Dietary Protein-Induced Increases in Urinary Calcium Are Accompanied by Similar Increases in Urinary Nitrogen and Urinary Urea: A Controlled Clinical Trial

To determine the usefulness of urinary urea as an index of dietary protein intake, 10 postmenopausal women were enrolled in and completed a randomized, double-blind, cross-over feeding trial from September 2008 to May 2010 that compared 10 days of a 45-g whey supplement with 10 days of a 45-g maltodextrin control. Urinary nitrogen, urinary calcium, urinary urea, and bone turnover markers were measured at days 0, 7, and 10. Paired sample t tests, Pearson's correlation statistic, and simple linear regression were used to assess differences between treatments and associations among urinary metabolites. Urinary nitrogen/urinary creatinine rose from 12.3 +/- 1.7 g/g (99.6 +/- 13.8 mmol/mmol) to 16.8 +/- 2.2 g/g (135.5 +/- 17.8 mmol/mmol) with whey supplementation, but did not change with maltodextrin. Whey supplementation caused urinary calcium to rise by 4.76 +/- 1.84 mg (1.19 +/- 0.46 mmol) without a change in bone turnover markers. Because our goal was to estimate protein intake from urinary nitrogen/urinary creatinine, we used our data to develop the following equation: protein intake (g/day)= 71.221+1.719x(urinary nitrogen, g)/creatinine, g) (R=0.46, R-2=0.21). As a more rapid and less costly alternative to urinary nitrogen/urinary creatinine, we next determined whether urinary urea could predict protein intake and found that protein intake (g/day)= 63.844+1.11x(urinary urea, g/creatinine, g) (R=0.58, R-2=0.34). These data indicate that urinary urea/urinary creatinine is at least as good a marker of dietary protein intake as urinary nitrogen and is easier to quantitate in nutrition intervention trials. Acad Nutr Diet. 2013;113:447-451.