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Article

Prior Protein Intake May Affect Phenylalanine Kinetics Measured in Healthy Adult Volunteers Consuming 1 g Protein · kg^-1 · d^-1

^a Departments of Nutritional Sciences and ^b Paediatrics, University of Toronto, ^c The Research Institute, The Hospital for Sick Children, Toronto, Ontario, Canada, M5G 1X8 and ^d Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, Canada, T6G 2P5

Study of the amino acid metabolism of vulnerable groups, such as pregnant women, children and patients, is needed. Our existing protocol is preceded by 2 d of adaptation to a low ¹³C formula diet at a protein intake of 1 g · kg^-1 · d^-1 to minimize variations in breath ¹³CO₂ enrichment and protein metabolism. To expand on our potential study populations, a less invasive protocol needs to be developed. We have already established that a stable background ¹³CO₂ enrichment can be achieved on the study day without prior adaptation to the low ¹³C formula. Therefore, this study investigates phenylalanine kinetics in response to variations in prior protein intake. Healthy adult subjects were each fed nutritionally adequate mixed diets containing 0.8, 1.4 and 2.0 g protein · kg^-1 · d^-1 for 2 d. On d 3, subjects consumed an amino acid-based formula diet containing the equivalent of 1 g protein · kg^-1 · d^-1 hourly for 10 h and primed hourly oral doses of L-[1-¹³C]phenylalanine for the final 6 h. Phenylalanine kinetics were calculated from plasma-free phenylalanine enrichment and breath ¹³CO₂ excretion. A significant quadratic response of prior protein intake on phenylalanine flux (P = 0.012) and oxidation (P = 0.009) was identified, such that both variables were lower following adaptation to a protein intake of 1.4 g · kg^-1 · d^-1. We conclude that variations in protein intake, between 0.8 and 2.0 g · kg^-1 · d^-1, prior to the study day may affect amino acid kinetics and; therefore, it is prudent to continue to control protein intake prior to an amino acid kinetics study.

KEY WORDS: • phenylalanine • dietary protein • humans • amino acids • stable isotopes

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