Introduction: Insulin resistance (IR) develops as early as age 1 to 3 yr in small-for-gestational-age (SGA) infants who show rapid "catch-up" postnatal weight gain. In contrast, greater insulin secretion is related to infancy height gains. We hypothesized that insulin-like growth factor I (IGF-I) levels could be differentially related to gains in length and weight, and also differentially related to IR and insulin secretion.

Methods: We measured serum IGF-I levels in a prospective study of 50 SGA (birth weight <5^th percentile) and 14 normal birth weight (AGA) newborns who had fasting bloods and auxology assessed at birth (48 h old), and IR (by HOMA), insulin secretion by short intra-venous glucose tolerance test (IVGTT), and IGF-I levels, at age 1 yr and 3 yr.

Results: SGA infants had similar mean length and weight at 3 yr compared with AGA infants. SGA infants had lower IGF-I levels at birth (P < 0.0001), but conversely they had higher IGF-I levels at 3 yr (P = 0.003) than AGA infants. Within the SGA group, at 1 yr IGF-I was associated with length gain from birth and insulin secretion (P < 0.0001); in contrast at 3 yr IGF-I was positively related to weight, BMI and insulin resistance.

Conclusions: IGF-I levels increased rapidly from birth in SGA, but not AGA children. During the key first year growth period IGF-I levels were related to -cell function and longitudinal growth. In contrast by 3 yr, when catch-up growth was completed, IGF-I levels were related to BMI and IR, and these higher IGF-I levels in SGA infants might indicate the presence of relative IGF-I resistance.