Evaluating the Factors Affecting Serum Phosphate Levels, Including Dietary Phosphate-containing Additives, in Chronic Kidney Disease.

ABSTRACT

Background: Hyperphosphataemia (serum phosphate>1.7mmol/L) in end stage renal disease is independently linked to higher rates of morbidity and mortality and effects 30% of haemodialysis (HD) patients in the UK[1,2]. Current dietary practices focus on reducing the intake of naturally sourced phosphate to 800-1000mg/day and highly nutritious foods are often limited. Phosphate-containing additives (PCA), of which there are 14 listed by the European Commission, may be contributing an additional 1000mg phosphate per day [3]; it is therefore important to have knowledge of this source of phosphate in the diet. The aim of the service evaluation was to assess the relationship between dialysis parameters, medication, dietary intake of phosphate and PCA on the management of hyperphosphataemia. Method: 24 established (>90days) adult HD patients were recruited from Kent and Canterbury Renal Unit. Intake of phosphate binders and dietary information were collected using a 3-day diet diary (not weighed) and a questionnaire to clarify intake of high phosphate foods and phosphate binders. Phosphate-containing additives were considered using the weight of processed foods containing at least one such additive, stated on packaging or online nutritional databases. In addition, efficiency of HD (Kt/V) and residual renal function using glomerular filtration rate (GFR) and urine output, were considered. DietPlan6 and SPSS were used for data analysis. Statistical tests assessed data distribution, Pearson correlation and any differences in variables between patients with and without hyperphosphataemia (T-test). Ethical approval was granted by the University of Surrey, NHS ethics not required as it was a service evaluation. Results: The average serum phosphate was 1.73±0.52mmol/L, 38% of patients had hyperphosphataemia (>1.7mmol/L) and 79% of patients were prescribed a phosphate binder. 19/24 patients returned the 3-day diet diary and questionnaire. Mean phosphate intake was 884±278mg/day with approximately 85% of patients consuming less phosphate, energy and protein per kilogram per day than recommended (Table 1). Processed foods containing PCA were positively correlated to serum phosphate levels (r2=0.234, p=0.036), however, correlation no longer existed once outliers were excluded (r2=0.114, p=0.184). Serum potassium also correlated with the intake of processed food (r2=0.326, p = 0.017). 47% of patients had inadequate dialysis (Kt/V<1.2) and 71% of patients produced less than 500ml of urine per day. Mean GFR was significantly lower in patients with hyperphosphataemia; 7.25ml/min vs 5.8ml/min (p=0.021). Discussion: The outliers regarding the consumption of phosphate-containing food additives were largely due to high consumption of high phosphate soft drinks (eg cola). Overall intake, including phosphate, maybe underestimated as weighed food diaries were not used, however the fact that both serum phosphate and potassium correlate with intake of processed food has implications for dietary advice and recommendations. Inadequate dialysis may also be an important factor in hyperphosphataemia and needs to be addressed by the multidisciplinary team. Conclusion: Maximising dialysis dose will improve both phosphate and potassium levels and should perhaps be considered before increasing dietary restrictions.Disclosure of phosphate and potassium content on food and drink packaging would help determine more accurately the impact of food additives on biochemistry levels in renal patients.