Glutamine and leucine nitrogen kinetics and their relation to urea nitrogen in newborn infants

Glutamine kinetics and its relation to transamination of leucine and urea synthesis were quantified in 16 appropriate-for-gestational-age infants, four small-for-gestational-age infants, and seven infants of diabetic mothers. Kinetics were measured between 4 and 5 h after the last feed (fasting) and in response to formula feeding using [5-¹⁵N]glutamine, [1-¹³C,¹⁵N]leucine, [²H₅]phenylalanine, and [¹⁵N₂]urea tracers. Leucine nitrogen and glutamine kinetics during fasting were significantly higher than those reported in adults. De novo synthesis accounted for ∼85% of glutamine turnover. In response to formula feeding, a significant increase (P = 0.04) in leucine nitrogen turnover was observed, whereas a significant decrease (P = 0.002) in glutamine and urea rate of appearance was seen. The rate of appearance of leucine nitrogen was positively correlated (r² = 0.59, P = 0.001) with glutamine turnover. Glutamine flux was negatively correlated (r² = 0.39, P = 0.02) with the rate of urea synthesis. These data suggest that, in the human newborn, glutamine turnover is related to a high anaplerotic flux into the tricarboxylic acid cycle as a consequence of a high rate of protein turnover. The negative relationship between glutamine turnover and the irreversible oxidation of protein (urea synthesis) suggests an important role of glutamine as a nitrogen source for other synthetic processes and accretion of body proteins.