Urinary and Serum Urea as Indicators of Protein Metabolism in Very Low Birthweight Infants Fed Varying Human Milk Protein Intakes

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Abstract
Urea concentrations in serum and urine were measured in 28 growing, very low birth weight, appropriate-for-gestational age infants fed varying human milk protein intakes (range 1.7 to 3.9 g/kg/day). We found a high correlation between serum urea values at the end of the study and mean protein intake (rs=0.85, p<0.001) and between urinary urea concentrations in eight-hour urine collections and protein intake (rs=0.81, p<0.001). All serum and urine urea values were below 1.6 and 18 mmol/l, respectively, at protein intakes less than 3 g/day. Higher protein intakes caused higher serum and urinary urea concentrations. We also found a strong correlation between the individual serum and urinary urea values at the end of the study (r3=0.90, p<0.001). The presented data are consistent with the growth data previously reported and indicate that inadequate or excessive protein intakes can be detected by measurement of urea concentrations in serum and/or urine. If urine urea samples alone can be used for estimating optimal protein intake, painful blood sampling procedures could be obviated.