Diurnal and daily variation in creatinine excretion among preadolescent girls consuming three levels of dietary protein
Fifteen healthy preadolescent girls served as subjects in this 34-day study. The subjects were equally divided into 3 groups which were rotated through a feeding schedule enabling each subject to consume 30, 60, and 90 grams of protein for periods of 10 or 11 days. All urine was collected daily throughout the experiment and analyzed for nitrogen and creatinine. On the third day of each period, urine was collected from 8 - 12 a.m., 12 - 4 p.m., and 8 p.m. - 8 a.m.
The mean amounts of creatinine excreted daily by subjects consuming 30, 60, and 90 grams of protein differs significantly at the 1% level. There was also significant diurnal variation at the 5% level in the urinary nitrogen/creatinine ratio among subjects consuming both 30 and 90 grams of protein. The heaviest group of excreted the greatest amount of creatinine, and the lightest group of subjects, the lowest mean of creatinine, in each period. The greatest mean diurnal creatinine excretion of subjects on Day 3 of each period occurred from 8 - 12 a.m.
In the author’s opinion, urinary nitrogen/creatinine ratio is insufficiently constant from voiding to voiding serve as a reliable index of protein nutriture in metabolic studies. The apparent contribution of lean body mass to creatinine excretion points to the need for experimental diets which are controlled for preformed creatine and creatinine in order to gain a greater understanding of the true effect of dietary protein level on creatinine excretion.