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Protein Calculation from Food Diaries of Adult Humans Underestimates Values Determined Using a Biological Marker^1,2,

Department of Nephrology ^* General Clinical Research Center, College of Medicine, University of Iowa, Iowa City, Iowa 52242

We evaluated the ability of a biological marker (nitrogen excretion expressed as protein) to accurately reflect the protein intake of 12 healthy subjects consuming a low protein diet (0.6 g protein/kg standard body wt). In this crossover study, protein intake was confirmed by chemically analyzing a duplicate of the constant diet each subject consumed for 3 d and by calculating protein content of self-selected diets recorded during two additional 3-d periods. Diet analysis matched excretion (difference 0.03 ± 0.04 g protein/kg standard body wt, means ± SEM). Self-selected intake manually calculated by subjects using educational materials matched the prescription [0.60 (0.42, 0.86) g protein/kg standard body wt, median (range)], but underestimated excretion by 0.18 ± 0.02 g protein/kg standard body wt (means ± SEM). Self-selected intake recalculated by the authors using a computerized database was only +0.05 (-0.08, +0.44) g protein/kg standard body wt higher than subjects' calculations, suggesting that discrepancies between databases and/or subject calculation errors only partially accounted for how greatly self-selected intake underestimated excretion. In a secondary analysis of self-selected intake, the three dietitian subjects consumed more energy and excreted less protein than nondietitians (137 ± 4.9 vs. 94 ± 3.5 kJ/standard body wt; 0.72 ± 0.02 vs. 0.83 ± 0.02 g protein/kg standard body wt), suggesting that adequate energy intake and/or additional training might improve agreement between intake and excretion. Thus, discrepancies between protein excretion and reported intake may reflect factors other than willful noncompliance.

KEY WORDS: • diet • diet records • patient compliance • urea • humans

¹ Supported by grant RR0059 of the General Clinical Research Centers Program, National Center for Research Resources, National Institutes of Health.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 16 September 1994. Revision accepted 23 March 1995.