The Impact of Dietary Protein on Urinary Oxalate Levels Utilizing the Nutritional Data System for Research (NDSR)
INTRODUCTION
The impact of dietary protein intake on urinary oxalate has not been clearly delineated. Potential reasons include inaccurate dietary assessment of oxalate, calcium, and vitamin C, as well as lack of control for the amount and source of protein. This study will be the first to use the NDSR to obtain a precise dietary assessment of these factors on self-selected rather than controlled diets. The objective of this study is to evaluate the impact of the amount and source of protein intake on urinary oxalate for patients with recent urolithiasis using the NDSR.
METHODS
We prospectively recruited patients from a single institution who either passed or underwent treatment for a calcium oxalate stone within 6 months and were subsequently referred for a complete metabolic work-up. Patients with metabolic predispositions to stone formation or taking medications that may impact urine parameters were excluded. Total dietary intake was obtained using a 3 day food record and 24 hour (24hr) dietary recall. A trained bionutritionist analyzed food records with participants using NDSR and conducted a 24hr dietary recall using a multiple pass method. Two 24hr urine samples were collected after enrollment, one of which was performed the same day of the 24 hour food recall. The combined 3 day dietary record with 24hr recall were compared to an average of the two urine samples; 24hr recall was compared to the same day urine sample. Linear and multiple regression analysis were used to determine the effect of the amount and source of protein intake on urinary oxalate, when accounting for dietary calcium, oxalate and vitamin C.
RESULTS
A total of 15 patients were evaluated. Mean age was 45.1 ± 18.3 years and BMI 26.0 ± 6.9 with a higher proportion of women (60%). Mean urinary oxalate level was 32.5 ± 12.0 mg/day. On linear regression, total and vegetable protein intake each trended towards a statistically significant association with urinary oxalate (β = 0.50, p = 0.06 and β = 0.50, p = 0.06). Multiple regression demonstrated a significant association between both total protein and dietary oxalate intake and urinary oxalate (β = 0.33, p = 0.03 and β = 0.06, p = 0.04). No statistically significant association between 24hr recall of total, animal, or vegetable dietary protein and urinary oxalate was observed.
CONCLUSION
The amount of dietary protein was positively associated with urinary oxalate levels, after accounting for confounders. Total protein as measured in a total dietary assessment with NDSR may hold more predictive value for increased risk of stone formation than a 24hr dietary recall alone.
Funding: Medical College of Wisconsin, Research Affairs Committee, New Faculty Pilot Grant