Fructose turnover and utilization in dependence on age were examined in 55 pre-term and term newborns appropriate for gestational age during continuous intravenous infusion of fructose (0.25–1.0 g . kg^––1 · h^––1). (1) The linear turnover range of fructose is exceeded during the first 3 days of life independently of gestational age with infusions of 1.0 g · kg^––1 · h^––1. This, however, is not the case in pre-term and term infants 10–12 days old. For the postnatal development of the fructose metabolism, the age of life is important, but not the gestational age. (2) During the first 3 days of life in pre-term and term infants, less than 10% of the fructose infused are eliminated. Thus, the fructose utilization is high even in the neonatal period, amounting to more than 90%. (3) The influence of fructose on the blood glucose concentration is not uniform and depends on fructose intake rate, gestational age, and age of life. Hypoglycemias were not observed.

Utilization, and total clearance of fructose were determined by methods of blood level kinetics in 40 preterm and term newborns appropriate and small for gestational age. In all groups the total clearance increases from 16.5 to 23.5 ml·kg^––1·min^––1 at the age of 12–72 h to 27.3–35.5 ml·kg^––1·min^––1 at the age of 10–12 days. Less than 10% of the fructose infused intravenously are eliminated in the urine.

Oral tolerance tests were performed with 2.0 and 3.0 g·kg^––1 fructose and the fructose and glucose blood levels were observed in preterm and term newborns appropriate for gestational age (AGA) and small for gestational age (SGA). Their age ranged from 12 to 72 h and from 10 to 12 days. The absorption rates of fructose – as compared with intravenous fructose infusions – in the groups examined amounted to 23–30%, but in preterm SGA newborns they reach a value of 67%. The elimination of fructose in the urine was less than 4% of the intake. Whereas preterm and term AGA newborns showed a clear increase of the glucose level irrespective of postnatal age, the glucose concentration was not influenced in preterm SGA newborns. Also in preterm SGA newborns the glucose blood level was increased by galactose during the first days of life, so that lack of increase of glucose after fructose in this group may possibly be due to a reduced gluconeogenesis at the level of hexose-1,6-diphosphatase.