UC Irvine

Background: Hyperphosphatemia is a known predictor of mortality in hemodialysis patients. Disruption of serum phosphorus levels have been related to subsequent disruption of parathyroid hormone and calcium homeostasis. These disruptions in mineral and hormone balance may lead to left ventricular fibrosis and hypertrophy, vascular calcification, and eventually sudden cardiac death. Hemodialysis often does not sufficiently remove enough phosphorus to reach recommended concentrations of serum phosphorus, making hyperphosphatemia common among hemodialysis patients.

Current renal dietary guidelines recommend for patients to consume a low phosphorus diet in order to reduce the risk of hyperphosphatemia. However, there are few studies to support this dietary practice. In the studies that exist, there have been conflicting reports concerning the outcomes of dietary phosphorus restriction. As lowering dietary phosphorus has the potential to concomitantly reduce intake of heart-healthy macro- and micronutrients, and increase risk of protein energy wasting, there is discussion for liberalization of diet with greater emphasis on serum phosphorus management with a focused reduction in inorganic phosphorus additives and phosphorus binder medications. This study utilized hemodialysis patients participating in the Malnutrition, Diet, and Racial Disparities in Chronic Kidney Disease (MADRAD) study.

Objectives: In this dissertation, I measured the association between absoulute dietary phosphorus intake, phosphorus/1000 kcal, and phosphorus-to-protein ratio and mortality in MADRAD study patients (Chapter 2); analyzed data from 3-day diet records collected from MADRAD study patients to gain insight into dietary phosphorus intakes and related dialysis patient clinical and sociodemographic characteristics (Chapter 3); and investigated dialysis patients’ perceptions of the renal diet, behaviors and attitudes of renal clinicians, and how they related to dietary adherence (Chapter 4).

Methods: Study participants are enrolled in the Malnutrition, Diet, and Racial Disparities in Chronic Kidney Disease Study. In Chapter 2, primary analyses included examining the association between measures of dietary phosphorus intake (absolute intake, phosphorus/1000 kcal, and phosphorus-to-protein ratio), categorized into tertiles and quartiles, and all-cause mortality. In Chapter 3, primary analyses involved conducting logistic regression analyses to examine the association of sociodemographic, comorbidity, dialysis treatment, health insurance, and dietary intake characteristics with likelihood of above median dietary intakes of phosphorus-to-total-protein ratio, phosphorus-to-plant-protein ratio, phosphorus/1000 kcal, and plant-protein-to-total-protein ratio, respectively. In Chapter 4, a semi-structured interview was conducted with two male hemodialysis patients and their renal clinicians were also observed.

Results: In Chapter 2, the lowest intakes of absolute dietary phosphorus intake, phosphorus/1000 kcal, and phosphorus-to-protein ratio were associated with mortality in unadjusted models and various models of expanded levels of adjustment. In expanded case-mix spline analyses that examined the association of daily dietary phosphorus as a continuous variable and mortality risk, there was a trend that daily dietary phosphorus intake in the lowest tertile was associated with higher death risk. In Chapter 3, I found that predictors of dietary phosphorus and protein intake included Black race, female sex, older age, and being single. In Chapter 4, I found that factors associated with patient dietary adherence included social support, self-efficacy, self-discipline, barriers, and cultural food norms. Renal clinicians were perceived as being supportive and encouraging in setting dietary goals and being accountable.

Conclusion: The finding that lower dietary phosphorus intake in dialysis patients is associated with greater death risk contradicts current recommendations for dialysis patients to eat a low phosphorus diet. Intake of dietary phosphorus is related to many modifiable and non-modifiable patient characteristics, and dietary recommendations may need to be liberalized with a focus on more enjoyable unprocessed whole foods and reductions in added phosphorus.