Energy substrate production in infants born small for gestational age

Aim: To investigate energy substrate production and its hormonal regulation in infants born small for gestational age. Methods: Eleven infants, aged 24.4 +/- 5.3 hour, were studied following a fast of 4.0 +/- 0.6 hour. Gestational age was 35.4 +/- 2.8 weeks and birth weight 1804 +/- 472 g (<-2 SD). Rates of glucose production and lipolysis were analyzed using [6,6-H-2(2)-glucose and [2-C-13]-glycerol. Results: Plasma levels of glucose and glycerol were 4.1 +/- 1.1 mmol L-1 and 224 +/- 79 mu mol L-1, respectively. Glucose appearance averaged 30.3 +/- 8.2 and glucose production rate 21.1 +/- 6.1 mu mol center dot kg(-1) minutes(-1). Glycerol production rate was 5.6 +/- 1.6 mu mol kg(-1) minutes(-1), correlating strongly to birth weight (r = 0.904, p < 0.001). Of the glycerol produced, 55 +/- 22% was converted to glucose, corresponding to 8 +/- 3% of the glucose production. Conclusions: Even though the infants could produce energy substrates, lipolysis was reduced and the glucose production was in the low end of the normal range compared with infants born appropriate for gestational age. The correlation between glycerol production and birth weight indicates that lipolysis depends on the amount of stored fat. Data on insulin and insulin-like growth factor binding protein 1 support the view that insulin sensitivity in these infants is reduced in the liver but increased peripherally.