Discordance seen with reporting of food oxalate values

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A study evaluating the oxalate content of foods as reported by two reputable sources shows discordance.

This finding is important clinically and also with respect to conducting and interpreting research, said Kristina L. Penniston, PhD, RDN, CD, at the 2016 World Congress of Endourology in Cape Town, South Africa.

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Dr. Penniston“The lack of agreement between these two sources is striking considering that regulating the oxalate content of food is an important component in managing some patients with hyperoxaluria. Patients need accurate information in order to successfully limit their oxalate intake and so that they do not unnecessarily restrict their intake of certain foods,” said Dr. Penniston, scientist in the department of urology at the University of Wisconsin School of Medicine & Public Health, and clinical nutritionist at the University of Wisconsin Hospital and Clinics, Madison.

“Furthermore,” she added, “when designing intervention trials in which oxalate intake is a variable or research outcome, we need to have standardized values for oxalate content in order to be able to compare and apply the results from different studies.”

Recognizing the absence of standardized food oxalate values, Dr. Penniston and colleagues at the University of Wisconsin compared the data reported on a publicly available database of food oxalate content (maintained by the Harvard University School of Public Health) with values from the Nutrition Data System for Research (NDSR), a nutrient analysis software program.

“The data on the Harvard website provide information for 536 foods analyzed using liquid chromatography, which is considered the gold-standard, state-of-the art method for oxalate analysis. The NDSR is designed to be used by registered dietitians for nutrient analysis and is considered a high-quality resource, but the oxalate values it lists are from an array of laboratories that used a variety of analytical methods,” Dr. Penniston explained.

“Considering the different sources and techniques that are used to measure oxalate content, it is not surprising at all to see variability in values. And, our study compared only two of the most reputable sources. Patients searching the Internet can find hundreds of sites claiming to give oxalate content, and we expect to see even more variability across all of those sites.”

Comparisons for the 536 foods found on the Harvard University website showed a lack of concordance overall between the two data sources compared in this study, and differences were in both directions; ie, the NDSR nutrient analysis software listed higher oxalate values for some foods yet lower oxalate values for others. Although there was some correspondence between the two data sources with respect to foods identified as having the highest oxalate content, the milligrams of oxalate per serving often differed significantly.

Next: Correlation especially low for fruits, cereals

Correlation especially low for fruits, cereals

Correlation between the two databases was particularly low for fruits and ready-to-consume breakfast cereals. Within those food categories, there were individual items that would be identified as having low oxalate content by one source but high oxalate content by the other.

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“The discrepancy for these types of foods is especially concerning considering that nutritional counseling in general advises eating a diet rich in fruits and whole grain foods,” Dr. Penniston said.

In order to put the differences in the reported oxalate values into context, mean daily oxalate intake was calculated from diet records for a group of 62 patients seen at the University of Wisconsin stone clinic. Using a 90-mg/d cutoff for oxalate intake, nearly 40% of patients would be incorrectly classified as being above or below this target depending on which database was used.

To address the problem, Dr. Penniston and colleagues are issuing a call to action to standardize food oxalate values.

“As a start, we hope that individuals in the urology community who are interested in stone research will work together so that we can reach agreement about sources that should be used for research studies and that will be listed on informational materials provided to patients,” she told Urology Times.