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

• Published in: Journal of the Academy of Nutrition and Dietetics 2013-03, Vol.113 (3), p.447-451
• Main Authors: Bihuniak, Jessica D., MS, RD, CDN, Simpson, Christine A., MS, Sullivan, Rebecca R., MS, RD, Caseria, Donna M., MS, RD, Kerstetter, Jane E., PhD, RD, Insogna, Karl L., MD
• Format: Article
• Language: English
• Subjects: administration & dosage; Biomarkers; Biomarkers - urine; Bone and Bones; Bone and Bones - metabolism; bone metabolism; calcium; Calcium - urine; clinical trials; creatinine; Creatinine - urine; Cross-Over Studies; dietary protein; Dietary Proteins; Dietary Proteins - administration & dosage; Dietary Proteins - pharmacokinetics; Double-Blind Method; equations; Female; Gastroenterology and Hepatology; Humans; Internal Medicine; linear models; metabolism; metabolites; Middle Aged; Milk Proteins; Milk Proteins - administration & dosage; Milk Proteins - pharmacokinetics; nitrogen; Nitrogen - urine; nutritional intervention; pharmacokinetics; Polysaccharides; Polysaccharides - administration & dosage; Polysaccharides - pharmacokinetics; Postmenopause; protein intake; urea; Urea - urine; urine; whey; Whey Proteins; women
• ISSN: 2212-2672; 2212-2680
• DOI: 10.1016/j.jand.2012.11.002

Abstract 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+1.719×(urinary nitrogen, g)/creatinine, g) ( R =0.46, R2 =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+1.11×(urinary urea, g/creatinine, g) ( R =0.58, R2 =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.