A study of patients who received dialysis at centers across the United States revealed no link between neighborhood food environment – including access to “healthy foods” and socioeconomic status – and hyperphosphatemia.
This finding led Vishnu S. Potluri, MD, MPH, of the Perelman School of Medicine at the University of Pennsylvania, and colleagues to suggest factors such as individual food preference, nutritional education and food labeling may have more of an impact on serum phosphorous management.“Foods rich in phosphorus are prevalent in the American diet,” Potluri and colleagues wrote. “Geographic areas with more poverty often have less availability of low-phosphorus alternatives, including fruits and vegetables. Because of the phosphorus-rich content of many types of food, we hypothesized that restrictions on food availability might affect phosphorus control for patients on dialysis, acknowledging that dietary choices are complex and depend on multiple factors, including gustatory preferences, sociocultural norms and comorbidities such as diabetes, income and nutrition literacy.”
For the study, researchers included 258,510 patients who received chronic hemodialysis between 2005 and 2013 (45% were women; 32% were Black patients; 15% were Hispanic patients).
Using the criteria defined by the CDC and Prevention Modified Retail Food Environment Index to categorize neighborhood availability of healthy food, Potluri and colleagues found patients had a median of 25 “less-healthy” food outlets available to them compared with a median of four “healthy” food outlets.
Despite this, researchers observed no association between living in a neighborhood with increased availability of healthy food and lower phosphorus levels. In addition, neighborhood income did not appear to affect phosphorous levels.
As for factors more closely associated with phosphorous levels, researchers determined patient age and race had an impact.
More specifically, they found older patients had a lower serum phosphorus than younger patients (serum phosphorus was 0.3 mg/dL lower for every 10-year increase in age), while patients who self-identified as Black or Hispanic had slightly lower phosphorous levels than white patients (0.2 mg/dL and 0.1 mg/dL, respectively).
“Understanding a patient’s income, food availability near where they live and their personal preferences can help develop personalized dietary plans to reduce dietary phosphorus intake,” the researchers wrote of their findings. “Future remedies to improve phosphorus control may require addressing these issues, as well as the pervasive problem of inadequate labeling.”
In terms of “inadequate labeling,” Potluri and colleagues highlighted that the FDA does not require food manufacturers to include phosphorous content.
“When the phosphorus content of food is listed on labels, the label can be challenging to interpret or the phosphorus content may be inaccurately stated,” they contended, adding “many patients on chronic dialysis have low health and nutrition literacy and limited income, creating further barriers to selecting meals low in phosphorus.”In the “Effect of neighborhood food environment and socioeconomic status on serum phosphorus level for patients on chronic dialysis,” SEC and S. PO4 levels were evaluated on more than 258,000 patients receiving hemodialysis across the United States. No correlation between SEC and S. PO4 was found. Instead, researchers found an inverse relationship between age and S. PO4 levels.
Though well thought out and documented, the study methods fall short in multiple areas which weakens the study.
The main fault with this study is the determination of patient SEC. Instead of obtaining SEC factors from individual participants, researchers obtained SEC information of the zip code where patients lived. Neighborhood SEC has little bearing on an individual’s SEC. It is the individual receiving dialysis, not the neighborhood.
In comparison to this study, Gutiérrez and colleagues have also looked at the relationship between SEC and S. PO4 levels multiple time. In his studies, individual SEC was determined using annual income, education level, employment status and poverty-to-income ratio. By doing this, he had a truer indication of SEC.
Secondly, researchers made multiple assumptions with regard to food procurement of the participants. They assumed participants shopped within their zip codes and they did not evaluate profiles of food outlets in surrounding areas. They have no information about patients’ social lives regarding food procurement. Older patients many need assistance with grocery shopping and have family members making food choices or they may be receiving community-sponsored meals (ie, Meals on Wheels, Mom’s Meals) that are adjusted for dialysis diet restrictions.Additionally, researchers had no insight into what foods were purchased by patients. Just because there are multiple “healthy” food outlets for a patient to shop, does not mean that the patient is choosing low phosphorus foods. Stores deemed “healthy” sell high phosphorus foods.
To understand participants’ diets, Gutiérrez utilized multiple food frequency questionnaires and 24-hour diet recalls giving him a fuller picture of the dietary PO4 sources.
Finally, only one S.PO4 level was obtained each month. Patients may have multiple S. PO4 results in a single month, and the researchers did not indicate which level was used for analysis. This brings into question the quality of the data. One dialysis unit may test S. PO4 levels once a month, bi-weekly or weekly. This must be considered when standardizing data collection. Also, S. PO4 is affected by non-nutritional factors including metabolic acidosis, poor urea kinetic results and missed treatments.
Potluri and colleagues can be congratulated for taking on the first analysis looking at SEC and S. PO4 of hemodialysis patients. Though there are many faults with their methods, their outcomes identified the need to increase education and support younger patients regarding S. PO4 management.
Gutiérrez O, et. al. Lower socioeconomic status associated with higher serum phosphate irrespective of race. J Am Soc. Nephrol. 21: 1953-1960. 2010.
Gutiérrez O, et. al. Impact of Poverty on Serum Phosphate Concentration in the Third National Health and Nutrition Examination Survey. J Ren Nutr. 21: 140-148. 2011.
Gutiérrez O, et. al. Associations of socioeconomic status and processed food intake with serum phosphorus concentrations in community-living adults: The multi-ethnic study of atherosclerosis (MESA). J Ren Nutr. 22: 480-489. 2012.