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9.16.2010

Disparities in Kidney Disease Linked to Diet

Racial disparities in kidney disease outcomes are among the most glaring in the U.S. health system in that blacks are up to four times more likely to progress to end stage renal disease than whites, and have much higher rates of cardiovascular disease and mortality in early chronic kidney disease. Until now the differences have largely been blamed on genetic susceptibility and socioeconomic inequalities leading to poor access to care. Miller School researchers have now found evidence that not being able to afford a healthy diet may also play a significant role.

The findings will be published in an upcoming issue of the Journal of the American Society of Nephrology in an article titled “Impact of Race and Socioeconomic Status on Serum Phosphate Concentrations in the Chronic Renal Insufficiency Cohort Study (CRIC).”

The researchers looked at levels of phosphate in the blood of patients participating in the CRIC Study, established by the National Institute of Diabetes and Digestive and Kidney Diseases to examine risk factors for kidney disease progression and cardiovascular disease in patients with chronic kidney disease. A total of 3,612 racially and ethnically diverse patients from seven clinical centers across the United States were enrolled in the study.

“Many studies have demonstrated that an elevated level of phosphate in the blood is associated with adverse outcomes in patients with chronic kidney disease and that blacks have higher phosphate levels than whites, but we did not understand why levels are higher in blacks,” said Myles Wolf, M.D., M.M.Sc., associate professor of medicine, director of clinical research in the Division of Nephrology and Hypertension and senior author of the study. “Our earlier work in the general population suggested that poverty was linked to a higher phosphate level, so we decided to delve deeper into that connection in the setting of chronic kidney disease.”

Among CRIC participants, the researchers found that those with the lowest income and those who were unemployed had higher phosphate concentrations in their blood than participants with higher income and those who were employed. Furthermore, there was no difference in phosphate levels by race when only blacks and whites in the lowest income group were compared. The investigators concluded that the known racial difference in phosphate levels is largely driven by differences in socioeconomic status.

“For those who are living in poverty, their access to healthy food choices is limited, so their diet tends to consist of processed and fast foods that are heavily enriched with highly absorbable phosphorus additives,” said Orlando M. Gutierrez, M.D., M.M.Sc., assistant professor of medicine and lead author of the study. “Because of our current food labeling regulations, the amount of phosphorus additives in food is not always listed, so people are unknowingly ingesting more phosphorus than they probably should.”

Phosphorus is a mineral found naturally in many foods, such as milk, cheese, beans and peanut butter, and is vital for the formation of bones and teeth. It is also crucial for energy production and formation of cell membranes, which are the outer wall of cells. Since the kidney is responsible for excreting excess phosphate, patients with chronic kidney disease are at risk of developing increased blood levels of phosphate if they ingest too much. Different kinds of phosphates are added to foods to give them a longer shelf life and make them look and taste better.

“These additives go by such names as monocalcium phosphate, monosodium phosphate, sodium aluminum phosphates, and consumers do not even realize they are ingesting them,” said Gutierrez. “They are generally found in large amounts in convenience foods, ready-to-eat and processed foods, which are much cheaper to buy.”

The researchers write that the findings of the study “may have important implications for efforts to stem the rising prevalence of kidney disease and one of its major complications, hyperphosphatemia (an abnormally high blood level of phosphate), among minorities and the very poor.” They go on to suggest that new nutritional assessment instruments need to be designed to better measure the intake of phosphorus additives.

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