2, P < .001) but slightly higher than those of controls (10.6 ± 3.2, P < .05). Patients with phosphorus serum level >5.5 mg/dL showed chronic kidney disease knowledge assessment tool for nutrition scores similar to those of patients with a serum phosphorus level <5.5 mg/dL. The prevalence of right answers to questions regarding knowledge of phosphorus was lower than that regarding knowledge of the other nutrients, both for patients (38.4% ± 17.8% vs. 57.3% ± 19.9%, P < .001) and nurses (55.6% ± 11.1% vs. 74.8% ± 11.7%, P < .001) as well as for controls (30.7% ± 14.5% vs. 60.1% ± 17.4%, P < .001).

CONCLUSIONS

Our study suggests that nutritional knowledge of hemodialysis patients, although higher than the general population, is lower for phosphorus with respect to the other nutrients, such as protein, sodium, and potassium. This occurs even in patients with hyperphosphatemia or those taking phosphate binder medications. Nurses showed the best scores; however, improvement is necessary, especially with regard to knowledge of phosphorus. Training programs on nutrition for nurses and on information for patients should be implemented. They can contribute to achievement of a more effective control of phosphate balance, reduction of costs, and improvement of the quality of care for hemodialysis patients.

Derangements in phosphate metabolism in chronic kidney diseases/endstage renal disease: therapeutic considerations

The changes in phosphate (PO(4)) metabolism across the spectrum of chronic kidney disease (CKD) and specific strategies to address these abnormalities by reducing PO(4) loads are discussed in this review. This review also addresses briefly the evidence for specific PO(4) serum targets in CKD and endstage renal disease (ESRD) and the potential for other biomarkers such as fibroblast growth factor-23 (FGF-23) to define disease and monitor the effectiveness of therapy. As renal function declines, single nephron excretion of PO(4) must increase to maintain PO(4) balance. Abnormalities in PO(4) metabolism occur early in CKD. Compensatory changes in renal PO(4) handling are sufficient to maintain a normal serum PO(4) level in early stages of CKD, but in more advanced CKD, these processes no longer suffice and overt hyperphosphatemia develops. The resulting increased PO(4) burden contributes directly to development of secondary hyperparathyroidism. The FGF-23 increases early in CKD, likely in response to abnormal PO(4) metabolism, and mediates processes that help restore serum PO(4) levels to normal in CKD stage 3 and in early stage 4. The increased PO(4) burden and subsequent overt hyperphosphatemia are associated with increased mortality and morbidity. Dietary PO(4) restriction, modification of dialysis prescriptions, and administration of oral PO(4) binders can restore PO(4) balance. As CKD progresses, population-based studies demonstrate that diet alone is typically not able to prevent or treat hyperphosphatemia. Dialysis modalities that are currently used often fail to remove sufficient PO(4) to prevent hyperphosphatemia in patients with an inadequately controlled dietary PO(4) load. This is particularly likely among patients without significant residual renal function. Thus, in the majority of ESRD patients, PO(4) binders remain the mainstay of therapy for hyperphosphatemia. All currently available PO(4) binders can restore serum PO(4) to the required level when administered appropriately and in conjunction with dietary PO(4) restrictions. PO(4) binders differ regarding their potential side-effects and impact on long-term patient-centered outcomes. Which of the PO(4) binders might result in the most favorable survival and cardiovascular morbidity profiles and which remain uncertain, remains a subject of considerable clinical investigation. Compelling observational and more limited randomized controlled trial (RCT) evidence support the view that PO(4) binders might differ in their effects on mortality and/or morbidity. The limited evidence from RCTs is mostly congruent with the findings from large observational studies. In particular, evidences from both epidemiologic and RCT support the view that excess calcium administration may independently increase the risk of cardiovascular disease in individuals with normal renal function and in patients with CKD and ESRD. Additional RCT evidence might help determine the degree at which any increased risk from oral calcium exposure can be mitigated with the use of noncalcium-based PO(4) binders. Judicious control of PO(4) early in CKD, possibly monitored by measures of FGF-23, could potentially reduce the risk of development of renal secondary hyperparathyroidism and all of the adverse clinical consequences of poorly controlled CKD-mineral and bone disorder. The mainstays of therapy are likely to include a balance of dietary restriction and PO(4) binders to reduce PO(4) input, and in ESRD patients, dialysis modalities to augment PO(4) output.

Extra-phosphate load from food additives in commonly eaten foods: a real and insidious danger for renal patients

BACKGROUND

Restriction of dietary phosphorus is a major aspect of patient care in those with renal disease. Restriction of dietary phosphorus is necessary to control for phosphate balance during both conservative therapy and dialysis treatment. The extra amount of phosphorus which is consumed as a result of phosphate-containing food additives is a real challenge for patients with renal disease and for dieticians because it represents a "hidden" phosphate load. The objective of this study was to measure phosphorus content in foods, common protein sources in particular, and comprised both those which included a listing of phosphate additives and those which did not.

METHODS

Determinations of dry matter, nitrogen, total and soluble phosphate ions were carried out in 60 samples of foods, namely cooked ham, roast breast turkey, and roast breast chicken, of which, 30 were with declared phosphate additives and the other 30 similar items were without additives.

RESULTS

Total phosphorus (290 ± 40 mg/100 g vs. 185 ± 23 mg/100 g, P < .001) and soluble phosphorus (164 ± 25 mg/100 g vs. 100 ± 19 mg/100 g, P < .001) content were higher in products containing additives than in foods without additives. No difference was detected between the 2 groups regarding dry matter (27.2 ± 2.0 g/100 g vs. 26.7 ± 1.9 g/100 g) or total nitrogen (3.15 ± 0.40 g/100 g vs. 3.19 ± 0.40 g/100 g). Consequently, phosphorus intake per gram of protein was much greater in the foods containing phosphorus additives (15.0 ± 3.1 mg/g vs. 9.3 ± 0.7 mg/g, P < .001).

CONCLUSIONS

Our results show that those foods which contain phosphate additives have a phosphorus content nearly 70% higher than the samples which did not contain additives. This creates a special concern because this extra amount of phosphorus is almost completely absorbed by the intestinal tract. These hidden phosphates worsen phosphate balance control and increase the need for phosphate binders and related costs. Information and educational programs are essential to make patients with renal disease aware of the existence of foods with phosphate additives. Moreover, these facts highlight the need for national and international authorities to devote more attention to food labels which should clearly report the amount of natural or added phosphorus.

Soy-protein consumption and kidney-related biomarkers among type 2 diabetics: a crossover, randomized clinical trial

OBJECTIVE

Renal disease is a major problem among diabetic patients. The type of protein consumed may affect alterations in kidney-related biomarkers in these patients. This study sought to assess the effects of soy-protein consumption on renal-related markers among type 2 diabetic patients with nephropathy.

METHODS

A crossover, randomized clinical trial was conducted among 14 patients (10 men and 4 women). One diet contained 0.8 g/kg protein (70% animal and 30% vegetable proteins), and a similar diet contained the same amount of protein with 35% animal protein, 35% soy protein, and 30% other vegetable proteins. These two diets were prescribed in each phase of the trial for 7 weeks. There was a 4-week washout between the two phases of the study.

RESULTS

Consumption of soy protein reduced urinary urea nitrogen (-0.9 +/- 0.8 vs. 0.2 +/- 0.6 mg/dL, respectively, SD; P < .001), proteinuria (-78 +/- 37 vs. 42 +/- 39 mg/day, respectively, SD; P < .001), blood sodium (-2 +/- 0.04 vs. 2.0 +/- 0.06 mg/dL, respectively, SD; P < .01), and serum phosphorus (-0.03 +/- 0.2 vs. 0.2 +/- 0.3 mg/dL, respectively, SD; P < .01) compared with animal protein. Serum and urinary creatinine, blood urea nitrogen, serum calcium, and potassium levels were not significantly changed in soy-protein versus anima-protein consumption.

CONCLUSION

Soy-protein consumption reduces proteinuria in type 2 diabetes with nephropathy.

Acute effects of different phosphorus sources on calcium and bone metabolism in young women: a whole-foods approach

The recommended dietary phosphorus intake is exceeded in the typical Western diet. However, few studies have been conducted on the bioavailability and metabolic consequences of dietary phosphorus from different food sources. In this study, acute effects of dietary phosphorus from three different food sources and a phosphate supplement on calcium and bone metabolism were investigated. Sixteen healthy women aged 20-30 years were randomized to five controlled 24-hour study sessions, each subject serving as her own control. At the control session, calcium intake was ca. 250 mg and phosphorus intake ca. 500 mg. During the other four sessions, phosphorus intake was about 1,500 mg, 1,000 mg of which was obtained from meat, cheese, whole grains, or a phosphate supplement, respectively. The foods served were exactly the same during the phosphorus sessions and the control session; only phosphorus sources varied. Markers of calcium and bone metabolism were followed. Analysis of variance with repeated measures was used to compare the study sessions. Only the phosphate supplement increased serum parathyroid hormone (S-PTH) concentration compared with the control session (P = 0.031). Relative to the control session, meat increased markers of both bone formation (P = 0.045) and bone resorption (P = 0.049). Cheese decreased S-PTH (P = 0.0001) and bone resorption (P = 0.008). These data suggest that the metabolic response was different for different foods.