This article appraisal is part of the EMiNEM Bone and Mineral Metabolism Series. Click here to reach the EMiNEM homepage on UKidney
To determine the effect of socioeconomic status on the relationship between race and serum phosphorus.
Study Design and Population:
A cross-sectional analysis of race, socioeconomic status and serum phosphate levels was performed for 2879 participants in the Chronic Renal Insufficiency Cohort (CRIC) Study. The CRIC study is a prospective cohort of patients (aged 21-74 years) with mild to moderate kidney disease and includes a racially and ethnically diverse group of patients. 3612 patients were initially enrolled in the study. Patients who did not include their income were excluded from the analysis.
Intervention or Observation:
Socioeconomic indices included: annual household income, highest level of education achieved and employment status.
Patients were asked to classify their race as black, white or other.
Laboratory values measured were phosphate, albumin, calcium, creatinine, PTH and hemoglobin A1C.
Diet was assessed using a validated food frequency questionnaire (the National Cancer Institute Diet History Questionnaire).
Lower socioeconomic status was independently associated with higher serum phosphate concentrations regardless of race.
In the study sample, mean serum phosphate significantly increased with decreasing levels of annual income. While slightly higher serum phosphate was noted in blacks in the highest income groups, both blacks and whites in the lowest income group were more than twice as likely to have a high serum phosphate.
The cross-sectional design of this study does not allow for causal relationships to be determined however it does indicate that there may be more than biological or cultural differences to explain racial differences in serum phosphate. It has been well demonstrated in the US that despite similar rates of CKD, blacks are much more likely to progress to ESRD, have higher CVD and mortality rates than whites. The authors suggest that higher low socioeconomic status and thus restricted access to healthy, unprocessed foods plays a large role in these racial disparities.
While there was no differences in estimated dietary phosphate intake across intake levels, the food frequency questionnaire used was not designed or validated for phosphate specifically and did not take into account phosphate additives which are regularly used in inexpensive convenience and fast foods. In addition dietary data was missing in 21% of the population. Unfortunately while diet characteristics such as calcium and phosphorus were included, frequency of fast food consumption was not.
Impact on Practice:
This study suggests that low-income is another risk factor for hyperphosphatemia and supports routine dietary counselling about processed and fast foods early in CKD. This study may help support patients and clinicians advocating for financial support for the increased cost and burden of the renal diet.
Reviewed by Reviewed by June Martin, RD